*Smithsonian Tropical Research Institute;
The Galton Laboratory, Department of Biology, University College London;
Instituto de Genética, Universidad de los Andes;
Department of Biology, University of Puerto Rico
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Abstract |
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Introduction |
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The development of fast-evolving loci to complement mtDNA has not proved easy in spite of an explosive growth of available sequence data. In this study, we develop two noncoding nuclear regions and describe their mode and tempo of evolution relative to the mitochondrial protein-coding genes, cytochrome oxidases I and II (COI and COII). These loci were developed as a tool to understand speciation and geographical differentiation in Heliconius butterflies. The passion-vine butterflies (Heliconiini) have undergone a recent radiation, with many closely related species that hybridize in the wild at low levels (Mallet, McMillan, and Jiggins 1998
). The group is well studied due to their bright coloration, impressive mimicry, close relationships with Passiflora host plants and geographic variability (Brown 1981
).
Phylogenetic relationships among the Heliconiini have been reworked at least 10 times in the last century, using morphological and ecological characters (Brown 1981
; Penz 1999
). More recently phylogenetic hypotheses based on mtDNA and nuclear sequences generally support most of the traditionally recognized species groups and show a number of species pairs separated by <4% mtDNA sequence divergence, implying divergence within the last two million years (Brower 1994
; Brower and Egan 1997
; Brower and DeSalle 1998
). Two Heliconius species in particular, Heliconius erato and Heliconius melpomene, are recognized as excellent model systems for the study of both intraspecific morphological differentiation and speciation (Sheppard et al. 1985
; Brower 1996b
; Mallet, McMillan, and Jiggins 1998
). Both species show parallel divergence into more than 20 geographic races across forests in Central America and South America, and their hybrid zones provide natural systems for the study of selection in the wild (Mallet and Barton 1989
). Furthermore, both taxa have very closely related sister species, which show strong but incomplete reproductive isolation, permitting the study of speciation while hybridization still occurs (Jiggins et al. 1996
, 2001b
).
There are a number of questions in Heliconius systematics and evolution that require the development of rapidly evolving nuclear markers. In particular, (1) Is Heliconius monophyletic? Monophyly is supported by recent morphological evidence (Penz 1999
), but has been challenged by mtDNA sequence data, which places Laparus doris and the entire genus Eueides within Heliconius (Brower 1994
). (2) What are the relationships between sister species and populations in the melpomene and erato species groups. The mtDNA data imply that H. melpomene is paraphyletic with respect to its sister species Heliconius cydno and fails to resolve relationships between H. erato and the closely related Heliconius himera (Brower 1996a
). Studies of pre- and postmating isolation between these sister species suggest they speciated recently as a result of divergence in habitat and color pattern (McMillan, Jiggins, and Mallet 1997
; Jiggins et al. 2001b
). This might have occurred in sympatry or parapatry with ongoing gene flow, but the alternative, allopatric divergence, cannot be ruled out. Recent studies in Drosophila have highlighted the value of multiple gene genealogies in differentiating between speciation models, as allopatric divergence is more likely to produce phylogenetic concordance at different loci (Wang, Wakeley, and Hey 1997
; Kliman et al. 2000
). Thus a multilocus phylogeny of species pairs can produce insights into speciation processes as well as clarify the relationships between taxa.
Our major aim is to develop nuclear gene regions to complement sequence data from the mitochondrial protein-coding genes. The only nuclear DNA marker used in Heliconius systematics to date, wingless (Brower and Egan 1997
), evolves slowly and is not sufficiently variable to address questions at or near the species boundary (Brower and DeSalle 1998
). As a consequence, we have developed primers for introns of two genes, triose-phospate isomerase (Tpi) and mannose 6-phosphate isomerase (Mpi), known to be highly variable as allozyme loci. Our experimental design takes advantage of the established phylogeny of the Heliconiini. We sampled taxa at different levels of evolutionary divergence, from geographic populations of the same species, through sister species within Heliconius, and finally outgroup taxa and compared the relative rate of sequence change in these genes with those in the mitochondrial COI and COII genes. The two mtDNA genes evolve rapidly and are already known to be informative in studies of divergence ranging from intraspecific biogeography of races to relationships among tribes and subfamilies in Heliconius (Brower 1994
, 1996b
). Specifically, our goals are (1) to compare evolutionary patterns between mtDNA and nuclear genes and between introns and exons within nuclear genes, (2) to determine phylogenetic levels at which these genes are informative, and (3) to describe genealogical relationships between races and sister species at the loci studied.
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Materials and Methods |
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Nuclear Loci Development
Primers for the genes of two enzymes, triose-phosphate isomerase (Tpi) and mannose-6-phosphate isomerase (Mpi), were developed in the laboratories of D. Heckel and W. O. McMillan. Tpi is an important enzyme for carbohydrate metabolism encoded by a sex-linked nuclear gene in lepidoptera (Logsden et al. 1995
). Mpi is encoded by an autosomal gene, and the expressed protein is highly polymorphic in lepidoptera (Jiggins et al. 1997
; Raijmann et al. 1997
; Beltrán 1999
). For both genes, we first designed degenerate PCR primers (table 2
) around conserved amino acid positions identified by comparing published sequence data from Drosophila and Heliothis for Tpi and Homo sapiens and Drosophila for Mpi. For Mpi, the region was amplified and sequenced from genomic DNA using these degenerate primers. For Tpi, the degenerate primers were then used to amplify the region from Heliconius cDNA made via reverse transcriptase from total mRNA. Amplified products in the targeted size range were cloned using pGEM®-T Easy Vector System (Promega) and sequenced as described above. The initial Heliconius sequence was aligned to published sequences, and Heliconius specific primers were designed that consistently amplified the Tpi region out of genomic DNA preparations.
For Tpi, the primers were situated in exons 3 and 4 of Heliothis (GenBank accession number U23080) and spanned intron 3 of the Tpi gene (table 2
). Previous work has shown that the region amplified is inherited in a Mendelian manner and is sex-linked in both melpomene and erato, as expected for the Tpi allozyme (Jiggins et al. 2001a
; A. Tobler et al. personal communication). Double-stranded DNA was synthesized in 25-µl reactions containing 2 µl of genomic DNA, 1x buffer, 3 mM MgCl2, 0.8 mM dNTPs, 0.5 mM of each primer, and 0.03 U/µl of Taq gold polymerase. DNA was amplified using the following step-cycle profile: 94°C for 7 min, 94°C for 45 s, 58°C for 45 s, 72°C for 1 min and 45 s for 10 cycles with the annealing temperature reduced 0.5°C per cycle, then 25 cycles with annealing temperature of 53°C. The products obtained from genomic DNA were run in a lowmelting point agarose gel and the bands excised and dissolved in gelase. For population and sister species samples in the H. melpomene and H. erato group, the gelase products were cloned to obtain the sequence for each allele, using pGEM®-T Easy Vector System II (Promega). The templates obtained from three to five colonies per individual were sequenced as above. For the remaining taxa gelase products were sequenced directly as for mtDNA.
The Mpi primers were situated in exons 3 and 4 (H. sapiens GenBank accession numbers AF227216 and AF227217) and amplified intron 3 (table 2 ). The region amplified by these primers segregates in a Mendelian manner and is inherited in complete linkage with the Mpi allozyme in broods of H. erato (A. Tobler, personal communication). The 25-µl PCR reaction mixture contained 2 µl of DNA, 1x buffer, 3 mM MgCl2, 0.8 mM dNTPs, 0.5 mM of each primer, and 0.03 u/µl of Amplitaq. Amplification was carried out using the following step-cycle profile: 94°C for 3 min, 94°C for 40 s, 55°C for 40 s, and 72°C for 45 s for 34 cycles. These products were cloned and sequenced as described above. For Mpi, 8 µl of double-stranded PCR product was run on a temporal temperature gradient gel using the BioRad "Dcode" system to confirm that no more than two alleles were amplified per individual. Gels contained 8% acrylamide and 1.75 tris-acetic acid-EDTA and were run from 46 to 53°C at a temperature ramp of 1°C/h. The results are summarized in table 1 .
Sequence Alignment
Chromatograms of mtDNA were edited and base calls checked using SEQUENCHER 3.1 (Gene Codes Corporation, Inc.). Following the verification of each sequence for an individual, protein-coding regions were aligned in SEQUENCHER across all taxa. Introns of nuclear sequences were aligned in Clustal W (Higgins and Sharp 1988
) and then adjusted manually to increase overall similarity. Due to strong sequence divergence and many indels, introns could only be aligned within the melpomene-silvaniform and the erato-sapho groups (see Supplementary Material).
Taq Error and Allele Selection
Sequencing of cloned PCR products is known to produce errors due to both single base substitution and recombination occurring during the PCR reaction (Wang and Wang 1997
; Bracho, Moya, and Barrio 1998
; Kobayashi, Tamura, and Aotsuka 1999
). We minimized this problem by sequencing at least three, and in most cases five, clones per individual and selecting a "consensus" sequence for each allele based on the parsimonious assumption that single-base Taq-induced error was likely to occur only once. In all cases comparisons of different sequences inferred to represent a single allele were compatible with this assumption. For Tpi, where more than 1 clone was compared with the deduced allele sequence, the distribution of errors was 15, 12, 9, and 2 clones with 0, 1, 2, and 3 single base pair errors, respectively. For Mpi the same distribution was 12, 11, 1, and 1. If undetected, such errors are unlikely to affect phylogenetic analyses as they would most likely be autapomorphic. One case of recombination between the two alleles in a single individual was also observed. In that case, when clone sequences were aligned, the pattern was that expected following a single recombination event, which presumably occurred during the PCR reaction, and was by chance selected for sequencing (Wang and Wang 1997
).
Phylogenetic Analysis
The nucleotide sequences for protein-coding mtDNA and nuclear DNA sequences were checked for reading-frame errors and termination codons and translated to functional peptide sequences in MacClade 4.0 (Maddison and Maddison 1997
). This program was also used to compute various sequence statistics including nucleotide transformation frequencies and variation among codon positions. Phylogenetic analyses and genetic distances (figs. 14) were calculated with PAUP* version 4.0b8 (Swofford 2000
). Models of sequence evolution were compared by means of likelihood ratio tests (G-tests) using ModelTest 3.04 (Posada and Crandall 1998
). PAUP* was then used to search for the maximum likelihood (ML) tree, based on the best fit model and parameter estimates given by ModelTest using a heuristic search with tree-bisectionreconnection (TBR). Confidence in each node was tested using the likelihood-ratio test implemented by PAUP*, which sequentially collapses branch lengths to zero and compares resulting topologies with the ML tree. For comparison, maximum parsimony (MP) trees were obtained using a heuristic search with TBR branch swapping. The consensus tree was calculated using majority rule. Confidence in each node was assessed by bootstrapping (1,000 replicates, heuristic search with TBR branch swapping). In figures 3
and 4
, branches were collapsed if they had less than 95% likelihood support (see above), bootstrap support of less than 50, or were not supported by an indel character.
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Results and Discussion |
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Of the 1,603 nucleotide sites examined, 440 (27%) were variable. Most of the variation occurred in the protein-coding regions. Twenty-five percent of sites were phylogenetically informative in COI and 22% in COII as compared with 6% in the tRNA-leucine. Within coding regions the total variability and variation per position is similar between the two CO subunits (table 3
). The GC content of COI + COII was 26%, comparable to that observed in other insects (Caterino and Sperling 1999
). As expected, >75% of the variation occurs at third positions (table 3
), and transitions were almost 10 times more frequent than were transversions (table 4
).
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The Tpi intron 3 exhibited considerable length variation, and alignment was only possible between closely related species. In the melpomene-silvaniform group, the Tpi intron 3 was completely absent in H. elevatus and ranged from 345 bp in H. melpomene cythera (allele 8073#2) to 457 bp in H. hecale. In the erato/sapho group (table 3 ) the same intron varied from 216 bp in Heliconius hecalesia to 244 bp in H. charithonia and H. clysonymus. As we were unable to align the Tpi intron across groups, analysis of this region was restricted to within-group comparisons. The melpomene-silvaniform group (alignment 1) and erato-sapho group (alignment 2) Tpi alignments are given in the supplementary material and are available upon request or at http://nmg.si.edu/cj/.
Mpi
We obtained 66 bp of Mpi exon for 43 alleles from 31 individuals representing 19 butterfly species (GenBank accession numbers AF413709AF413751; notation: Mpi-1 and Mpi-2 refer to alleles). Twenty-seven base pairs corresponded to positions 1601 to 1627 of exon 3 in the H. sapiens reference sequence (AF227216), and 39 bp to positions 417 to 455 of exon 4 in the H. sapiens sequence (AF227217). As with Tpi exons, there were no indels in the Mpi exons, and GC content was 40%. However, in stark contrast to Tpi, Mpi exons showed an unexpectedly high rate of nonsynonymous changes. Across 40 alleles from 19 species sequenced for both Mpi and Tpi, there were 6 amino acid replacements across 51 codon positions in Tpi, compared with 13 amino acid changes across only 31 amino acid positions in Mpi. The different amino acid replacement pattern between the two genes coincides with the higher proportion of first and second position changes recorded for Mpi (table 2
).
Mpi intron 3 also exhibited considerable length variation, and again intron alignment was only possible between closely related species. In the melpomene-silvaniform group, Mpi intron 3 ranged from 114 bp in H. m. melpomene (allele 437#1) to 388 bp in H. pachinus; however, length variation within even single populations of each species was almost as great. In the erato-sapho group (table 3 ) the same intron varied from 411 bp in H. erato hydara (allele 440#1) to 464 bp in H. himera (allele 8076#2). The difficulty of alignment between distantly related taxa meant that all intron-based analysis was restricted to within-group comparisons. The melpomene-silvaniform group (alignment 3) and erato-sapho group (alignment 4) Mpi alignment files are given in the supplementary material and are available upon request or at http//nmg.si.edu/cj/.
Patterns of Length Variation at Nuclear Loci
There were two kinds of length variation in Mpi and Tpi introns. First, there was variation in the length of repeated elements. Some of these repeats were homopolymers; for example, there was a poly-A repeat starting at position 34 of the Tpi intron in the melpomene-silvaniform group that varied from five to nine bases in length (supplementary material alignment 1). In other cases there were repeated elements that were interrupted or complex. For example in the Mpi intron there was a microsatellite region showing extensive variation around a CACACA motif. In general this type of indel variation provided little phylogenetic information and could not be easily mapped onto the Mpi and Tpi likelihood trees (see below).
Second, we observed insertions and deletions that were not associated with repeated elements. In virtually all cases, these indels were synapomorphic and therefore provided additional support for nuclear gene phylogenies based only on nucleotide substitutions (figs. 3 and 4 ). For example, a 7-bp insertion at Tpi position 263 in alignment 1 was common to three melpomene alleles from French Guiana that represent a monophyletic clade based on the sequence data (fig. 3a ). However, in the erato group, some indels at both Tpi and Mpi were not concordant with the sequence-based ML trees. Nonetheless, a tree constrained such that each Tpi indel represented a unique evolutionary event was not a significantly worse fit to the Tpi sequence data than the initial ML tree (SH test; Delta = 14.83, P = 0.06). We therefore recalculated the ML tree with a constraint based on the two discordant indels, which forced erato to be paraphyletic with respect to himera and grouped the H. erato cyrbia alleles to form a monophyletic group (fig. 3b ). However, in the case of Mpi in the erato group there was no way to make the phylogeny consistent with all indels. The 7-bp deletion at position 473 (shown as a filled oval in fig. 4b ) grouped alleles 2842#1 (H. himera), 590 (H. hecalesia) and 2923 (H. charithonia), whereas a 2-bp deletion at position 222 grouped H. hecalesia and H. charithonia with H. sara, Heliconius eleuchia, and H. sapho. The 2-bp indel was consistent with the ML tree, whereas the 7-bp indel appears to be genuinely homoplasious (fig. 4b ).
Natural recombination was, in contrast, apparently rare. Another apparently homoplasious indel, a 5-bp deletion at position 353 (open oval), appears to result from natural recombination. The erato 2980#1 allele appears to be a recombinant between 2980#2 and an allele which was not sampled, similar to 2981#1 (see Supplementary Material). Recombination could generate high levels of homoplasy, reducing phylogenetic signal. However, MP-based consistency indices calculated for the two species groups, pictured in figures 3 and 4 (excluding uninformative characters), were higher in the Tpi (0.77 and 0.65, respectively) and Mpi (0.80 and 0.64) regions than in the mitochondrial CO genes (0.36, fig. 2 ). Thus, homoplasy was lower in the nuclear genes.
Rate Comparisons Between Mitochondrial and Nuclear Genes
Rates of molecular evolution in the intron region of both Tpi and Mpi were high and similar to the mitochondrial coding genes that are typically used in inter- and intraspecific phylogenetic studies (Brower 1994
, 1996b
). Mean divergence between the melpomene-cydno group and the silvaniform species is 5.4% at mitochondrial COI and COII, 4.1% at Tpi, and 5.4% at Mpi; divergence for the same genes between the sapho and erato clades is 9%, 7.9%, and 12.3%, respectively.
Rates of evolution between genes were compared by plotting pairwise uncorrected sequence divergence of mitochondrial COI and COII against nuclear allele divergence for the same individuals (fig. 1 ). In the case of heterozygotes, one allele was randomly selected for each individual. Sequence divergence was very similar between CO and Tpi when all codon positions are included (data not shown). When only CO third positions were compared with Tpi, the intron was evolving at approximately one-third the rate of CO, suggesting that the neutral substitution rate was faster in the mitochondrion (fig. 1 ). In contrast, Mpi showed cases of high divergence between individuals carrying closely related mtDNA haplotypes, in part reflecting much higher within-population polymorphism at this nuclear locus. Nonetheless, there was a crude correlation between CO and Mpi distance, with the slope suggesting that the Mpi intron is evolving at approximately half the synonymous rate of CO.
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Phylogenetic Analysis
The level of variation observed in the nuclear loci produced well-supported genealogical relationships between alleles sampled from closely related species and geographic races of the same species (figs. 3
and 4
). Phylogenetic resolution was somewhat less at nuclear loci compared with the mtDNA (compare fig. 2
with figs. 3
and 4
), primarily due to the shorter length of these sequences. In addition, there is a large amount of phylogenetically informative indel variation in our intron sequences, which increases confidence in the tree topologies presented (figs. 3
and 4
). We found only two cases where indels were not concordant with our ML trees, both in the Mpi data for the erato group. These discrepancies may indicate recombination, but the generally low levels of homoplasy in our data suggest that recombination is rare and does not inhibit phylogenetic reconstruction.
Nonetheless, the high rate of molecular evolution, particularly the extensive length variation in the intron of both gene regions, restricts the phylogenetic utility of these loci to very closely related species or populations. Sequences are impossible to align among more distantly related taxa, and, even among those sequences that can be aligned, large deletions occasionally destroy phylogenetic signal. The use of introns therefore proves to be something of a lottery, since the region of interest may have been lost in some taxa. However, levels of variation were fairly high even in the short regions of exon examined in this study, suggesting that longer fragments of nuclear coding sequence would provide considerable phylogenetic information for resolving deeper level phylogenetic relationships (Brower and DeSalle 1998
; Regier et al. 1998
).
Analysis of Heliconius and Related Genera
Although our nuclear sequence data are not ideally suited for phylogenetic analysis at the generic level, these data, in combination with the additional mitochondrial sequence, warrant a reassessment of some outstanding questions in Heliconius phylogeny. The ML tree based on COI + COII accords reasonably well with previous phylogenetic analyses based on sequence, morphological, and ecological data (Brown 1981
; Brower 1994
; Brower and Egan 1997
) and includes three taxa not studied in previous molecular analyses, H. hecalesia, Heliconius hierax, and Eueides lineata.
In our ML tree (fig. 2
) based on COI and COII, Eueides is a sister taxon to Heliconius, as expected on the basis of ecology, morphology, and combined mitochondrial and nuclear gene sequences (Brower and Egan 1997
). This contrasts with an earlier parsimony analysis of a smaller portion of the mitochondrial COI and COII regions, which placed Eueides within the genus Heliconius. Mpi was uninformative at this level as it could not be amplified in outgroup taxa, but trees inferred from the 155 bp of Tpi exon data using both ML and parsimony methods show a monophyletic Heliconius clade with Eueides outside Heliconius, with bootstrap support of only 56% (data not shown). However, the overall support for reciprocal monophyly of the two genera is weak. Analyses based either on the mtDNA data alone (SH test; Delta = 7.27, P = 0.47) or including previously published wingless sequences with our Mpi, Tpi, and CO data (SH test; Delta = 8.21, P = 0.22) fail to exclude the possibility that Eueides falls within Heliconius.
Laparus doris has traditionally been considered a monotypic sister genus to Heliconius (Brown 1981
; Penz 1999
). However, our mtDNA data provide strong statistical support for previous results (Brower and Egan 1997
) based on COI, COII, and wingless which group doris within Heliconius in a clade which includes Heliconius wallacei (fig. 2
; SH test; Delta = 62.47, P = 0.002). Combined analysis of the nuclear coding sequence from the wingless, Tpi, and Mpi genes provides no further resolution, as the likelihood test based on these data fails to reject a tree in which doris lies outside Heliconius (SH test; Delta = 2.42, P = 0.41).
Within Heliconius, there was an unresolved trichotomy at the base of the genus (fig. 2
), with no compelling support at COI and COII for two major divisions (Riffarth 1901
; Emsley 1965
). Heliconius sapho, H. sara, and H. eleuchia, and H. charithonia are considered to share the characteristic pupal-mating behavior with the erato group (Brown 1981
; Lee et al. 1992
). However, our ability to align Tpi and Mpi introns in H. ricini, H. charithonia, H. sapho, and allies, with those of the erato-himera group but not the melpomene-silvaniform group gives support for the traditional grouping.
The melpomene Group
The evolutionary relationships between H. melpomene and the H. cydno group (cydno, pachinus, and heurippa) varied depending on the gene region analyzed (table 5
). Heliconius melpomene and H. cydno were reciprocally monophyletic sister taxa in the mtDNA ML tree, with an average uncorrected divergence of 3.3%. This contrasts with the paraphyly of melpomene with respect to cydno described previously on the basis of a shorter region of the COI + COII genes (Brower 1996b
). In Brower's study, paraphyly was due to a single branch, melpomene from French Guiana, placed basally to cydno and the rest of melpomene. Parsimony analysis also supports reciprocal monophyly of melpomene and cydno, and branches leading to both groups show high bootstrap support (fig. 2
). However, it is interesting to note that despite strong bootstrap support, ML topology tests were only able to reject the hypothesis that both groups were polyphyletic (table 5 ). This is in part because the monophyly of H. melpomene was supported by only two transitions. More surprisingly, four transition substitutions (three C-T and one A-G) and three A-T transversions provide no significant support for cydno group monophyly (table 5 ); the SH test would seem to be overly conservative.
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Lastly, the Tpi ML tree (fig. 3a ) showed melpomene as a monophyletic group, with cydno alleles basal and paraphyletic with respect to melpomene, with an average uncorrected divergence of 3.3% between the species. The silvaniforms form a distinct clade and were used to root the melpomene + cydno clade, with an average divergence of 4.1% between the two groups. However, between melpomene and cydno there was very little phylogenetic resolution provided by the Tpi data, and it was not possible to reject the alternative hypotheses of polyphyly or paraphyly between the species at this locus (table 5 ).
The erato group
The phylogenetic relationship between H. erato and H. himera also varied depending on the gene region examined. In the mtDNA tree, himera forms a monophyletic group nested within different geographic races of H. erato (fig. 2
). Mean uncorrected divergence between himera and all other erato was 3.2%. Nonetheless, a tree where the two species were forced to be reciprocally monophyletic could not be rejected in likelihood tests (table 5
). A similar pattern was demonstrated in our Tpi sequence data (table 5
). In the Tpi tree (fig. 4a
) H. himera forms a monophyletic group within erato. The unconstrained ML tree showed both species monophyletic, and support for the paraphyly of erato came from an 18-bp deletion at position 211 (fig. 4a
) shared by all himera and erato alleles with the exception of H. erato petiverana 2980#2. In contrast, in the Mpi genealogy (fig. 4b
) alleles from both species are clearly mixed. There were highly divergent alleles in both groups, and topologies that forced either species to be monophyletic were not supported by the data (table 5
).
Conclusions Regarding Relationships Between Sister Species
In conclusion, genetic variation in the maternally inherited mitochondrial genome and the sex-linked Tpi gene clustered together by species. Heliconius melpomene and H. cydno showed an average of 3.3% uncorrected sequence divergence at COI and COII genes and 3.1% uncorrected divergence at the Tpi region. Heliconius erato and H. himera showed similar levels of divergence at both loci (3.2% at both CO and Tpi). Assuming a rate of mitochondrial evolution of 1.1% to 1.2% per lineage per million years (Brower 1994
) this suggests that both species pairs diverged from each other within the last 1
million years. However, both H. erato and H. melpomene are more widely distributed than their respective sister species, and neither the mtDNA or Tpi genealogies exclude the possibility that geographic populations of one species are paraphyletic with respect to the sister species (figs. 24
). In contrast, the Mpi genealogies in both species pairs failed to show structure consistent with species boundaries, despite considerable resolution and well-supported nodes within the trees (figs. 3b
and 4b
). Our data, therefore, show marked discordance between gene genealogies and species boundaries at different loci.
Why are the Genealogies Discordant?
Gene trees are not the same as species trees, and the discordance between allelic genealogies observed may simply reflect differences in expected coalescence times among loci (Tajima 1983
; Pamilo and Nei 1988
; Takahata 1989
; Nichols 2001
). Of the three loci examined in this study, the autosomal Mpi locus has the largest genetic effective population size and is expected to harbor ancestral shared variation for longer time periods than sex-linked or maternally inherited genes such as Tpi and CO. In particular, maternally inherited mitochondrial genes will coalesce on average four times faster than autosomal genes do. We can use the mtDNA data to predict the coalescence time of nuclear genes following the three-times rule (Tavaré 1984
; Palumbi, Cipriano, and Hare 2001
). In the absence of gene flow, coalescence theory predicts nuclear allele coalescence within a species for a majority of autosomal nuclear loci when the branch length leading to the mtDNA sequences of that species is three times longer than the average within-species mtDNA sequence diversity (or two times as long for an X-linked locus). Our data show that all species in the erato-himera and melopomene-cydno groups have mtDNA branch length to diversity ratios less than 2 (table 6
). Therefore, a majority of nuclear loci are expected to show polyphyletic patterns between these species pairs.
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Even in the absence of nuclear allele coalescence within species, it should still be possible to detect the signature of recent introgression between species. Because the three gene regions studied here are evolving at similar rates (fig. 1
), the observation of far more closely related alleles between melpomene and cydno at Mpi than at the other two loci likely results from recent introgression between species. Indeed, both of the sister species pairs studied here are known to hybridize in the wild. Heliconius melpomene and cydno are broadly sympatric, with hybrids forming perhaps 0.1% of overlapping populations (Jiggins et al. 2001b
). Furthermore, there is hybrid female sterility between the species, associated with the sex-linked Tpi gene in one direction of backcross (Naisbit et al. 2001
). This hybrid sterility might be expected to prevent introgression at both mtDNA and Tpi (Sperling 1994
), while allowing the flow of some nuclear genes. At least for melpomene and cydno, the pattern observed is therefore consistent with the genetic architecture of reproductive isolation. It seems likely that both gene flow and balancing selection have played a role: the latter could maintain high allelic diversity within populations, whereas the former would favor the "capture" of new alleles following rare interspecific hybridization.
In conclusion, phylogenies of recently evolved species, which may still exchange genes, are inevitably difficult to resolve. The markers studied here provide well-supported gene genealogies, but the general lack of concordant reciprocal monophyly between closely related species and the disagreements between loci highlight the importance of multiple locus comparisons in resolving sister species relationships. Fast-evolving nuclear genes such as those described here are likely to become an important tool for phylogenetic analysis. Furthermore, it is clear that biologically and ecologically relevant species may sometimes not be recognizable under phylogenetic (Cracraft 1989
) or genealogical species concepts (Baum and Shaw 1995
). Speciation does not necessarily isolate all regions of the genome and therefore cannot be expected to produce instantaneous reciprocal monophyly.
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Supplementary Material |
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Acknowledgements |
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Footnotes |
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Keywords: coalescence
hybridization
phylogeny
mitochondrial DNA
nuclear genes
Taq error
Address for correspondence and reprints: Margarita Beltrán, Smithsonian Tropical Research Institute, AA2072, Balboa, Panama. E-mail: beltranm{at}naos.si.edu
.
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