Department of Genetics, Rutgers University
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Gene flow between incipient species is a component of the divergence-with-gene-flow models of speciation (i.e., sympatric or parapatric models) (Maynard Smith 1966
; Endler 1977
; Felsenstein 1981
; Rice and Hostert 1993
). Interestingly, these models have the consequence that incipient or hybridizing species can become divergent over some part of the genome although they may continue to share variation at others (Wang, Wakeley, and Hey 1997
). This is so because some regions of the genome may introgress more readily than others (Clarke, Johnson, and Murray 1996
; della Torre et al. 1997
; Wang, Wakeley, and Hey 1997
; Rieseberg, Whitton, and Gardner 1999
; Jiang et al. 2000
; Noor et al. 2001
). Natural selection is expected to preclude gene flow at regions of the genome that are associated with (or linked to genes for) species-specific adaptations. Thus, natural selection can maintain species that are distinct from each other at some genes, in spite of persistent gene flow at other genes.
Under divergence-with-gene-flow models, natural selection has a direct role in generating and strengthening barriers to gene flow, and therefore a direct role in generating species. The role of natural selection in these models differs sharply from that in the classic and most accepted genetic model of speciation, the Dobzhansky-Muller model (Dobzhansky 1937
; Muller 1940
) (which was originally described by Bateson [Orr 1996
]), in which natural selection plays an indirect role in speciation. In that model, reproductive isolation is simply the result of incompatibilities between gene variants that have arisen independently in each species and that are deleterious in a different genetic background.
Recently, speciation studies have taken advantage of several modern population genetic and phylogenetic techniques to analyze multilocus DNA sequence data (Bernardi, Sordino, and Powers 1993
; Hey and Kliman 1993
; Burton and Lee 1994
; Hey 1994
; Hilton and Hey 1997
; Wang, Wakeley, and Hey 1997
; Hare and Avise 1998
; Kliman et al. 2000
). The overall approach involves detailed population genetic analysis of species divergence for each of the several loci as well as an analysis of patterns that appear to be common among loci. This general methodology has been called divergence population genetics (DPG) (Kliman et al. 2000
). By including multiple loci, the approach permits inferences regarding historical gene flow and natural selection that have acted on some, but not all genes. It is, therefore, possible to investigate whether different regions of the genome of incipient species have undergone more gene flow than others. This makes the DPG approach a powerful one to assess the importance of gene flow and natural selection during species divergence.
Drosophila pseudoobscura and D. persimilis are a classic species pair for the study of speciation (Dobzhansky 1936
; Dobzhansky and Epling 1944;
Powell 1983
; Orr 1987
; Wang, Wakeley, and Hey 1997
; Noor et al. 2001
). It is estimated that the species started to diverge about 500,000 years ago (Aquadro et al. 1991
; Wang, Wakeley, and Hey 1997
), and reproductive isolation is not complete. F1 hybrid females are fertile, but F1 hybrid males are sterile; backcross hybrid males are fertile, but some of the hybrid backcross females are sterile (Dobzhansky 1936
; Orr 1987, 1989
); and there is geographic variation in D. pseudoobscura for the degree of premating isolation with D. persimilis (Noor 1995b
). Hybridization does occur in nature, as a small number of backcross hybrid individuals have been collected in the field (Dobzhansky 1973
; Powell 1983
). Therefore, there is the potential for gene introgression across species via backcross of hybrid females to the parental species. Although there are fixed inversion differences on chromosome XL and chromosome 2, which should impede gene introgression at loci located in these chromosome regions (Tan 1935
; Dobzhansky and Epling 1944;
Anderson, Ayala, and Michod 1977
; Moore and Taylor 1986
), a study of hybrids using 14 codominant molecular markers (microsatellites and RFLPs) found no evidence of major barriers decreasing the potential for gene flow across most of the autosomal chromosomes (Noor et al. 2001
). A DPG study of three loci found evidence of gene flow for one locus (Adh) located in the fourth chromosome (Wang, Wakeley, and Hey 1997
).
In 1963 D. pseudoobscura was found to have a closer relative, D. pseudoobscura bogotana, which occurs in allopatry in Colombia (Dobzhansky et al. 1963
). These two subspecies are estimated to have begun diverging about 200,000 years ago (Wang, Wakeley, and Hey 1997
). Although there is very little premating isolation between these subspecies (Noor 1995a
), hybrid D. pseudoobscura-D. p. bogotana males are fertile when D. pseudoobscura is the mother but sterile when D. p. bogotana is the mother.
Here we address the question of how much gene flow among these taxa has occurred historically and how has it varied for different regions of the genome by collecting and analyzing sequence data from 11 loci. We consider these data, together with previously collected data, using a broad population genetic approach which includes a new method for assessing gene flow.
![]() |
Materials and Methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
Loci
DNA sequences were collected for 11 loci (table 1
). Nine of these are noncoding regions that flank or include microsatellite markers (or both) developed for D. pseudoobscura (Noor, Schug, and Aquadro 2000
), and two are protein coding genes (bcd and rh1). The sequences of three loci (X010, 4002 and bcd) contain the microsatellite, but the repeats were not included in the analyses. Previously reported sequences from Adh/Adh-dup (Schaeffer and Miller 1992b;
Wang, Wakeley, and Hey 1997
), per (period), and Hsp82 (Schaeffer and Miller 1992b;
Wang, Wakeley, and Hey 1997
) were also included in the analyses. The D. pseudoobscura Adh/Adh-dup data set consists of a subset of 10 sequences from the Apple Hill population (Schaeffer and Miller 1992a, 1992b
). These 14 loci are scattered across the genome of D. pseudoobscura (table 1
, fig. 2
). Chromosomal locations and recombinational distances among the microsatellite markers have been previously reported (Noor, Schug, and Aquadro 2000
; Noor and Smith 2000
). The cytological location of the markers was determined by in situ hybridization using the method of Lim (1993)
.
|
|
For DNA sequencing, a PCR reaction was performed using one of the primers with an M13 forward tail and the other with an M13 reverse tail. For fragments longer than 1 kbp, internal M13-tailed primers were designed to carry out secondary PCR amplifications of two smaller overlapping fragments. The PCR fragments were either gel purified (QIAGEN), column purified (MILLIPORE), or diluted 1/10 and sequenced bidirectionally using fluorescently labeled M13 primers in a LI-COR 4200 automated sequencer (Lincoln, Neb.). The sequence files were edited and assembled using the program ALIGN-IR (LI-COR, Lincoln, Neb.). PCR and sequencing primer information is available upon request.
bcd and rh1 Sequencing
Primers were designed to amplify a 1.4-kbp PCR fragment, including introns 13 and exons 2 and 3 of the bcd (bicoid) gene. The sequence data used for the analyses encompasses positions 7762147 of the complete D. pseudoobscura sequence (Seeger and Kaufman 1990
). Primers were designed to amplify a 1.5-kbp PCR fragment, including introns 24 and exons 25 from the rh1 (Rhodopsin 1) gene. The sequenced region corresponds to positions 6112055 of the published D. pseudoobscura sequence (Carulli and Hartl 1992
). Two smaller overlapping fragments were amplified using M13-tailed primers and sequenced as described previously.
Data Analyses
The sequences from each homologous data set were initially aligned with the program PileUp (Wisconsin Package v. 10, Genetics Computer Group, Madison, Wis.). Manual alignments were further performed in some data sets to improve the PileUp alignments. BLAST searches (Altschul et al. 1990
) against the genome sequence of D. melanogaster were performed for each microsatellite-flanking region of D. pseudoobscura using the tool available at the Berkeley Drosophila genome project web site (http://www.fruitfly.org). Basic polymorphism analyses were performed with the program SITES (Hey and Wakeley 1997
). Indels were not included in the analyses. The data from D. miranda were primarily used to root the variation found among the other species. Analyses of molecular variance (AMOVA) (Excoffier, Smouse, and Quattro 1992
) were carried out with the Arlequin computer program (Schneider, Roessli, and Excoffier 2000
). McDonald-Kreitman tests (McDonald and Kreitman 1991
) were performed using the data from all four species, counting a given site as polymorphic if it was variable in any one of the species and performing the G-tests of independence using Williams' correction (Sokal and Rohlf 1981
, p. 704). The polymorphism data were fitted to a model of speciation with no gene flow (Wakeley and Hey 1997
) using the method described by Wang, Wakeley, and Hey (1997)
. A new method to assess gene flow using patterns of linkage disequilibrium (LD) is described subsequently (see Results). We discuss the results based on the traditional phylogeny of the species ([pseudoobscura, bogotana], persimilis). We focus primarily on the pseudoobscura-bogotana and pseudoobscura-persimilis comparisons because D. pseudoobscura and D. p. bogotana are the most closely related species and D. pseudoobscura and D. persimilis are partially sympatric.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Description of Intraspecific Variation
Polymorphism analyses are summarized in table 2
. Consistent with previous observations based on data from three genes (Wang, Wakeley, and Hey 1997
), D. pseudoobscura and D. p. bogotana have the most and the least nucleotide variation, respectively. The only loci showing exceptions to that pattern are X009 and Adh, where D. persimilis has more variation than D. pseudoobscura, and 4003, where both taxa have similar levels of variation. These observations suggest a larger historic effective population size for D. pseudoobscura, which is consistent with its more extensive geographic distribution and agree with previous findings showing that this species is highly polymorphic (Riley, Kaplan, and Veuille 1992
; Schaeffer and Miller 1992b;
Veuille and King 1995
; Wells 1996
; Hamblin and Aquadro 1999
). The most noteworthy observation in the protein-coding genes is the complete lack of replacement polymorphism and fixed replacement differences at rh1. This is not unexpected, however, given that rh1 is a very conserved gene in Drosophila (Carulli and Hartl 1992
).
|
AMOVA analyses (Excoffier, Smouse, and Quattro 1992
) show that, with respect to sequence variation at these loci, these taxa are largely panmictic throughout their geographical range (table 3 ). The distribution of variation is similar across most loci, with almost all variation caused by within-population and between-species variation. In X009 there is a significant covariance component attributable to differences among populations (FSC = 0.3546 and P = 0.007) that explains about 15% of the total variation. No evidence of population structure is observed for X009 when only D. p. bogotana and D. persimilis are compared (FSC = -0.1482 and P = 0.5), but the covariance component is significant in the D. pseudoobscura-D. persimilis and D. pseudoobscura-D. p. bogotana comparisons (FSC = 0.3946 and P = 0.01; FSC = 0.4578 and P = 0.009). The evidence of population structure in D. pseudoobscura based on X009 is caused by an interesting pattern of haplotype structure in this locus, where the first 246 bp of the aligned sequence of three out of four haplotypes from one sympatric (Mather) and one allopatric (AFC) locality are quite different from the rest of D. pseudoobscura and D. p. bogotana haplotypes but almost identical to those of D. persimilis. If the analyses are repeated without that region, the evidence of population structure disappears (FSC = -0.0175 and P = 0.16; FST = 0.4603 and P < 0.001; FCT = 0.4695 and P = 0.009). These results are generally consistent with earlier allozyme (Prakash, Lewontin, and Hubby 1969
; Singh 1983
; Keith et al. 1985
), RFLP (Riley, Hallas, and Lewontin 1989
), sequence (Schaeffer and Miller 1992a;
Wang and Hey 1996
), and microsatellite (Noor, Schug, and Aquadro 2000
) data supporting the lack of geographic population structure in D. pseudoobscura.
|
The McDonald-Kreitman test (McDonald and Kreitman 1991
) uses a contrast similar to that of the HKA test but examines different types of sites that are interspersed with each other over the sequence of a locus. This test examines whether the ratio of silent to replacement variation is the same for polymorphisms as it is for fixed differences between species. Under the assumption that these two kinds of variation are selectively neutral, the ratios are expected to be the same. The McDonald-Kreitman test revealed no departure from the neutral model at bcd (G = 1.434, P = 0.231), Hsp82 (G = 1.824, P = 0.177), or per (G = 0.535, P = 0.464). The test for the Adh region, comprised of Adh (G = 1.726, P = 0.189) and Adh-dup (G = 1.374, P = 0.241), is significant (G = 3.882, P = 0049) but only before correcting for multiple tests. Although it is impossible to assign with confidence the cause of the departure from neutrality, the observed pattern suggests that there may be an excess of replacement differences between species at this locus.
We also examined whether the pattern of variation at each locus within each species was consistent with the neutral model. Table 2 shows the value of Tajima's D (Tajima 1989b
), which is proportional to the difference between two estimates of the population mutation parameter
, the mean pairwise differences between the sampled sequences (
), and Watterson's estimator
. Under a neutral model with constant population size, both estimators have the same expected value. In our sample, Tajima's D was negative in almost all the cases, but its value was significantly different from zero only in the X010 sample from D. pseudoobscura and D. persimilis and in the 4002 sample from D. persimilis (table 2
). Negative values of D are expected in the presence of purifying selection, or following a selective sweep, or in samples from populations that are expanding in size (Tajima 1989a, 1989b
). To test whether the average value of Tajima's D, within each species, departs significantly from zero, we conducted a test using the same simulations used in the HKA test. For D. pseudoobscura and D. persimilis the mean values of D (
) were less than all of the means found in 10,000 simulations (
= -1.100, P < 0.0001;
= -1.009, P < 0.0001, respectively), whereas the D. p. bogotana value was not significantly different from zero (
= -0.372, P = 0.124). The consistency of the negative value of D across loci suggests a demographic explanation because demographic forces affect all loci simultaneously. A recent population expansion in D. pseudoobscura and D. persimilis is the likely explanation for this general pattern.
A relative rate test for multiple sequences (Li and Bousquet 1992
) was used to examine whether there is evidence of differences in the rate of substitution among taxa. After correcting for multiple comparisons, the tests show evidence of rate heterogeneity across taxa in the sequences from 4002 and per. The sequences of D. p. bogotana have evolved faster than the sequences of D. persimilis (4002: Z = 6.222 and P < 0.0001; per: Z = 4.805959 and P < 0.0001); and the sequences of D. pseudoobscura have evolved faster than those of D. persimilis (4002 Z = 4.883 and P < 0.0001; per: Z = 2.835 and P = 0.0046). However, the fact that in some loci the outgroup D. miranda shares variation with the ingroup species (variation that probably predates the divergence of these taxa) reduces the utility of this test. For instance, the significant result for the D. pseudoobscura-D. persimilis comparison of 4002 can be explained by the fact that the sequence of D. miranda (Mather28) is identical to the sequences of eight D. persimilis lines.
In conclusion, neutral model assumptions, including selective neutrality and constant rate of mutation accumulation, are not generally violated by the data. However, the consistent negative values of Tajima's D suggest that the assumption of constant population size might not be correct for these data.
Shared Variation and Sequence Divergence
Under the null speciation model (see subsequently), two very recently diverged species are expected to share some polymorphisms that were present in the ancestral population. As the species diverge from each other, genetic drift within each species leads to an accumulation of fixed differences and a loss of shared polymorphisms. With observations from a number of loci, one expects to find a negative correlation between fixed differences and shared polymorphisms across loci. In particular, for a locus that has no history of recombination and no recurrent mutation, shared polymorphisms and fixed differences are mutually exclusive (Wakeley and Hey 1997
). The loss of shared polymorphisms and the accumulation of fixed differences is expected to occur more rapidly at loci involved in adaptive divergence or at loci linked to such regions. On the other hand, if the strict isolation model is not correct and gene flow has occurred, then the divergence of a given locus will be retarded. That happens because gene flow removes, and prevents the accumulation of, fixed differences at the same time as it introduces shared polymorphisms.
Table 4 shows the number of shared and fixed differences between species. The expected negative relationship between the two quantities is observed, and in several genes the species pairs share large numbers of polymorphisms. Markers from chromosomes 2 and 4 show the largest counts of shared polymorphisms. Of the three markers showing no shared polymorphisms between D. pseudoobscura and D. persimilis, one is located in a region spanned by a fixed inversion difference (2002), and the other two (X010 and 4002) have few low-frequency polymorphisms. Shared polymorphism can also be caused by recurrent mutation (homoplasy). However, homoplasy can only explain a fairly small fraction of the observed shared polymorphisms in most of the genes (table 4 ).
|
|
The assumptions of the isolation model are violated if genes have been exchanged between species. Gene flow will elevate the numbers of shared polymorphisms and reduce both the number of exclusive polymorphisms and the number of fixed differences between taxa. Furthermore, if gene flow occurs at some loci and not at others, it will elevate the variance among loci in numbers of shared polymorphisms and fixed differences. This last mentioned idea has been used as the basis for a test of gene flow by Wang, Wakeley, and Hey (1997)
(hereafter referred as WWH). They used a simple measure (the difference between the highest and lowest counts of shared polymorphisms among a set of loci plus the difference between the highest and lowest counts of fixed differences observed over the same group of loci) and compared the observed value to a simulated distribution. Alternatively, one can use a
2 statistic to measure the overall fit of the data to the isolation model (Kliman et al. 2000
). This
2 statistic compares the observed and expected counts of each type of polymorphic site (exclusive polymorphisms for each species, shared polymorphisms, and fixed differences between the species) over all the loci. The expected counts are obtained using the methods described by Wakeley and Hey (1997)
and Wang, Wakeley, and Hey (1997)
.
The results of simulations assessing the significance of the observed values of the test statistics for the D. pseudoobscura-D. persimilis and D. pseudoobscura-D. p. bogotana comparisons are shown in table 6
. The isolation model is not rejected for any of the two comparisons when the 2 statistic is used, as the observed values of the statistic do not depart exceptionally from those of the simulated distribution. However, use of the WWH test statistic leads to a clear rejection of the isolation model for the D. pseudoobscura-D. persimilis comparison (P = 0.015) but not for the D. pseudoobscura-D. p. bogotana comparison (P = 0.06). The WWH values and test results obtained here closely resemble those found with just three loci (Wang, Wakeley, and Hey 1997
), and the simulation results are very similar.
|
LD Tests of Gene Flow
In the two explanations of shared polymorphisms: (1) persistence since species coancestry, and (2) gene flow, we have differing expectations regarding patterns of LD within loci. According to the persistence model, shared polymorphisms are relatively old, at least as old as the time of population splitting, and they will have had more time, relative to nonshared polymorphisms, to recombine with other polymorphisms within each species. The general expectation is that LD among shared polymorphisms within species may be closer to zero than LD among other nonshared polymorphisms or between shared polymorphisms and nonshared polymorphisms. However, if polymorphisms have been introduced by gene flow at some time after the species began to diverge, then there will have been less time for recombination, and thus more LD is expected among these polymorphisms and between these polymorphisms and nonshared polymorphisms. Furthermore, we can also generate predictions regarding the sign of LD. If polymorphisms are rooted by an outgroup, they can be sorted into ancestral and derived character states. Among rooted polymorphisms, positive LD occurs when both ancestral bases or both derived bases of two polymorphic sites appear together more often than expected on the basis of their frequencies. Negative LD occurs when there is an excess of haplotypes carrying an ancestral base at one site and a derived base at the other site.
Figure 3 depicts two populations, each with two exclusive polymorphisms that have arisen since the onset of isolation. In general, gene flow between two populations need not change the haplotype distribution as it may involve haplotypes that are already shared. However, if gene flow moves polymorphisms that are exclusive to one population into the other, then it creates shared polymorphisms. The LD among these new shared polymorphisms, within the recipient species, will tend to be positive as the derived bases that immigrated together are preferentially associated with one another (fig. 3 ). Consider too the LD between these shared polymorphisms (C and D in fig. 3 ) and the other nonshared, exclusive polymorphisms (A and B in fig. 3 ). The introgressed haplotype will tend to carry ancestral bases at those sites where exclusive polymorphisms have arisen in the recipient species. Thus, the derived bases that come in via gene flow and cause shared polymorphisms will tend to be linked to ancestral bases at sites that support exclusive polymorphisms in the recipient species. A negative LD between shared polymorphisms and exclusive polymorphisms is expected.
|
In principle, x can be calculated for any measure of LD. In selecting a measure, it was necessary to consider the way in which different measures of LD vary as functions of allele frequencies. The simple isolation model assumes a constant population size, and the simulations under this model generate a particular distribution of allele frequencies. However, the broadly negative values of Tajima's D suggest that the species have undergone a recent population expansion. Regardless of the cause, the allele frequency distributions in these data sets are markedly shifted toward low-frequency polymorphisms. Thus, for statistical tests of the observed values of x, we selected a measure of LD that will be less sensitive to allele frequencies. We have chosen D', which is equal to the conventional measure of LD divided by the maximum possible value given the allele frequencies (Lewontin 1964
).
The actual expected sign and value of x, both with and without gene flow, is difficult to assess as it will depend on the relative ages of shared and nonshared polymorphisms and their relative allele frequencies, which will change depending on the time since population splitting. The argument behind x is not quantitative, and we do not have an expression for its expected value under a null isolation model. To test whether observed values of x are consistent with the null model, we used the same computer simulations of the isolation model that were used to test the WWH and 2 statistics (table 6
). These simulations used the estimates of the population recombination rate listed in table 2 . Table 7
shows the observed values of x for each locus, calculated using D', for the species pairs D. pseudoobscura-D. persimilis and D. pseudoobscura-D. p. bogotana as well as the results for the overall mean and standard deviation (SD) of x. If there are only a small number of shared or exclusive polymorphisms, then these quantities cannot be calculated, and this was the case for several loci. The observed values are consistently positive across loci and across species comparisons, suggesting gene flow according to the argument presented previously. All species in both contrasts have observed values in the upper tail of the simulated distribution, but the overall mean value of x is significantly high only in D. persimilis, suggesting that this species has been the recipient of gene flow. Interestingly, although D. pseudoobscura did not have an overall significantly elevated mean of x, it did have an elevated standard deviation of x in both species contrasts.
|
Cladistic and Qualitative Assessments of Gene Flow
Recent gene flow can also be inferred using cladistic and qualitative approaches. The cladistic approach (Slatkin and Maddison 1989
) uses gene genealogies to estimate levels of gene flow. Unfortunately, the high levels of recombination in these data make it impossible to build accurate gene genealogies for each locus, thus reducing the utility of this approach. The qualitative approach looks for regions of sequence or full haplotypes that are atypical for the species on hand but that are identical or very similar to sequences that are typical for the other species. No haplotypes were shared between any of the species, although several loci (4002, X009, 3002, bcd, per) show partial regions of sequence that resemble the typical sequence from the other species. The lack of full shared haplotypes indicate that the putative gene flow events occurred sufficiently long ago that there has been enough time since then for recombination to occur.
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
DPG analyses of the large multilocus sequence data set reported here have allowed us to generate an initial genome-wide portrait of the history of divergence of three closely related species: D. pseudoobscura, D. persimilis, and D. p. bogotana. The large variation across loci in patterns of fixed differences and shared polymorphisms leads us to reject the null model of speciation for D. pseudoobscura and D. persimilis but not for D. pseudoobscura and D. p. bogotana. We argue for gene flow as the main cause for the rejection of the isolation model in D. pseudoobscura and D. persimilis. However, factors other than gene flow could also increase the variance in fixed differences and shared polymorphisms across loci and in principle could lead us to reject that null model. Two models, in particular, natural selection at a subset of loci and population structure in the ancestor, could generate data patterns not consistent with the isolation model.
With regard to natural selection, HKA and McDonald-Kreitman tests found no evidence of selection in the data. HKA tests do not reject neutrality in any of the relevant ingroup comparisons, and, based on the McDonald-Kreitman test, the evidence for selection in Adh is weak. Nonneutral patterns were observed only in 4002 and X010 (significant Tajima's D). The significant negative value of Tajima's D in 4002 and X010 could be caused not only by selection but also by population expansion, a more plausible explanation supported by the consistently negative value of Tajima's D across loci. Therefore, we have no evidence that natural selection could have generated the observed variance in shared and fixed differences across loci in these data.
An informative comparison regarding the effect of selection is the recent DPG study of the D. simulans group (D. simulans, D. sechellia, D. mauritiana) using data from 14 genes (Kliman et al. 2000
). In that study, the McDonald-Kreitman test was significant in three genes and the HKA test was significant for all three species. Despite the evidence of directional selection at about half of the loci the isolation model was not rejected, in contrast to the present case of D. pseudoobscura and D. persimilis where there is little evidence of directional selection, and yet the isolation model is rejected.
Population structure in the ancestral species could also increase the variance among genes. However, this explanation is, in effect, our conclusion, for we argue for a model in which an ancestral population diverged into two populations and engaged in gene flow during the process to lead to their becoming separate species. Thus, at some point, the distinction between that scenario and our explanation is a semantic one concerning when, during the history, it was appropriate to consider separate populations as separate species. It also bears noting, in this context, that populations that first experienced divergence were probably separated by considerable distance, or else selection against gene flow must have been quite strong. The reason is that these flies are highly mobile, and today we find no evidence of population structure at any of these loci over a range of 600 miles.
On balance, the simplest model of divergence, consistent with the data, is one that includes gene flow between D. pseudoobscura and D. persimilis. Additional evidence also supports this model (for additional discussion see Noor, Johnson, and Hey [2000]
). First, D. pseudoobscura and D. persimilis are partially sympatric, they can hybridize in the lab, and F1 hybrids have been collected in the wild (Dobzhansky 1973
; Powell 1983
). Second, new data from regions of no recombination (mitochondrial and dot chromosome loci) provide clear evidence of gene flow between the two species (full haplotype sharing) (C. A. Machado and J. Hey, unpublished data). Third, the contrasting situation provided by the comparison between D. p. bogotana and D. pseudoobscura, provides indirect evidence to support our explanation. There, the isolation model is not rejected, providing a case that is quite consistent with the known history of geographical isolation between the two subspecies.
Regarding the timing of gene flow, the data do not suggest the occurrence of recent and pervasive gene flow between D. pseudoobscura and D. persimilis. Although they suggest that gene flow has occurred at a number of the surveyed loci, the lack of more evident cases of recent introgression (e.g., the sharing of complete haplotypes) suggests that what is observed mostly reflects older gene flow events. This may be surprising, given the potential for introgression via backcross of hybrid females, and the fact that most of the genome of these taxa can introgress between species (Noor et al. 2001
). However, previous observations suggest low levels of hybridization in nature among these taxa (Dobzhansky 1951, 1973
; Powell 1983
), which are probably because of sexual isolation caused by strong female species discrimination (Merrell 1954
; Noor 1996
), a trait that probably evolved to reinforce isolation mechanisms between the two taxa (Noor 1995b
).
Another piece of evidence showing that most of the gene flow is not recent is the observation that the proportions of shared polymorphism over the total number of polymorphisms are almost identical in sympatric and allopatric populations of D. pseudoobscura (0.1616 vs. 0.1636). This is not surprising, given the high level of gene flow found among D. pseudoobscura populations, but if interspecific gene flow were currently ongoing at high rates, then we might see more evidence of it in sympatric populations.
Comparing Divergence and Isolation Mapping Studies of D. pseudoobscura and D. persimilis
Recently, Noor and co-workers (2001)
used 14 codominant markers to map genomic regions associated with reproductive isolation (isolation map) between D. pseudoobscura and D. persimilis. All the markers linked to or located in the chromosomal inversions in the left and right arms of the X-chromosome (XL, XR) and the center of the second chromosome were strongly associated with barriers to gene exchange (fig. 2
). A weak effect was observed in the center of the third chromosome, and the fourth and fifth chromosomes showed no detectable effects (fig. 2
). These general results demonstrate that in laboratory conditions most of the genome of these two species can introgress.
Our results can also be interpreted as a kind of mapa divergence map showing which parts of the genome have diverged between species and which parts show evidence of gene flow. We can then ask: how does this divergence map compare with the isolation map developed by Noor et al. (2001)
? If divergence is less for some genes because of gene flow, then we expect a correspondence between the two types of maps. Several of the markers used by Noor et al., correspond to the same microsatellite or RFLP loci for which we have collected flanking sequence data (X009, 2002, 2003, 3002, 4002, 4003, and Adh). We did not sequence any markers located within the XL fixed inversion, but two loci (X008 and per) are located on that same chromosome arm, with one of them (X008) being physically close to the XL inversion breakpoint (fig. 2
). Interestingly, the X008 data show the largest number of fixed differences between D. pseudoobscura and D. persimilis and two shared polymorphisms that can be explained on the basis of recurrent mutation (table 4
). The period locus did show evidence of one instance of gene flow some time ago, with a portion of one haplotype explaining all of the shared polymorphism (Wang and Hey 1996
).
Three of the sequenced loci are located in the right arm of the X chromosome (X009, Hsp82 and X010) (fig. 2 ), but none of these maps within the XR inversion (which is fixed among D. pseudoobscura and nonSex-Ratio (SR) XR D. persimilis strains). As expected, the Hsp82 and X010 data suggest a fairly old cessation of gene flow between D. pseudoobscura and D. persimilis (these loci have the lowest values of population migration rates and the largest numbers of fixed differences after X008), whereas shared partial haplotypes suggest some recent introgression at X009 (not shown).
The locus located in the fixed inversion of the second chromosome (2002) revealed no shared polymorphisms and a large number of fixed differences, consistent with complete isolation or an old termination of gene flow between D. pseudoobscura and D. persimilis (table 4 ). The other loci from the second chromosome (2001, rh1, bcd, and 2003) show several shared polymorphisms and no fixed differences between D. pseudoobscura and D. persimilis (table 4 ). Two of the loci (rh1 and 2001) show high values of x, the measure of LD associated with shared polymorphisms (table 7 ). Interestingly, the same two loci show just one shared polymorphism but several fixed differences between D. pseudoobscura and D. p. bogotana. This observation suggests an older time for the cessation of gene flow at these loci between D. pseudoobscura and D. p. bogotana than between D. pseudoobscura and D. persimilis, which is supported by both estimates of net sequence divergence and population migration rates (table 5 ).
The one other locus that is associated with an inversion is 3002, located in a region of the third chromosome where several inversions are known to occur. Interestingly, data from that locus show no fixed differences and a large number of shared polymorphisms between D. pseudoobscura and D. persimilis (table 4 ). The fact that D. pseudoobscura and D. p. bogotana also have a larger number of shared polymorphisms (13) may suggest that some of the shared variation between D. pseudoobscura and D. persimilis is ancestral. However, the data also show regions of the 3002 sequence from several D. pseudoobscura strains that resemble D. persimilis sequences (not shown), and in those regions all exclusive polymorphisms of D. pseudoobscura correspond to fixed derived bases in D. persimilis, suggesting recent introgression. The pattern observed in 3002 is intriguing, given its genomic location and the isolation mapping results which found a weak effect for reproductive isolation in that region of the genome (Noor et al. 2001
). However, there are, in principle, no barriers for gene flow to occur across all the third chromosome of these species because both share the standard inversion arrangement, which is the most common third chromosome inversion arrangement of D. pseudoobscura in regions of sympatry with D. persimilis (Anderson et al. 1991
; Powell 1992
).
Apart from 4002, the other markers located on the fourth chromosome (4003 and Adh) have the largest numbers of shared polymorphisms and the highest estimates of population migration rate between D. pseudoobscura and D. persimilis (tables 4 and 5
). This observation is consistent with the findings of Noor et al. (2001)
and with the fact that this chromosome is colinear between the two species. The locus 4002 revealed a shared microsatellite allele with 15 dinucleotide repeats, a repeat number typical for D. pseudoobscura but quite different from that of D. persimilis, where the longest allele has only 10 repeats.
Thus, divergence and isolation maps are fairly consistent with each other. Genes that are located in genomic regions not associated with isolation phenotypes (Noor et al. 2001
) show more evidence of introgression or more recent cessation of gene flow than those that are located in (or that are closely linked to) genomic regions associated with isolation phenotypes. This pattern strongly suggests the action of natural selection preventing introgression at these regions. There are, however, two potential incongruences between the maps. First, the sequence data suggest the occurrence of gene flow and possibly recent introgression at X009, a locus near the XR inversion and which is significantly associated with several isolation phenotypes. The data also suggest some gene flow and recent introgression at 3002, a locus located in the third chromosome inversion which is weakly associated with one isolation phenotype. One explanation for the apparent incongruence is that high levels of historical recombination and possibly not very large selection effects allowed X009 and 3002 to introgress, despite their linkage to isolation factors. In addition, it is important to note that a similar comparison between the maps of D. pseudoobscura and D. p. bogotana is expected to show less congruence because of the history of old geographic isolation between the subspecies. If the current state of allopatry also existed during earlier stages of divergence, then gene flow should not have occurred at any loci.
Limitations of the Current Methods and Future Developments
The tools of our DPG approach have some limitations, particularly regarding the causes of shared polymorphisms. In this study, we have used tests of the isolation model of species divergence (WWH), patterns of LD, and qualitative assessments of shared haplotypes, to try to assess the impact of gene flow. However, none of these tests are ideal. The qualitative assessments are subjective, and the WWH and LD methods are strongly affected by the amount of recombination that is occurring (Wang, Wakeley, and Hey 1997
). In order to carry out these tests, the simulations employed the
estimates of 4Nc, the population recombination rate (Hey and Wakeley 1997
) from table 2
. These estimates are expected to underestimate the true value, on average (Hey and Wakeley 1997
), which makes the statistical tests conservative with regard to rejection of the null model (which has no gene flow). If recombination is increased, then the variance of the WWH statistic and the LD measures under the null model goes down, and the apparent significance of the observations increases (results not shown, but available upon request). Nevertheless, the strong dependence of the tests on ad hoc estimates of recombination (i.e., recombination is not estimated simultaneously with other parameters) is a limitation.
The LD test described here is a useful addition to the basic DPG methodology. One of its main advantages is that it permits inferences on the direction of introgression for each locus, unlike the WWH test which addresses the pattern of variation for all loci simultaneously. However, although the overall LD test was significant across loci, the independent tests for each locus were significant for only two loci in the D. pseudoobscura-D. persimilis comparison (X009, 2001). These observations suggest that this test might not be powerful for studying species like D. pseudoobscura and D. persimilis that show large levels of recombination and for which gene flow at many loci seems to have ceased some time ago. Further, because the LD test is not based on explicit quantitative arguments, we have no expressions for the expected value of x under the null isolation model, and we do not know much about its power. Recent simulation results suggest that using patterns of LD among shared and exclusive polymorphic sites may not be a statistically powerful approach to test for gene flow, particularly when recombination is high (F. Depaulis, personal communication).
Another limitation is that we do not at present have ways to fit models with isolation and gene flow and to estimate the timing and magnitude of gene flow given such models. Developing such models is crucial, given the apparent inappropriateness of speciation models without gene flow.
In the future, divergence population genetics can be expected to rely more on maximum likelihood (ML) methods for testing the fit of the data to strict isolation model and constantgene flow model of speciation. In principle, for the case of two diverged populations one could construct multilocus coalescent models that take into account mutation, recombination, population size changes, time since divergence, plus no migration (isolation model), constant levels of migration across loci (constantgene flow model), or different levels of migration across loci (differentialgene flow model) (Wakeley and Hey 1998
). Having the likelihood of the data, given each model and the estimated ML parameters, one could then compare the adequacy of the different models using likelihood ratio tests. Coalescent ML models that include migration and that can be adapted to multilocus cases have been implemented for the case of two populations (Beerli and Felsenstein 1999
) or multiple populations (Beerli and Felsenstein 2001
) with symmetric and nonsymmetric levels of migration. Recent developments using the Markov Chain Monte Carlo approach to better explore the genealogy space (Beerli and Felsenstein 1999
; Nielsen 2000
; Beerli and Felsenstein 2001
) also provide hope for the implementation of better and more complex models in the near future.
Sequence Availability
Sequences have been deposited in GenBank with accession numbers AF450504AF451008.
![]() |
Acknowledgements |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
Footnotes |
---|
Present address: Department of Biology, Kean University
Present address: Department of Biological Sciences, University of Cincinnati
Abbreviations: DPG, divergence population genetics.
Keywords: Drosophila pseudoobscura
speciation
polymorphism
natural selection
gene flow
reproductive isolation
Address for correspondence and reprints: Jody Hey, Department of Genetics, Rutgers University, Nelson Biological Labs, 604 Allison Road, Piscataway, New Jersey 08854-8082. jhey{at}mbcl.rutgers.edu
.
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Altschul S. F., W. Gish, W. Miller, E. W. Myers, D. J. Lipman, 1990 Basic local alignment search tool J. Mol. Biol 215:403-410[ISI][Medline]
Anderson E., 1949 Introgressive hybridization Wiley, New York
Anderson E., L. Hubricht, 1938 The evidence for introgressive hybridization Am. J. Bot 25:396-402
Anderson W. W., J. Arnold, D. G. Baldwin, et al. (21 co-authors) 1991 Four decades of inversion polymorphism in Drosophila pseudoobscura Proc. Natl. Acad. Sci. USA 88:10367-10371[Abstract]
Anderson W. W., F. J. Ayala, R. E. Michod, 1977 Chromosomal and allozymic diagnosis of three species of Drosophila J. Hered 68:71-74[ISI][Medline]
Aquadro C. F., A. L. Weaver, S. W. Schaeffer, W. W. Anderson, 1991 Molecular evolution of inversions in Drosophila pseudoobscura: the amylase gene region Proc. Natl. Acad. Sci. USA 99:305-309
Ashburner M., 1989 Drosophila, a laboratory manual Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York
Beerli P., J. Felsenstein, 1999 Maximum-likelihood estimation of migration rates and effective population numbers in two populations using a coalescent approach Genetics 152:763-773
. 2001 Maximum likelihood estimation of a migration matrix and effective population sizes in n subpopulations by using a coalescent approach Proc. Natl. Acad. Sci. USA 98:4563-4568
Bernardi G., P. Sordino, D. A. Powers, 1993 Concordant mitochondrial and nuclear DNA phylogenies for populations of the teleost fish Fundulus heteroclitus Proc. Natl. Acad. Sci. USA 90:9271-9274[Abstract]
Burton R. S., B. N. Lee, 1994 Nuclear and mitochondrial gene genealogies and allozyme polymorphism across a major phylogeographic break in the copepod Tigriopus californicus Proc. Natl. Acad. Sci. USA 91:5197-5201[Abstract]
Carulli J. P., D. L. Hartl, 1992 Variable rates of evolution among Drosophila opsin genes Genetics 132:193-204
Clark A. G., 1997 Neutral behavior of shared polymorphism Proc. Natl. Acad. Sci. USA 94:7730-7734
Clarke B. C., M. S. Johnson, J. Murray, 1996 Clines in the genetic distance between two species of island land snails: how molecular leakage can mislead us about speciation Philos. Trans. R. Soc. Lond. Ser. B 351:773-784[ISI]
della Torre A., L. Merzagora, J. R. Powell, M. Coluzzi, 1997 Selective introgression of paracentric inversions between two sibling species of the Anopheles gambiae complex Genetics 146:239-244
Dobzhansky T., 1936 Studies of hybrid sterility. II. Localization of sterility factors in Drosophila pseudoobscura hybrids Genetics 21:113-135
. 1937 Genetics and the origin of species Columbia University Press, New York
. 1951 Experiments on sexual isolation in Drosophila X. Reproductive isolation between Drosophila pseudoobscura and Drosophila persimilis under natural and under laboratory conditions Proc. Natl. Acad. Sci. USA 37:792-796[ISI]
. 1973 Is there gene exchange between Drosophila pseudoobscura and Drosophila persimilis in their natural habitats? Am. Nat 107:312-314[ISI]
Dobzhansky T., T. Epling, 1944 Taxonomy, geographic distribution and ecology of Drosophila pseudoobscura and its relatives Pp. 146 in T. Dobzhansky and T. Epling, eds. Contributions to the genetics, taxonomy, and ecology of Drosophila pseudoobscura and its relatives. Carnegie Institute of Washington, Washington, D.C
Dobzhansky T., A. S. Hunter, O. Pavlovsky, B. Spassky, B. Wallace, 1963 Genetics of an isolated marginal population of Drosophila pseudoobscura Genetics 48:91-103
Dobzhansky T., C. C. Tan, 1936 Studies on hybrid sterility III. A comparison of the gene arrangement in two species Z. Indukt. Abstammungs.-Vererbungsl 72:88-114
Endler J. A., 1977 Geographic variation, speciation, and clines Princeton University Press, Princeton, NJ
Excoffier L., P. E. Smouse, J. M. Quattro, 1992 Analysis of molecular variance inferred from metric distances among DNA haplotypes: applications to human mitochondiral DNA restriction data Genetics 131:479-491
Felsenstein J., 1981 Skepticism towards Santa Rosalia, or why are there so few kinds of animals Evolution 35:124-138[ISI]
Hamblin M. T., C. F. Aquadro, 1999 DNA sequence variation and the recombinational landscape in Drosophila pseudoobscura: a study of the second chromosome Genetics 153:859-869
Hare M. P., J. C. Avise, 1998 Population structure in the american oyster as inferred by nuclear gene genealogies Mol. Phylogenet. Evol 15:119-128
Hey J., 1994 Bridging phylogenetics and population genetics with gene tree models Pp. 435447 in B. Schierwater, B. Streit, G. P. Wagner, and R. DeSalle, eds. Molecular ecology and evolution: approaches and applications. Birkhauser Verlag, Basel, Switzerland
Hey J., R. M. Kliman, 1993 Population genetics and phylogenetics of DNA sequence variation at multiple loci within the Drosophila melanogaster species complex Mol. Biol. Evol 10:804-822[Abstract]
Hey J., J. Wakeley, 1997 A coalescent estimator of the population recombination rate Genetics 145:833-846
Hilton H., J. Hey, 1997 A multilocus view of speciation in the Drosophila virilis group reveals complex histories and taxonomic conflicts Genet. Res 70:185-194[ISI]
Hilton H., R. M. Kliman, J. Hey, 1994 Using hitchhiking genes to study adaptation and divergence during speciation within the Drosophila melanogaster species complex Evolution 48:1900-1913[ISI]
Hudson R. R., M. Kreitman, M. Aguade, 1987 A test of neutral molecular evolution based on nucleotide data Genetics 116:153-159
Hudson R. R., M. Slatkin, W. P. Maddison, 1992 Estimation of levels of gene flow from DNA sequence data Genetics 132:583-589
Jiang C. X., P. W. Chee, X. Draye, P. L. Morrell, C. W. Smith, A. H. Paterson, 2000 Multilocus interactions restrict gene introgression in interspecific populations of polyploid Gossypium (cotton) Evolution 54:798-814[ISI][Medline]
Keith T. P., L. D. Brooks, R. C. Lewontin, J. C. Martinez-Cruzado, D. L. Rigby, 1985 Nearly identical allelic distributions of xanthine dehydrogenase in two populations of Drosophila pseudoobscura 2:206216
Kliman R. M., P. Andolfatto, J. A. Coyne, F. Depaulis, M. Kreitman, A. J. Berry, J. McCarter, J. Wakeley, J. Hey, 2000 The population genetics of the origin and divergence of the Drosophila simulans complex species Genetics 156:1913-1931
Lewontin R. C., 1964 The interaction of selection and linkage. I. General considerations; heterotic models Genetics 49:49-67
Li P., J. Bousquet, 1992 Relative-rate test for nucleotide substitutions between two lineages Mol. Biol. Evol 9:1185-1189
Lim J. K., 1993 In situ hybridization with biotinylated DNA Dros. Inf. Serv 72:73-77
Maynard Smith J., 1966 Sympatric speciation Am. Nat 100:637-650[ISI]
McDonald J. H., M. Kreitman, 1991 Adaptive protein evolution at the Adh locus in Drosophila Nature 351:652-654[ISI][Medline]
Merrell D. J., 1954 Sexual isolation between Drosophila persimilis and Drosophila pseudoobscura Am. Nat 88:93-99[ISI]
Moore B. C., C. E. Taylor, 1986 Drosophila of southern California III Gene arrangements of Drosophila persimilis J. Hered 77:313-323[ISI]
Muller H. J., 1940 Bearings of the Drosophila work on systematics Pp. 185268 in J. Huxley, ed. The new systematics. Clarendon Press, Oxford, U.K
Nei M., 1987 Molecular evolutionary genetics Columbia University Press, New York
Nielsen R., 2000 Estimation of population parameters and recombination rates from single nucleotide polymorphisms Genetics 154:931-942
Noor M. A., 1995a. Incipient sexual isolation in Drosophila pseudoobscura bogotana Ayala & Dobzhansky (Diptera:Drosophilidae) Pan-Pac. Entomol 71:125-129
. 1995b. Speciation driven by natural selection in Drosophila Nature 375:674-675[ISI][Medline]
. 1996 Absence of species discrimination in Drosophila pseudoobscura and D. persimilis males Anim. Behav 52:1205-1210[ISI]
Noor M. A., N. A. Johnson, J. Hey, 2000 Gene flow between Drosophila pseudoobscura and D. persimilis Evolution 54:2174-2175[ISI][Medline]
Noor M. A., M. D. Schug, C. F. Aquadro, 2000 Microsatellite variation in populations of Drosophila pseudoobscura and Drosophila persimilis Genet. Res 75:25-35[ISI][Medline]
Noor M. A., K. R. Smith, 2000 Recombination, statistical power, and genetic studies of sexual isolation in Drosophila J. Hered 91:99-103
Noor M. A. F., K. L. Grams, L. A. Bertucci, Y. Almendarez, J. Reiland, K. R. Smith, 2001 The genetics of reproductive isolation and the potential for gene exchange between Drosophila pseudoobscura and D. persimilis via backcross hybrid males Evolution 55:512-521[ISI][Medline]
Noor M. A. F., J. R. Wheatley, K. A. Wetterstrand, H. Akashi, 1998 Western North America obscura-group Drosophila collection data, summer 1997 Dros. Inf. Serv 81:136-137
O'Tousa J. E., W. Baehr, R. L. Martin, J. Girsh, W. L. Pak, M. L. Abblebury, 1985 The Drosophila ninaE gene encodes an opsin Cell 40:839-850[ISI][Medline]
Offringa R., F. van der Lee, 1995 Isolation and characterization of plant genomic DNA sequences via (inverse) PCR amplification Methods Mol. Biol 49:181-195[Medline]
Orr H. A., 1987 Genetics of male and female sterility in hybrids of Drosophila pseudoobscura and D. persimilis Genetics 116:555-563
. 1989 Genetics of sterility in hybrids between two subspecies of Drosophila Evolution 43:180-189[ISI]
. 1996 Dobzhansky, Bateson, and the genetics of speciation Genetics 144:1331-1335
Powell J. R., 1983 Interspecific cytoplasmic gene flow in the absence of nuclear gene flow: evidence from Drosophila Proc. Natl. Acad. Sci. USA 80:492-495[Abstract]
. 1992 Inversion polymorphisms in Drosophila pseudoobscura and Drosophila persimilis Pp. 73126 in C. B. Krimbas and J. R. Powell, eds. Drosophila inversion polymorphism. CRC Press, Boca Raton, Fla
Prakash S., R. C. Lewontin, J. L. Hubby, 1969 A molecular approach to the study of genic heterozygosity in natural populations IV. Patterns of genic variation in central, marginal and isolated populations of Drosophila pseudoobscura Genetics 61:841-858
Rice W. R., E. E. Hostert, 1993 Laboratory experiments on speciation: what have we learned in forty years? Evolution 47:1637-1653[ISI]
Rieseberg L. H., J. Whitton, K. Gardner, 1999 Hybrid zones and the genetic architecture of a barrier to gene flow between two sunflower species Genetics 152:713-727
Riley M. A., M. E. Hallas, R. C. Lewontin, 1989 Distinguishing the forces controlling genetic variation at the Xdh locus in Drosophila pseudoobscura Genetics 123:359-369
Riley M. A., S. R. Kaplan, M. Veuille, 1992 Nucleotide polymorphism at the xanthine dehydrogenase locus in Drosophila pseudoobscura Mol. Biol. Evol 9:56-69[Abstract]
Schaeffer S. W., C. F. Aquadro, 1987 Nucleotide sequence of the Adh region of Drosophila pseudoobscura: evolutionary change and evidence for an ancient gene duplication Genetics 117:61-73
Schaeffer S. W., E. L. Miller, 1992a. Estimates of gene flow in Drosophila pseudoobscura determined from nucleotide sequence analysis of the alcohol dehydrogenase region Genetics 132:471-480
. 1992b. Molecular population genetics of an electrophoretically monomorphic protein in the alcohol dehydrogenase region of Drosophila pseudoobscura Genetics 132:163-178
Schneider S., D. Roessli, L. Excoffier, 2000 Arlequin: a software for population genetics data analysis Genetics and Biometry Lab, Department of Anthropology, University of Geneva
Seeger M. A., T. C. Kaufman, 1990 Molecular analysis of the bicoid gene from Drosophila pseudoobscura: identification of conserved domains within coding and noncoding regions of the bicoid mRNA EMBO J 9:2977-2987[Abstract]
Segarra C., G. Ribó, M. Aguadé, 1996 Differentiation of Muller's chromosomal elements D and E in the Obscura group of Drosophila Genetics 144:139-146
Singh R. S., 1983 Genetic differentiation for allozyme and fitness characters between mainland and Bogota populations of Drosophila pseudoobscura Can. J. Genet. Cytol 25:590-604[ISI]
Slatkin M., W. P. Maddison, 1989 A cladistic measure of gene flow inferred from the phylogenies of alleles Genetics 123:603-613
Sokal R. R., F. J. Rohlf, 1981 Biometry: the principles and practice of statistics in biological research W. H. Freeman, San Francisco
Steinemann M., S. Steinemann, 1992 Degenerating Y chromosome of Drosophila miranda: a trap for retrotransposons Proc. Natl. Acad. Sci. USA 89:7591-7595[Abstract]
Stocker A. J., C. D. Kastritsis, 1972 Developmental studies in Drosophila III. The puffing patterns of the salivary gland chromosomes of D. pseudoobscura Chromosoma 37:139-176[ISI][Medline]
Sturtevant A. H., E. Novitski, 1941 The homologies of the chromosome elements in the genus Drosophila Genetics 26:517-541
Tajima F., 1989a. The effect of change in population size on DNA polymorphism Genetics 123:597-601
. 1989b. Statistical method for testing the neutral mutation hypothesis by DNA polymorphism Genetics 123:585-595
Tan C. C., 1935 Salivary gland chromosomes in the two races of Drosophila pseudoobscura Genetics 20:392-402
Veuille M., L. M. King, 1995 Molecular basis of polymorphism at the esterase-5B locus in Drosophila pseudoobscura Genetics 141:255-262
Wakeley J., J. Hey, 1997 Estimating ancestral population parameters Genetics 145:847-855
. 1998 Testing speciation models with DNA sequence data Pp. 157175 in R. DeSalle and B. Schierwater, eds. Molecular approaches to ecology and evolution. Birkhäuser Verlag, Basel
Wang R. L., J. Hey, 1996 The speciation history of Drosophila pseudoobscura and close relatives: inferences from DNA sequence variation at the period locus Genetics 144:1113-1126
Wang R. L., J. Wakeley, J. Hey, 1997 Gene flow and natural selection in the origin of Drosophila pseudoobscura and close relatives Genetics 147:1091-1106
Watterson G. A., 1975 On the number of segregating sites in genetical models without recombination Theor. Popul. Biol 7:256-276[ISI][Medline]
Wells R. S., 1996 Nucleotide variation at the Gpdh locus in the genus Drosophila Genetics 143:375-384