Department of Wildlife and Fisheries Sciences, Texas A&M University
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Abstract |
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Introduction |
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Rodents provide an ideal opportunity to address ecological and evolutionary hypotheses, given the prevalence of well-accepted monophyletic groups of closely related, yet ecologically and morphologically diverse assemblages of species. Among South American hystricognath rodents (i.e., Caviomorpha), the monophyly of the superfamily Cavioidea is well supported (Woods 1993
; Nedbal, Honeycutt, and Schlitter 1996
; Huchon, Catzeflis, and Douzery 1999
). This closely related assemblage, including 33 species (Nowak 1999
, p. 1663), retains an extraordinary diversity in behavior, habitat utilization, morphology, and life-history strategies (Cabrera and Yepes 1960
, p. 25; Kleiman 1974
; Mares and Genoways 1982
, pp. 187, 377) (table 1
). The rapid and extensive radiation of caviomorph rodents in the early Oligocene or late Eocene (3040 MYA; Wyss et al. 1993
) also complicates interpretation of phylogenetic relationships among these lineages, because of the high degree of parallelism seen in morphological and serological characters (Hartenberger 1985
; Nedbal, Honeycutt, and Schlitter 1996
). As a result, taxonomic designations, particularly at the familial level, have been inconsistent and widely debated (Cabrera 1961
; Anderson and Jones 1984
, p. 402; Corbet and Hill 1991
, p. 199; Wilson and Reeder 1993
, p. 778; McKenna and Bell 1997
, p. 191). Currently, little is known about relationships among families and genera of cavioid rodents. A more accurate phylogenetic perspective of these relationships will allow for detailed studies of the evolution of life-history traits within this diverse superfamily. Using an independently derived molecular phylogeny, it may be possible to ascertain whether certain morphological, ecological, or behavioral traits (or all of them) characterizing lineages are the result of shared ancestry or which could potentially be the result of independent evolution in response to similar environmental conditions.
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At the molecular level, a reliable phylogenetic framework is pertinent to testing different hypotheses accounting for observed heterogeneity in rates of molecular substitution among closely related lineages. Although these findings have been criticized by some (e.g., Slowinski and Arbogast 1999
), several recent molecular studies have reported lineage-specific rates of substitution that appear to be correlated with life-history traits, body size, and metabolic rates (Martin and Palumbi 1993
; Mooers and Harvey 1994
; Martin 1995
; Mindell et al. 1996
). The original explanation for this rate heterogeneity is that species with shorter generation times have a greater number of DNA replications per year, thus incurring an increased chance of replication error per unit time (Li, Tanimura, and Sharp 1987
; Ohta 1993
; Mooers and Harvey 1994
; Li et al. 1996
). An alternative explanation is the metabolic rate hypothesis, which attributes a positive correlation between metabolic rate and rate of nucleotide substitution to the effects of oxidative DNA damage (Shigenaga, Gimeno, and Ames 1989
).
The order Rodentia provides an excellent model for detailed studies of molecular rate heterogeneity. Although contested by some (e.g., Easteal 1990
), several molecular studies have revealed an overall faster rate of nucleotide substitution in rodents relative to other mammalian lineages (Li, Tanimura, and Sharp 1987
; Li et al. 1990
; Gissi et al. 2000
). However, little is known about lineage-specific rate heterogeneity within the order Rodentia. Although O'hUigin and Li (1992)
observed rate homogeneity among muroid rodents, their comparisons were limited to three taxa. Huchon, Catzeflis, and Douzery (1999)
detected rate heterogeneity within and among several rodent families and suggested a lack of support for a generation time effect. However, no statistical analyses were performed to support or refute this observation. Given the diversity of body size, metabolic rate, generation time, and life-history strategies within rodents, more detailed studies of rates of molecular evolution need to focus on specific monophyletic groups and their associated life-history traits. Species within Cavioidea show 100-fold differences in body size and as much as threefold differences in metabolic rate (Silva and Downing 1995
, p. 195; Lovegrove 2000
), providing an ideal opportunity for detailed assessments of the influence of body size and its correlates on rates of molecular evolution.
The objectives of this paper are to derive a molecular phylogeny for members of the superfamily Cavioidea. Sequences from two nuclear genes (growth hormone receptor (GHR) exon #10 and transthyretin (TTH) intron #1) and one mitochondrial gene (12S rRNA) are analyzed separately and in combination, and the resultant phylogeny is used to test several hypotheses. First, relationships depicted by the combined tree are compared to those based on previous taxonomic treatments derived from morphology. Second, the molecular phylogeny is used to examine the influence of shared ancestry versus ecological constraints on the origin of complex social systems. Finally, the phylogeny and branch lengths are used to investigate lineage-specific rates of molecular evolution and potential correlates with life-history traits, including body size, gestation time, and metabolic rate.
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Materials and Methods |
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Nucleotide Sequencing
Total genomic DNA was isolated using DNeasyTM Tissue Kits (Qiagen). Double-stranded DNA amplification products were sequenced directly with ABI PRISMTM (Perkin-Elmer) Big Dye Terminatior Cycle Sequencing Kits and Applied Biosystems (Perkin-Elmer) 377 automated DNA sequencer. Sequencing primers were chosen to give complete overlap of sequences, reading in both directions.
Primers GHRend, GHREXON10, and GHR50F (table 2
) were used to PCR amplify exon #10 of GHR, under the following conditions: (1) hot start of one cycle at 95°C for 5 min; (2) five cycles with denaturation at 95°C for 1 min, annealing at 61°C for 1 min, and extension at 72°C for 1 min; (3) four sets of five cycles at the same denaturation and extension conditions but with lowering of the annealing temperature each time (59, 57, 55, and 53°C); (4) a single set of 10 cycles with an annealing temperature of 53°C; and (5) a final extension for one cycle at 72°C for 10 min. Two internal primers, GHR10c and GHRendC, were used to obtain completely overlapping sequences. Combinations of primers TTHF2, TTHR2, CFx, CRx, and CRy were used to amplify intron #1 of TTH (table 2
). The PCR thermal profile was the same as that used to amplify GHR. The complete 12S rRNA gene, consisting of approximately 1,000 base pairs (bp), was amplified using primers L651 and 12GH. Internal primers from Nedbal, Allard, and Honeycutt (1994)
were used to sequence both strands (table 2
). PCR conditions were similar to those described in Nedbal, Allard, and Honeycutt (1994)
and Nedbal, Honeycutt, and Schlitter (1996)
. Designation of all heavy (H) and light (L) strand primers refers to positions in the mouse mitochondrial genome (Bibb et al. 1981
).
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Maximum Parsimony Analyses
All maximum parsimony (MP) analyses were conducted using PAUP*4.0b8 (Swofford 1999
). Pairwise uncorrected p distances were calculated to assess within and among species differences. Data sets were then reduced to one representative of each species, which was the same individual for all the three gene sequences. Prior to performing MP analyses, all data sets were examined for evidence of saturation by plotting percent change in transitions or transversions between taxa as a function of HKY (Hasegawa, Kishino, and Yano 1995
) distances. Because of differential rates at codon positions, saturation plots for GHR were analyzed separately for the first, second, and third codon positions. Likewise, stems and loops were assessed independently for 12S rRNA.
Genes were analyzed separately prior to total evidence analysis; combinability was assessed using the partition homogeneity test (PHT; 1,000 replications, = 0.05; Cunningham 1997
). All MP analyses were performed using branch-and-bound search methods. In all cases, both bootstrap replication (1,000 replicates using a full heuristic search; Felsenstein 1985
) and Bremer decay indices (Bremer 1994
) were used to assess support for individual nodes.
Maximum Likelihood Analyses
All maximum likelihood (ML) analyses were implemented in PAUP*4.0b8 (Swofford 1999
) and were conducted on both separate and combined data. The MP tree was used for selection of an appropriate model of evolution for likelihood analyses by first assessing likelihood scores for a nested array of models (Sullivan and Swofford 1997
; Posada and Crandall 1998
). Substitution models included F81 (Felsenstein 1981
), HKY (Hasegawa, Kishino, and Yano 1985
), and general time reversible (GTR; Yang 1994a
). Among-site rate variation models were then tested in a nested manner, under the appropriate substitution model (see Posada and Crandall 1998
). Significance in gain of likelihood under increasingly complex models and patterns of rate variation were measured using likelihood ratio tests (LRTs; Yang, Goldman, and Friday 1995
), assuming a chi-square distribution of scores (degrees of freedom [df] are equal to the difference in number of parameters estimated under the different models), for all pairwise comparisons and Bonferroni correction (Rice 1989
) for multiple testing. With the determined model of choice, heuristic searches were performed using tree-bisection-reconnection (TBR) branch swapping with 10 random addition replicates. Because of computational limitations, 100 bootstrap replicates were implemented using the fast stepwise-addition method (PAUP*4.0b8).
Hypothesis Testing
To address the idea of environmental constraints in the evolution of cavioid rodents, as previously suggested by Lacher (1981)
, KH-tests (Kishino and Hasegawa 1989
) were used to compare the tree generated from sequence data to trees consistent with Lacher's original hypothesis. In addition, patterns of correlated character evolution between habitat characteristics and sociality were assessed with the concentrated changes test (MacClade 3.1; Maddison and Maddison 1992
) applied to both previous morphological phylogenies (Quintana 1998
; da Silva Neto 2000
) and the molecular phylogeny obtained in this paper.
Because methods for assessing lineage-specific substitution rates perform optimally under different situations (Sorhannus and Van Bell 1999
; Bickel 2000
; Bromham et al. 2000
), three different approaches were used to evaluate rate heterogeneity. First, Tajima's relative rate test (RRT; Tajima 1993
) was used in pairwise comparisons of taxa to a reference outgroup. This method requires no mathematical model and minimizes the effects of sampling bias. The test does, however, suffer from lack of power, requiring a large number of variable sites and a closely related outgroup (Bromham et al. 2000
). In addition, the nonindependent triplet comparisons require Bonferroni correction for multiple testing, thus rendering the test even more conservative. Second, the two cluster test (TCT) and branch length test (BLT; Takezaki, Rzhetsky, and Nei 1995
; LINTRE program: http://www.bio.psu.edu/People/Faculty/Nei/Lab/) were used to evaluate several lineages simultaneously, allowing identification of either single or multiple lineages that are evolving significantly fast or slow compared with the average rate for all taxa. This method is sensitive to unbalanced taxonomic sampling (Sorhannus and Van Bell 1999
) as well as unbalanced tree topologies (Robinson et al. 1998
). Finally, the LRT (Felsenstein 1988
) was used to compare likelihood scores of a topology derived under the assumption of a molecular clock to one that does not assume a molecular clock. This method suffers from a lack of power when few variable sites are available (Sorhannus and Van Bell 1999
). It also fails to identify specific lineages contributing to the heterogeneity.
A nonparametric correlation approach was used to assess the nature of the relationship of substitution rate heterogeneity (i.e., deviation from a molecular clock) to either generation time (or some indicator thereof) or metabolic rate. Tests for serial independence (TFSI; Abouheif 1999
) were applied to assess possible associations between body mass, gestation time, or metabolic rate (or all the three) with phylogenetic history. Because this is a parametric test, traits were transformed, when necessary, to obtain normality. Subsequently, Spearman rank correlation analyses, with correction for tied values (Sokal and Rohlf 1998
, p. 598), were used to identify significant relationships, on the premise that differences in life-history parameters are expected to yield differences in branch length estimates. If, for example, there is a negative effect of generation time (measured here as gestation time or body size; Eisenberg 1981
, p. 241; Calder 1984
, pp. 1, 285) on substitution rate, then one might expect lineages with longer generation times (longer gestation times and larger body size) to have shorter branch lengths (i.e., ML branch length estimates) than the average (i.e., clock-constrained branch lengths) for this group of taxa. Likewise, lineages with shorter generation times (shorter gestation times and smaller body size) should have positive branch length values (i.e., individual branch length estimates minus clock-constrained branch length). The same approach was taken for assessing the dependence of rates of evolution on metabolic rate.
To assess whether the distribution of rate heterogeneity was similar among genes analyzed, correlation statistics were implemented. Branch length correlation, total evolution correlation, and correlated rates of evolution were estimated using Pearson product moment correlations, Spearman rank correlations, and binomial tests for overall pattern across taxa (Omland 1997
). For binomial tests, patterns of rate similarities between the genes were obtained from clade contrasts because of the lack of power in using only terminal sister taxa comparisons.
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Results |
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A total of 814 bp from GHR was used in an equally weighted parsimony analysis, which resulted in one most parsimonious tree (fig. 1a
) of length 300 (consistency index [CI] = 0.85, retention index [RI] = 0.81). The placements of Hydrochaeris and Kerodon were unstable, and a tree just one step longer (length = 301) was consistent with the ML topology (fig. 1b
). Using the MP tree topology, LRTs suggested that HKY + (Yang 1994b
) was the most appropriate model of evolution for ML analysis. A heuristic search with TBR branch swapping and 10 random additions was implemented and all appropriate parameters estimated. The -ln likelihood (L) score of the best tree was 2,759, the transition to transversion (ti/tv) ratio was estimated to be 2.50, and the gamma shape parameter (
) was estimated at 0.70.
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The TTH data set consisted of 1,004 bp (after exclusion of 3 bp of questionable positional homology; positions 6163) plus 22 indels. Because the tree topology was not affected by their presence, indels were excluded from subsequent analyses. MP analysis under equal weights yielded one tree of length 604 (CI = 0.82, RI = 0.80; fig. 2a
). This tree was subsequently used to estimate ML parameters under three different substitution models. The most appropriate model for the ML analysis was GTR + . All applicable parameters were estimated, using the heuristic search option and TBR branch swapping with 10 random additions. The -ln L score of the best tree was 4,439, with
= 1.46. This tree topology (fig. 2b
) was identical to the MP tree.
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Although the complete 12S rRNA was examined, a total of 161 ambiguous base pairs (5762, 7585, 115121, 161166, 217231, 289293, 299303, 314326, 367382, 477481, 653660, 739746, 751760, 776781, 881915) were removed, leaving 807 bp for phylogenic analysis. The MP analysis, with loops weighted twice as much as stems (accounting for compensatory changes occurring in stem regions), yielded one most parsimonious tree of length 899 (CI = 0.56, RI = 0.59; fig. 3a
). However, all deep-level nodes were quite unstable, as indicated by bootstrap values. A GTR + + Inv model was chosen, using the LRT for nested models of evolution. Tree searches were performed as above, resulting in a best -ln L score of 3,748, with
= 0.47, and the estimated proportion of invariable sites (Inv) 0.39. Because of computational limitations, only 50 bootstrap replicates, using the fast stepwise-addition method, were performed. The ML topology (fig. 3b
) was inconsistent with all other analyses, with the Galea species being located at the base of the Cavioidea clade. However, constraining the 12S rRNA MP topology did not result in a significantly less likely topology (-ln L = 3,755; KH-test; P = 0.11).
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Discussion |
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This is the first detailed phylogenetic analysis of the entire superfamily Cavioidea. Previous studies were either broader in scope, with sparse taxonomic sampling within Cavioidea (e.g., Woods 1982
; Wallau, Schmitz, and Perry 2000
; Huchon and Douzery 2001
), or focused primarily on relationships within the family Caviidae with a priori designation of Hydrochaeridae and Dasyproctidae as outgroups (e.g., Quintana 1998
; da Silva Neto 2000
). In this study, all 11 genera (Cabrera 1961
), six of which are monotypic, are represented. Although sometimes included within the superfamily Cavioidea (Patterson and Wood 1982
; McKenna and Bell 1997
), based on morphological evidence, the monotypic Dinomyidae (Dinomys branickii) has been excluded from this analysis. Because of its taxonomic inconsistency and uncertain phylogenetic affiliation as a member of the ingroup, Cavioidea, or possibly as a member of any closely related outgroup (see White and Alberico 1992
), it is inappropriate to include this taxon in our analysis (see Swofford et al. 1996
). Our exclusion of Dinomys is further supported by recent molecular phylogenetic analyses suggesting that this taxon is not a member of a monophyletic Cavioidea (D. L. Rowe and R. L. Honeycutt, unpublished data; Adkins et al. 2001
; Huchon and Douzery 2001
). Preliminary analyses (D. L. Rowe and R. L. Honeycutt, unpublished data) suggest Octodontoidea as the sister group to Cavioidea. However, in only one case (12S rRNA) did outgroup selection (Erethizontoidea vs. Chinchilloidea) influence ingroup relationships. C. piliroides (an octodontid) thus served as a single outgroup taxon.
The general congruence among the independent molecular data sets, lack of multiple most parsimonious topologies, overall consistency under different methods of analysis, and strong support for all nodes (bootstrap and Bremer decay indices) in the combined analysis imply a robust phylogeny. Both separate and combined analyses provide strong support for the nontraditional placement of the family Hydrochaeridae within the Caviidae, rendering the family Caviidae paraphyletic. Furthermore, our data suggest a paraphyletic Caviinae because of the placement of Kerodon, a member of the Caviinae, with the subfamily Dolichotinae. Interestingly, Hydrochaeris and Kerodon consistently group as sister taxa, a relationship previously suggested by Woods (1984)
and dos Reis (1994)
. This is, however, inconsistent with the majority of morphological interpretations and is somewhat surprising, given the readily apparent differences in morphology and life-history strategies. Kerodon is a small-bodied rodent, adapted to rocky outcrops in arid environments, and has small litter sizes (13, average = 1.5). On the contrary, Hydrochaeris is a very large-bodied aquatic specialist with adaptations for a semiaquatic lifestyle, and has large litter sizes (18, average = 5) in relation to other caviomorph rodents (Mares and Genoways 1982
).
The data also strongly support designation of two separate families, Agoutidae and Dasyproctidae. Enforcing monophyly, using the combined data set, results in significant changes in tree scores; KH-tests are highly significant for both MP and ML. Although the position of the two families within the Cavioidea clade differs with the three independent data sets, the combined data lends relatively strong support to the position of Agoutidae at the base of the cavioid clade. Enforcing a topology with Dasyproctidae as the basal clade resulted in significant changes in tree scores.
Our findings with regard to the genera Agouti and Stictomys warrants further phylogeographic investigation. Although five subspecies of Agouti have been recognized (Cabrera 1961
), the preliminary data here suggest that the A. paca specimens from Mexico and Bolivia may be as genetically distinct from one another as either is from Stictomys.
Habitat and Behavior
The evolution of morphological and behavioral differences in the family Caviidae has been proposed to be strongly linked to variable habitat requirements with species occupying habitats characterized by restricted resources (i.e., patchy distribution) or increased susceptibility to predation demonstrating a tendency toward increased sociality (Lacher 1981
). For instance, most members of the subfamily Caviinae (e.g., Galea, Microcavia, and Cavia) share numerous morphological similarities and lack recurring pairbonds. For the most part, these three genera occupy productive and diverse habitats with abundant food and shelter. Individuals are unable to monopolize resources, tend to disperse, and demonstrate low social tolerance (i.e., social interactions do not occur repeatedly between specific males and females). Kerodon, a member of the Caviinae, occupies areas where resources are distributed among habitat patches (e.g., rock piles). Unlike other members of the Caviinae, Kerodon demonstrates a harem-based mating system and high social tolerance. Members of the subfamily Dolichotinae occur in high plains, deserts, and open grasslands of southern South America. They are highly adapted for cursorial life, their offspring are vulnerable to predation, and adequate den sites are limited (i.e., patchily distributed). Presumably, as a consequence of this increased risk to predation and resource limitations in terms of den sites, several mated pairs use a single den site for raising pups, and these pairs demonstrate high social tolerance (Taber and Macdonald 1992
). Based on the assumption that Kerodon is a member of the Caviinae, Lacher (1981)
suggested that the social structure shared between Kerodon and members of the Dolichotinae were the consequence of habitat constraints resulting from increased risk of predation and the distribution of resources. If one accepts the traditional taxonomy (fig. 5b
), whereby Kerodon is phylogenetically part of the Caviinae, the concentrated changes test supports Lacher's (1981)
contention that environmental constraints have contributed to the evolution of social behavior in this assemblage of rodents.
In contrast, the molecular phylogeny (fig. 4
) and the shortest tree consistent with a monophyletic Caviinae (see da Silva Neto 2000
) (fig. 5a
) suggest that sociality did not have two independent origins in response to similar environmental constraints. The molecular data suggest that Kerodon is a member of a behaviorally social, monophyletic clade (Dolichotis, Pediolagus, Kerodon, and Hydrochaeris). Interestingly, Kerodon and Hydrochaeris are sister taxa and both have harem-based polygynous breeding systems (Lacher 1981
; Macdonald 1981
). Like Kerodon, Hydrochaeris is a habitat specialist (i.e., open water is a patchily distributed resource, especially during the dry season). Although all of the highly social taxa within this rodent assemblage are habitat specialists, they are also all members of a monophyletic clade. The concentrated changes test suggests that the probability of sociality mirroring habitat specialization is quite likely by chance alone in this phylogeny, suggesting that the ancestor to this clade could have been highly social, allowing occupation of harsh environmental niches with patchily distributed resources or high predation pressure (or both). To better ascertain the potential importance of environmental constraints on social behavior, a broader taxonomic sampling must be encompassed (e.g., the entire caviomorph radiation), in which social behavior has potentially arisen multiple times.
Rate Heterogeneity
RRTs and LRTs detected significant rate heterogeneity among taxa in all three independent data sets. TFSI, measuring the degree of nonrandomness in continuous variables, suggested that the life-history traits assessed (log body size, gestation time, and metabolic rate) were not strongly correlated with their phylogenetic histories. Correlation analyses provided no support for the metabolic rate hypothesis, with even the mitochondrial gene failing to show a relationship between metabolic rate and variation in rates of molecular evolution. The 12S rRNA branch lengths also showed no significant correlation with body mass or gestation time. However, our inability to detect a clear pattern may be the result of the complex substitution pattern observed for 12S rRNA. Stems and loops demonstrate very different patterns, owing to compensatory changes occurring in stem regions, indels in the loop regions, rapid saturation of transitions in loops or heterogeneity in ti/tv bias (or all) between the two regions (Nedbal, Honeycutt, and Schlitter 1996
).
Both the GHR (nuclear exon) and TTH (intron) rates were negatively correlated with gestation time and average adult body mass. This is consistent with the generation time hypothesis, with longer generation times (measured here as gestation time) being associated with slower rates of evolution. However, the significant functional relationship between body size and gestation time makes it difficult to assess exactly what factors are contributing to the rate heterogeneity. If body size effects are controlled, then a significant correlation with gestation time is seen for TTH but not for GHR. The difference between GHR and TTH could arguably be attributed to selective constraints imposed on the coding gene. Third positions are theoretically more likely to be independent of selective constraints and may be expected to show a more similar pattern to the TTH intron. However, most changes in GHR were at third positions, and using only those changes did not significantly alter branch length estimations (i.e., clock minus no-clock values remain negative or positive; data not shown). This suggests that generation time effects may be more pronounced in the patterns seen in the TTH intron, whereas body size effects may be primarily responsible for the rate patterns seen in GHR (see Bromham, Rambaut, and Harvey 1996
). Evidence for a body-maintenance effect has been proposed, whereby maintenance of a large body (more cells and cell generations than a small body) necessitates a higher degree of DNA copy fidelity and repair (Promislow 1994
; Bromham, Rambaut, and Harvey 1996
). Others also have suggested that the cost of change in replication fidelity may vary with life history or genome size, with the total energetic cost likely to be greater in species with larger genome sizes (Drake et al. 1998
).
Overall, there appears to be a single underlying mechanism or multiple mechanisms acting simultaneously in a concerted manner. The significant branch length correlation and total evolution correlation (Omland 1997
) suggest that rates are variable within but correlated between the data sets. More simply, there is rate heterogeneity among lineages in both GHR and TTH data sets, and this variability is occurring in a similar pattern for both genes. The total evolution contrasts provide further support for the observed similarity in patterns of rate heterogeneity.
Our data support the hypothesized (Li, Tanimura, and Sharp 1987
; Ohta 1993
; Mooers and Harvey 1994
; Li et al. 1996
) trend toward slower rates of molecular change in taxa with longer generation times. Consistent with the generation time hypothesis, the nuclear data sets showed a pronounced gestation time effect, and possible body size effect, on molecular evolution but no effect of metabolic rate. This is consistent with the nature of nuclear DNA replication; nuclear replication is linked to cell division, which is often correlated with body size and generation time (Bromham, Rambaut, and Harvey 1996
), whereas mitochondrial DNA can replicate independently of cell division (often many times during the lifetime of a cell). The majority of the rate heterogeneity detected here was attributable to silent changes in that they involved an intron for TTH and primarily changes at the third codon position for GHR. These observations suggest that differences in mutation rate are contributing to molecular rate variation.
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Supplementary Material |
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Acknowledgements |
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Footnotes |
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Keywords: environmental constraints
phylogeny
rate heterogeneity
Rodentia
systematics
Address for correspondence and reprints: Rodney L. Honeycutt, 210 Nagle Hall, 2258 TAMUS, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, Texas 77843-2258. rhoneycutt{at}tamu.edu
.
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