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PubMed1989

The effects of assortative mating and migration on cytonuclear associations in hybrid zones.

We examine the influence of nonrandom mating and immigration on the evolutionary dynamics of cytonuclear associations in hybrid zones. Recursion equations for allelic and genotypic cytonuclear disequilibria were generated under models of (1) migration alone, assuming hybrid zone matings are random with respect to cytonuclear genotype; and (2) migration in conjunction with refined epistatic mating, in which females of the pure parental species preferentially mate with conspecific males. Major results are as follows: (a) even the slightest migration removes the dependency of the final outcome on initial conditions, producing a unique equilibrium in which both pure parental genotypes are maintained in the hybrid zone; (b) in contrast to nuclear genes, the dynamics of cytoplasmic allele frequencies appear robust to changes in the assumed mating system, yet are particularly sensitive to gene flow; (c) continued immigration can generate permanent cytonuclear disequilibria, whether mating is random or assortative; and (d) the order of population censusing (before versus after reproduction by immigrants) can have a dramatic effect on the magnitude but not the pattern of cytonuclear disequilibria. Using the maximum likelihood method, the parameter space of migration rates and assortative mating rates was examined for best fit to observed cytonuclear disequilibria data in a hybrid population of Hyla tree frogs. An epistatic mating model with a total immigration rate of about 32% per generation produces equilibrium gene frequencies and cytonuclear disequilibria consistent with the empirical observations.

Animals↗
PubMed1993

Population genetics of tandem repeats in centromeric heterochromatin: unequal crossing over and chromosomal divergence at the Responder locus of Drosophila melanogaster.

The Responder (Rsp) locus in Drosophila melanogaster is the target locus of segregation distortion and is known to be comprised of a tandem array of 120-bp repetitive sequences. In this study, we first determined the large scale molecular structure of the Rsp locus, which extends over a region of 600 kb on the standard sensitive (cn bw) chromosome. Within the region, small Rsp repeat arrays are interspersed with non-Rsp sequences and account for 10-20% of the total sequences. We isolated and sequenced 32 Rsp clones from three different chromosomes. The main results are: (1) Rsp repeats isolated from the same chromosome are not more similar than those from different chromosomes. This implies either that there are more homologous exchanges at the Rsp locus than expected or, alternatively, that the second chromosomes of D. melanogaster have diverged from one another more recently at the centromeric heterochromatin than at the nearby euchromatin. (2) The repeats usually have a dimeric structure with an average difference of 16% between the left and right halves. The differences allow us to easily identify the products of unequal exchanges. Despite the large differences between the two halves, exchanges have occurred frequently and the majority of them fall within a 29-bp interval of identity between the two halves. Our data thus support the suggestion that recombination depends on short stretches of complete identity rather than long stretches of general homology. (3) Frequent unequal crossover events obscure the phylogenetic relationships between repeats; therefore, different parts of any single repeat could often have different phylogenetic histories. The high rate of unequal crossing over may also help explain the evolutionary dynamics of the Rsp locus.

Animals↗
PubMed1994

The effect of an intronic polymorphism on alcohol dehydrogenase expression in Drosophila melanogaster.

Several lines of evidence indicate that natural selection controls the frequencies of an allozyme polymorphism at the alcohol dehydrogenase (Adh) locus in Drosophila melanogaster. However, because of associations among sequence polymorphisms in the Adh region, it is not clear whether selection acts directly (or solely) on the allozymic site. This problem has been approached by using in vitro mutagenesis to distinguish among the effects on Adh expression of individual polymorphisms. This study shows that a polymorphism within the first Adh intron (delta 1) has a significant effect on the level of ADH protein. Like the allozyme, delta 1 shows a geographic cline in frequency, indicating that it may also be a target of natural selection. These results suggest that multisite selection models may be required to understand the evolutionary dynamics of individual loci.

Alcohol Dehydrogenase↗
PubMed1995

Inferring weak selection from patterns of polymorphism and divergence at "silent" sites in Drosophila DNA.

Patterns of codon usage and "silent" DNA divergence suggest that natural selection discriminates among synonymous codons in Drosophila. "Preferred" codons are consistently found in higher frequencies within their synonymous families in Drosophila melanogaster genes. This suggests a simple model of silent DNA evolution where natural selection favors mutations from unpreferred to preferred codons (preferred changes). Changes in the opposite direction, from preferred to unpreferred synonymous codons (unpreferred changes), are selected against. Here, selection on synonymous DNA mutations is investigated by comparing the evolutionary dynamics of these two categories of silent DNA changes. Sequences from outgroups are used to determine the direction of synonymous DNA changes within and between D. melanogaster and Drosophila simulans for five genes. Population genetics theory shows that differences in the fitness effect of mutations can be inferred from the comparison of ratios of polymorphism to divergence. Unpreferred changes show a significantly higher ratio of polymorphism to divergence than preferred changes in the D. simulans lineage, confirming the action of selection at silent sites. An excess of unpreferred fixations in 28 genes suggests a relaxation of selection on synonymous mutations in D. melanogaster. Estimates of selection coefficients for synonymous mutations (3.6 < magnitude of Nes < 1.3) in D. simulans are consistent with the reduced efficacy of natural selection (magnitude of Nes < 1) in the three- to sixfold smaller effective population size of D. melanogaster. Synonymous DNA changes appear to be a prevalent class of weakly selected mutations in Drosophila.

Animals↗
PubMed1996

Reduced variation in Drosophila simulans mitochondrial DNA.

We investigated the evolutionary dynamics of infection of a Drosophila simulans population by a maternally inherited insect bacterial parasite, Wolbachia, by analyzing nucleotide variability in three regions of the mitochondrial genome in four infected and 35 uninfected lines. Mitochondrial variability is significantly reduced compared to a noncoding region of a nuclear-encoded gene in both uninfected and pooled samples of flies, indicating a sweep of genetic variation. The selective sweep of mitochondrial DNA may have been generated by the fixation of an advantageous mitochondrial gene mutation in the mitochondrial genome. Alternatively, the dramatic reduction in mitochondrial diversity may be related to Wolbachia.

Animals↗
PubMed1997

Ancestral polymorphism of Mhc class II genes in mice: implications for balancing selection and the mammalian molecular clock.

To investigate the evolutionary dynamics at Mhc class II DR genes of mice (genus Mus), we sequenced the peptide binding regions (PBRs) of 41 DRB (= E beta) genes and eight DRA (= E alpha) genes from 15 strains representing eight species. As expected trees of these PBR sequences imply extensive maintenance of ancestral DRB alleles across species. We use a coalescent simulation model to show that the number of interspecific coalescent events (c) observed on these trees was higher than the number expected for neutral genealogies and similar sample sizes and is more consistent with balancing selection that with neutrality. Patterns of ancestral polymorphism in mouse DRB alleles were also used to examine the tempo of synonymous substitution in the PBR of mouse class II genes. Both absolute and relative rate tests on DRA and DRB genes imply increased substitution rates at two- and fourfold degenerate sites of mice and rats relative to primates, and decreased rates for the DRB genes of primates relative to ungulate and carnivore relatives. Thus rates of synonymous substitution at Mhc DR genes in mammals appear to be subject to generation time effects in ways similar to those found at other mammalian genes.

Alleles↗
PubMed2001

Polymorphism at the ribosomal DNA spacers and its relation to breeding structure of the widespread mushroom Schizophyllum commune.

The common split-gilled mushroom Schizophyllum commune is found throughout the world on woody substrates. This study addresses the dispersal and population structure of this fungal species by studying the phylogeny and evolutionary dynamics of ribosomal DNA (rDNA) spacer regions. Extensive sampling (n = 195) of sequences of the intergenic spacer region (IGS1) revealed a large number of unique haplotypes (n = 143). The phylogeny of these IGS1 sequences revealed strong geographic patterns and supported three evolutionarily distinct lineages within the global population. The same three geographic lineages were found in phylogenetic analysis of both other rDNA spacer regions (IGS2 and ITS). However, nested clade analysis of the IGS1 phylogeny suggested the population structure of S. commune has undergone recent changes, such as a long distance colonization of western North America from Europe as well as a recent range expansion in the Caribbean. Among all spacer regions, variation in length and nucleotide sequence was observed between but not within the tandem rDNA repeats (arrays). This pattern is consistent with strong within-array and weak among-array homogenizing forces. We present evidence for the suppression of recombination between rDNA arrays on homologous chromosomes that may account for this pattern of concerted evolution.

Base Sequence↗
PubMed2002

General models of multilocus evolution.

In 1991, Barton and Turelli developed recursions to describe the evolution of multilocus systems under arbitrary forms of selection. This article generalizes their approach to allow for arbitrary modes of inheritance, including diploidy, polyploidy, sex linkage, cytoplasmic inheritance, and genomic imprinting. The framework is also extended to allow for other deterministic evolutionary forces, including migration and mutation. Exact recursions that fully describe the state of the population are presented; these are implemented in a computer algebra package (available on the Web at http://helios.bto.ed.ac.uk/evolgen). Despite the generality of our framework, it can describe evolutionary dynamics exactly by just two equations. These recursions can be further simplified using a "quasi-linkage equilibrium" (QLE) approximation. We illustrate the methods by finding the effect of natural selection, sexual selection, mutation, and migration on the genetic composition of a population.

Emigration and Immigration↗
PubMed2003

Influence of two Wolbachia strains on population structure of East African Drosophila simulans.

Drosophila simulans is hypothesized to have originated in continental East Africa or Madagascar. In this study, we investigated evolutionary forces operating on mitochondrial DNA (mtDNA) in populations of D. simulans from Zimbabwe, Malawi, Tanzania, and Kenya. Variation in mtDNA may be affected by positive selection, background selection, demographic history, and/or any maternally inherited factor such as the bacterial symbiont Wolbachia. In East Africa, the wRi and wMa Wolbachia strains associate with the siII or siIII mitochondrial haplogroups, respectively. To ask how polymorphism relates to Wolbachia infection status, we sequenced 1776 bp of mitochondrial DNA and 1029 bp of the X-linked per locus from 79 lines. The two southern populations were infected with wRi and exhibited significantly reduced mtDNA variation, while Wolbachia-uninfected siII flies from Tanzania and Kenya showed high levels of mtDNA polymorphism. These are the first known populations of D. simulans that do not exhibit reduced mtDNA variation. We observed no mitochondrial variation in the siIII haplogroup regardless of Wolbachia infection status, suggesting positive or background selection. These populations offer a unique opportunity to monitor evolutionary dynamics in ancestral populations that harbor multiple strains of Wolbachia.

Africa, Eastern↗
PubMed2005

Maternal effects on encystment in crosses between two geographic strains of Artemia franciscana.

Maternal effects can have environmental or genetic causes. A method that can be used to demonstrate the genetic basis of a maternal effect is to look for grandfather effects in a backcross following reciprocal crosses. The absence of a grandfather effect would exclude a chromosomal basis for the maternal effect when the male sex is heterogametic (XX-XY sex determination system). However, in organisms in which the female is heterogametic (ZW-ZZ sex determination system), the absence of a grandfather effect does not rule out a chromosomal basis of the maternal effect, since the genes responsible for that effect can be located in the W chromosome, which is transmitted matrilineally. Conversely, the absence of a grandfather effect would point to a W-chromosome basis for the trait, provided that a maternal effect has been previously demonstrated. Distinguishing between W-located and autosome or Z-located maternal effects is important to understand the evolutionary dynamics of a trait. Here we report on a study of the chromosomal basis of maternal effects on two life-history traits related to encystment in the branchiopod crustacean Artemia franciscana, in which females are heterogametic. We performed crosses of two populations that differ in the number of cysts they produce. The proportion of encysted broods showed a maternal effect and was not affected by the grandfather's genotype, pointing to a W-chromosome basis. The average number of encysted offspring per brood showed a strong paternal effect and also a slight maternal effect. This trait also showed a grandfather effect, which suggests that the geographical variation has an autosomal or Z-chromosomal basis.

Animals↗
PubMed2025

Chromosome-Level Genome Assembly of Eden's Whale Clarifies the Taxonomy and Speciation of Bryde's Whale Complex.

Eden's whale (Balaenoptera edeni), a poorly understood baleen cetacean, has long been shrouded in taxonomic ambiguity due to limited genomic resources, obscuring its distinction from closely related species and its position within the cetacean Tree of Life. In this paper, we present a high-quality chromosomal-level genome of B. edeni and conduct comparative genomic analyses to address long-standing taxonomic confusion and elucidate speciation of balaenopterids. Our phylogenomic analysis and demographic reconstruction reveal that B. edeni is a distinct sister to Bryde's whale (Balaenoptera brydei), sharing a common ancestor that diverged approximately 7.84 million years ago during the late Miocene. Their genetic divergence exceeds typical intraspecific variation in whales, supporting the reinstatement of B. brydei as a valid species. Chromosomal syntenic analyses suggest that macro-fragment inversions contributed to speciation in balaenopterid whales and uncover unexpected large-scale complex genome rearrangements in Bryde's whale, offering novel insights into cetacean genome evolution. Functional enrichment analysis of inverted regions between B. edeni and Balaenoptera musculus indicates their predominant association with metabolism and biosynthesis, as well as responses to various substances, stress, and stimuli. These genomic resources for B. edeni not only lay a critical foundation for comparative genetic and evolutionary research of cetaceans but also advance our understanding of the taxonomy and evolutionary dynamics of the Bryde's whale complex, with broader implications for baleen whale conservation and biodiversity.

Animals↗
PubMed2003

Mitochondrial DNA diversity in South America and the genetic history of Andean highlanders.

We analyzed mtDNA sequence variation in 590 individuals from 18 south Amerindian populations. The spatial pattern of mtDNA diversity in these populations fits well the model proposed on the basis of Y-chromosome data. We found evidence of a differential action of genetic drift and gene flow in western and eastern populations, which has led to genetic divergence in the latter but not in the former. Although it is not possible to identify a pattern of genetic variation common to all South America, when western and eastern populations are analyzed separately, the mtDNA diversity in both regions fits the isolation-by-distance model, suggesting independent evolutionary dynamics. Maximum-likelihood estimates of divergence times between central and south Amerindian populations fall between 13,000 and 19,000 years, which is consistent with a Pleistocenic peopling of South America. Moreover, comparison of among-population variability of mtDNA and Y-chromosome DNA seems to indicate that South America is the only continent where the levels of differentiation are similar for maternal and paternal lineages.

Chromosomes, Human, Y↗
PubMed2004

Successful lateral transfer requires codon usage compatibility between foreign genes and recipient genomes.

We present evidence supporting the notion that codon usage (CU) compatibility between foreign genes and recipient genomes is an important prerequisite to assess the selective advantage of imported functions, and therefore to increase the fixation probability of horizontal gene transfer (HGT) events. This contrasts with the current tendency in research to predict recent HGTs in prokaryotes by assuming that acquired genes generally display poor CU. By looking at the CU level (poor, typical, or rich) exhibited by putative xenologs still resembling their original CU, we found that most alien genes predominantly present typical CU immediately upon introgression, thereby suggesting that the role of CU amelioration in HGT has been overemphasized. In our strategy, we first scanned a representative set of 103 complete prokaryotic genomes for all pairs of candidate xenologs (exported/imported genes) displaying similar CU. We applied additional filtering criteria, including phylogenetic validations, to enhance the reliability of our predictions. Our approach makes no assumptions about the CU of foreign genes being typical or atypical within the recipient genome, thus providing a novel unbiased framework to study the evolutionary dynamics of HGT.

Archaea↗
PubMed2004

Structural and evolutionary analyses of the Ty3/gypsy group of LTR retrotransposons in the genome of Anopheles gambiae.

The recent availability of the genome of Anopheles gambiae offers an extraordinary opportunity for comparative studies of the diversity of transposable elements (TEs) and their evolutionary dynamics between two related species, taking advantage of the existing information from Drosophila melanogaster. To this goal, we screened the genome of A. gambiae for elements belonging to the Ty3/gypsy group of long-terminal repeat (LTR) retrotransposons. The A. gambiae genome displays a rich diversity of LTR retrotransposons, clearly greater than D. melanogaster. We have characterized in detail 63 families, belonging to five of the nine main lineages of the Ty3/gypsy group. The Mag lineage is the most diverse and abundant, with more than 30 families. In sharp contrast with this finding, a single family belonging to this lineage has been found in D. melanogaster, here reported for the first time in the literature, most probably consisting of old inactive elements. The CsRn1 lineage is also abundant in A. gambiae but almost absent from D. melanogaster. Conversely, the Osvaldo lineage has been detected in Drosophila but not in Anopheles. Comparison of structural characteristics of different families led to the identification of several lineage-specific features such as the primer-binding site (PBS), the gag-pol translational recoding signal (TRS), which is extraordinarily diverse within the Ty3/gypsy retrotransposons of A. gambiae, or the presence/absence of specific amino acid motifs. Interestingly, some of these characteristics, although in general well conserved within lineages, may have evolved independently in particular branches of the phylogenetic tree. We also show evidence of recent activity for around 75% of the families. Nevertheless, almost all families contain a high proportion of degenerate members and solitary LTRs (solo LTRs), indicative of a lower turnover rate of retrotransposons belonging to the Ty3/gypsy group in A. gambiae than in D. melanogaster. Finally, we have detected significant overrepresentations of insertions on the X chromosome versus autosomes and of putatively active insertions on euchromatin versus heterochromatin.

Amino Acid Motifs↗
PubMed2005

Evolution of the trnF(GAA) gene in Arabidopsis relatives and the brassicaceae family: monophyletic origin and subsequent diversification of a plastidic pseudogene.

Recently, we used the 5'-trnL(UAA)-trnF(GAA) region of the chloroplast DNA for phylogeographic reconstructions and phylogenetic analysis among the genera Arabidopsis, Boechera, Rorippa, Nasturtium, and Cardamine. Despite the fact that extensive gene duplications are rare among the chloroplast genome of higher plants, within these taxa the anticodon domain of the trnF(GAA) gene exhibit extensive gene duplications with one to eight tandemly repeated copies in close 5' proximity of the functional gene. Interestingly, even in Arabidopsis thaliana we found six putative pseudogenic copies of the functional trnF gene within the 5'-intergenic trnL-trnF spacer. A reexamination of trnL(UAA)-trnF(GAA) regions from numerous published phylogenetic studies among halimolobine, cardaminoid, and other cruciferous taxa revealed not only extensive trnF gene duplications but also favor the hypothesis about a single origin of trnF pseudogene formation during evolution of the Brassicaceae family 16-21 MYA. Conserved sequence motifs from this tandemly repeated region are codistributed nonrandomly throughout the plastome, and we found some similarities with a DNA sequence duplication in the rps7 gene and its adjacent spacer. Our results demonstrate the potential evolutionary dynamics of a plastidic region generally regarded as highly conserved and probably cotranscribed and, as shown here for several genera among cruciferous plants, greatly characterized by parallel gains and losses of duplicated trnF copies.

Arabidopsis↗
PubMed2005

Evolution of proteins and gene expression levels are coupled in Drosophila and are independently associated with mRNA abundance, protein length, and number of protein-protein interactions.

Organismic evolution requires that variation at distinct hierarchical levels and attributes be coherently integrated, often in the face of disparate environmental and genetic pressures. A central part of the evolutionary analysis of biological systems remains to decipher the causal connections between organism-wide (or genome-wide) attributes (e.g., mRNA abundance, protein length, codon bias, recombination rate, genomic position, mutation rate, etc) as well as their role-together with mutation, selection, and genetic drift-in shaping patterns of evolutionary variation in any of the attributes themselves. Here we combine genome-wide evolutionary analysis of protein and gene expression data to highlight fundamental relationships among genomic attributes and their associations with the evolution of both protein sequences and gene expression levels. Our results show that protein divergence is positively coupled with both gene expression polymorphism and divergence. We show moreover that although the number of protein-protein interactions in Drosophila is negatively associated with protein divergence as well as gene expression polymorphism and divergence, protein-protein interactions cannot account for the observed coupling between regulatory and structural evolution. Furthermore, we show that proteins with higher rates of amino acid substitutions tend to have larger sizes and tend to be expressed at lower mRNA abundances, whereas genes with higher levels of gene expression divergence and polymorphism tend to have shorter sizes and tend to be expressed at higher mRNA abundances. Finally, we show that protein length is negatively associated with both number of protein-protein interactions and mRNA abundance and that interacting proteins in Drosophila show similar amounts of divergence. We suggest that protein sequences and gene expression are subjected to similar evolutionary dynamics, possibly because of similarity in the fitness effect (i.e., strength of stabilizing selection) of disruptions in a gene's protein sequence or its mRNA expression. We conclude that, as more and better data accumulate, understanding the causal connections among biological traits and how they are integrated over time to constrain or promote structural and regulatory evolution may finally become possible.

Animals↗
PubMed2005

Adaptive molecular evolution in the opsin genes of rapidly speciating cichlid species.

Cichlid fish inhabit a diverse range of environments that vary in the spectral content of light available for vision. These differences should result in adaptive selective pressure on the genes involved in visual sensitivity, the opsin genes. This study examines the evidence for differential adaptive molecular evolution in East African cichlid opsin genes due to gross differences in environmental light conditions. First, we characterize the selective regime experienced by cichlid opsin genes using a likelihood ratio test format, comparing likelihood models with different constraints on the relative rates of amino acid substitution, across sites. Second, we compare turbid and clear lineages to determine if there is evidence of differences in relative rates of substitution. Third, we present evidence of functional diversification and its relationship to the photic environment among cichlid opsin genes. We report statistical evidence of positive selection in all cichlid opsin genes, except short wavelength-sensitive 1 and short wavelength-sensitive 2b. In all genes predicted to be under positive selection, except short wavelength-sensitive 2a, we find differences in selective pressure between turbid and clear lineages. Potential spectral tuning sites are variable among all cichlid opsin genes; however, patterns of substitution consistent with photic environment-driven evolution of opsin genes are observed only for short wavelength-sensitive 1 opsin genes. This study identifies a number of promising candidate-tuning sites for future study by site-directed mutagenesis. This work also begins to demonstrate the molecular evolutionary dynamics of cichlid visual sensitivity and its relationship to the photic environment.

Animals↗
PubMed2005

Genetic diversity and the evolutionary history of plant immunity genes in two species of Zea.

Plant pathogenesis-related genes (PR genes) code for enzymes, enzyme inhibitors, and other peptides that confer resistance to pathogens and herbivores. Although several PR genes have been the subject of molecular population genetic analyses, a general understanding of their long-term evolutionary dynamics remains incomplete. Here we analyze sequence data from 17 PR genes from two closely related teosinte species of central Mexico. In addition to testing whether patterns of diversity at individual loci depart from expectations under a neutral model, we compared patterns of diversity at defense genes, as a class, to nondefense genes. In Zea diploperennis, the majority of defense genes have patterns of diversity consistent with neutral expectations while at least two genes showed evidence of recent positive selection consistent with arms-race models of antagonistic coevolution. In Zea mays ssp. parviglumis, by contrast, analyses of both defense and nondefense genes revealed strong and consistent departures from the neutral model, suggestive of nonequilibrium population dynamics or population structure. Nevertheless, we found a significant excess of replacement polymorphism in defense genes compared to nondefense genes. Although we cannot exclude relaxed selective constraint as an explanation, our results are consistent with temporally variable (transient or episodic) selection or geographically variable selection acting on parviglumis defense genes. The different patterns of diversity found in the two Zea species may be explained by parviglumis' greater distribution and population structure together with geographic variation in selection.

Base Sequence↗