Evolution Of Reproductive Proteins Research Articles

Duplication, Selection and Gene Conversion in aDrosophila mojavensisFemale Reproductive Protein Family

Protein components of the Drosophila male ejaculate, several of which evolve rapidly, are critical modulators of reproductive success. Recent studies of female reproductive tract proteins indicate they also are extremely divergent between species, suggesting that reproductive molecules may coevolve between the sexes. Our current understanding of intersexual coevolution, however, is severely limited by the paucity of genetic and evolutionary studies on the female molecules involved. Physiological evidence of ejaculate-female coadaptation, paired with a promiscuous mating system, makes Drosophila mojavensis an exciting model system in which to study the evolution of reproductive proteins. Here we explore the evolutionary dynamics of a five-paralog gene family of female reproductive proteases within populations of D. mojavensis and throughout the repleta species group. We show that the proteins have experienced ongoing gene duplication and adaptive evolution and further exhibit dynamic patterns of pseudogenation, copy number variation, gene conversion, and selection within geographically isolated populations of D. mojavensis. The integration of these patterns in a single gene family has never before been documented in a reproductive protein.

Orthology, function and evolution of accessory gland proteins in the Drosophila repleta group.

The accessory gland proteins (Acps) of Drosophila have become a model for the study of reproductive protein evolution. A major step in the study of Acps is to identify biological causes and consequences of the observed patterns of molecular evolution by comparing species groups with different biology. Here we characterize the Acp complement of Drosophila mayaguana, a repleta group representative. Species of this group show important differences in ecology and reproduction as compared to other Drosophila. Our results show that the extremely high rates of Acp evolution previously found are likely to be ubiquitous among species of the repleta group. These evolutionary rates are considerably higher than the ones observed in other Drosophila groups' Acps. This disparity, however, is not accompanied by major differences in the estimated number of Acps or in the functional categories represented as previously suggested. Among the genes expressed in accessory glands of D. mayaguana almost half are likely products of recent duplications. This allowed us to test predictions of the neofunctionalization model for gene duplication and paralog evolution in a more or less constrained timescale. We found that positive selection is a strong force in the early divergence of these gene pairs.

Reproductive protein evolution within and between species: maintenance of divergent ZP3 alleles in Peromyscus

In a variety of animal taxa, proteins involved in reproduction evolve more rapidly than nonreproductive proteins. Most studies of reproductive protein evolution, however, focus on divergence between species, and little is known about differentiation among populations within a species. Here we investigate the molecular population genetics of the protein ZP3 within two Peromyscus species. ZP3 is an egg coat protein involved in primary binding of egg and sperm and is essential for fertilization. We find that amino acid polymorphism in the sperm-combining region of ZP3 is high relative to silent polymorphism in both species of Peromyscus. In addition, while there is geographical structure at a mitochondrial gene (Cytb), a nuclear gene (Lcat) and eight microsatellite loci, we find no evidence for geographical structure at Zp3 in Peromyscus truei. These patterns are consistent with the maintenance of ZP3 alleles by balancing selection, possibly due to sexual conflict or pathogen resistance. However, we do not find evidence that reinforcement promotes ZP3 diversification; allelic variation in P. truei is similar among populations, including populations allopatric and sympatric with sibling species. In fact, most alleles are present in all populations sampled across P. truei's range. While additional data are needed to identify the precise evolutionary forces responsible for sequence variation in ZP3, our results suggest that in Peromyscus, selection to maintain divergent alleles within species contributes to the pattern of rapid amino acid substitution observed among species.

EST analysis of male accessory glands from Heliconius butterflies with divergent mating systems

BackgroundHeliconius butterflies possess a remarkable diversity of phenotypes, physiologies, and behaviors that has long distinguished this genus as a focal taxon in ecological and evolutionary research. Recently Heliconius has also emerged as a model system for using genomic methods to investigate the causes and consequences of biological diversity. One notable aspect of Heliconius diversity is a dichotomy in mating systems which provides an unusual opportunity to investigate the relationship between sexual selection and the evolution of reproductive proteins. As a first step in pursuing this research, we report the generation and analysis of expressed sequence tags (ESTs) from the male accessory gland of H. erato and H. melpomene, species representative of the two mating systems present in the genus Heliconius.ResultsWe successfully sequenced 933 ESTs clustering into 371 unigenes from H. erato and 1033 ESTs clustering into 340 unigenes from H. melpomene. Results from the two species were very similar. Approximately a third of the unigenes showed no significant BLAST similarity (E-value <10-5) to sequences in GenBank's non-redundant databases, indicating that a large proportion of novel genes are expressed in Heliconius male accessory glands. In both species only a third of accessory gland unigenes were also found among genes expressed in wing tissue. About 25% of unigenes from both species encoded secreted proteins. This includes three groups of highly abundant unigenes encoding repetitive proteins considered to be candidate seminal fluid proteins; proteins encoded by one of these groups were detected in H. erato spermatophores.ConclusionThis collection of ESTs will serve as the foundation for the future identification and evolutionary analysis of male reproductive proteins in Heliconius butterflies. These data also represent a significant advance in the rapidly growing collection of genomic resources available in Heliconius butterflies. As such, they substantially enhance this taxon as a model system for investigating questions of ecological, phenotypic, and genomic diversity.

Causes and consequences of the evolution of reproductive proteins

Proteins involved in reproduction often evolve rapidly, raising the possibility that changes in these proteins contribute to reproductive isolation between species. We review the evidence for rapid and adaptive change in reproductive proteins in animals, focusing on studies in recently diverged vertebrates. We identify common patterns and point out promising directions for future research. In particular, we highlight the ways that integrating the different but complementary approaches of evolutionary and developmental biology will provide new insights into fertilization processes.

Sexual Selection and the Adaptive Evolution of Mammalian Ejaculate Proteins

An elevated rate of substitution characterizes the molecular evolution of reproductive proteins from a wide range of taxa. Although the selective pressures explaining this rapid evolution are yet to be resolved, recent evidence implicates sexual selection as a potentially important explanatory factor. To investigate this hypothesis, we sought evidence of a high rate of adaptive gene evolution linked to postcopulatory sexual selection in muroid rodents, a model vertebrate group displaying a broad range of mating systems. Specifically, we sequenced 7 genes from diverse rodents that are expressed in the testes, prostate, or seminal vesicles, products of which have the potential to act in sperm competition. We inferred positive Darwinian selection in these genes by estimation of the ratio of nonsynonymous (d(N), amino acid changing) to synonymous (d(S), amino acid retaining) substitution rates (omega = d(N)/d(S)). Next, we tested whether variation in this ratio among lineages could be attributed to interspecific variation in mating systems, as inferred from the variation in these rodents' relative testis sizes (RTS). Four of the 7 genes examined (Prm1, Sva, Acrv1, and Svs2, but not Svp2, Msmb, or Spink3) exhibit unambiguous evidence of positive selection. One of these, the seminal vesicle-derived protein Svs2, also shows some evidence for a concentration of positive selection in those lineages in which sperm competition is common. However, this was not a general trend among all the rodent genes we examined. Using the same methods, we then reanalyzed previously published data on 2 primate genes, SEMG1 and SEMG2. Although SEMG2 also shows evidence of positive selection concentrated in lineages subject to high levels of sperm competition, no such trend was found for SEMG1. Overall, despite a high rate of positive selection being a feature of many ejaculate proteins, these results indicate that the action of sexual selection potentially responsible for elevated evolutionary rates may be difficult to detect on a gene-by-gene basis. Although the extreme diversity of reproductive phenotypes exhibited in nature attests to the power of sexual selection, the extent to which this force predominates in driving the rapid molecular evolution of reproductive genes therefore remains to be determined.

Gene Duplication and Adaptive Evolution of Digestive Proteases in Drosophila arizonae Female Reproductive Tracts

It frequently has been postulated that intersexual coevolution between the male ejaculate and the female reproductive tract is a driving force in the rapid evolution of reproductive proteins. The dearth of research on female tracts, however, presents a major obstacle to empirical tests of this hypothesis. Here, we employ a comparative EST approach to identify 241 candidate female reproductive proteins in Drosophila arizonae, a repleta group species in which physiological ejaculate–female coevolution has been documented. Thirty-one of these proteins exhibit elevated amino acid substitution rates, making them candidates for molecular coevolution with the male ejaculate. Strikingly, we also discovered 12 unique digestive proteases whose expression is specific to the D. arizonae lower female reproductive tract. These enzymes belong to classes most commonly found in the gastrointestinal tracts of a diverse array of organisms. We show that these proteases are associated with recent, lineage-specific gene duplications in the Drosophila repleta species group, and exhibit strong signatures of positive selection. Observation of adaptive evolution in several female reproductive tract proteins indicates they are active players in the evolution of reproductive tract interactions. Additionally, pervasive gene duplication, adaptive evolution, and rapid acquisition of a novel digestive function by the female reproductive tract points to a novel coevolutionary mechanism of ejaculate–female interaction.

Duplication and Selection on Abalone Sperm Lysin in an Allopatric Population

While gene duplication is a major source of evolutionary novelty, the importance of this process in reproductive protein evolution has not been widely investigated. Here, we report the first known case of gene duplication of abalone sperm lysin in an allopatric subspecies found in the Eastern Atlantic, Haliotis tuberculata coccinea. Mass spectrometry identified both copies of the lysin protein in testis tissue, and 3-dimensional structural modeling suggests that both proteins remain functional. We also detected positive selection acting on both paralogs after duplication and found evidence of a recent selective sweep. Because H. t. coccinea occurs in geographic isolation from other abalone species, these findings suggest that the evolution of lysin is not driven to create reproductive barriers to unfit hybrid formation with an overlapping species. Instead, sexual selection or sexual conflict acting during abalone fertilization could be responsible for the recent positive selection on this protein. The presence of multiple, rapidly evolving lysin genes in H. tuberculata presents an opportunity to study the early stages of diversification of a protein whose function is well understood.

Adaptive Evolution of Fertilization Proteins within a Genus: Variation in ZP2 and ZP3 in Deer Mice (Peromyscus)

Rapid evolution of reproductive proteins has been documented in a wide variety of taxa. In internally fertilized species, knowledge about the evolutionary dynamics of these proteins between closely related taxa is primarily limited to accessory gland proteins in the semen of Drosophila. Investigation of additional taxa and functional classes of proteins is necessary in order to determine if there is a general pattern of adaptive evolution of reproductive proteins between recently diverged species. We performed an evolutionary analysis of 2 egg coat proteins, ZP2 and ZP3, in 15 species of deer mice (genus Peromyscus). Both of these proteins are involved in egg-sperm binding, a critical step in maintaining species-specific fertilization. Here, we show that Zp2 and Zp3 gene trees are not consistent with trees based on nonreproductive genes, Mc1r and Lcat, where species formed monophyletic clades. In fact, for both of the reproductive genes, intraspecific amino acid variation was extensive and alleles were sometimes shared across species. We document positive selection acting on ZP2 and ZP3 and identify specific amino acid sites that are likely targets of selection using both maximum likelihood approaches and patterns of parallel amino acid change. In ZP3, positively selected sites are clustered in and around the region implicated in sperm binding in Mus, suggesting changes may impact egg-sperm binding and fertilization potential. Finally, we identify lineages with significantly elevated rates of amino acid substitution using a Bayesian mapping approach. These findings demonstrate that the pattern of adaptive reproductive protein evolution found at higher taxonomic levels can be documented between closely related mammalian species, where reproductive isolation has evolved recently.

Rapid evolution of reproductive proteins in abalone and Drosophila.

Observations from different taxa, including plants, protozoa, insects and mammals, indicate that proteins involved in reproduction evolve rapidly. Several models of adaptive evolution have been proposed to explain this phenomenon, such as sexual conflict, sexual selection, self versus non-self recognition and pathogen resistance. Here we discuss the potential role of sexual conflict in the rapid evolution of reproductive genes in two different animal systems, abalone (Haliotis) and Drosophila. In abalone, we reveal how specific interacting sperm-egg proteins were identified and discuss this identification in the light of models for rapid protein evolution and speciation. For Drosophila, we describe the genomic approaches taken to identify male accessory gland proteins and female reproductive tract proteins. Patterns of protein evolution from both abalone and Drosophila support the predicted patterns of rapid protein evolution driven by sexual conflict. We stress however that other selective pressures may contribute to the rapid evolution that is observed. We conclude that the key to distinguishing between sexual conflict and other mechanisms of protein evolution will be an integration of genetic, experimental and theoretical data.

Evolution of reproductive proteins from animals and plants

Sexual reproduction is a fundamental biological process common among eukaryotes. Because of the significance of reproductive proteins to fitness, the diversity and rapid divergence of proteins acting at many stages of reproduction is surprising and suggests a role of adaptive diversification in reproductive protein evolution. Here we review the evolution of reproductive proteins acting at different stages of reproduction among animals and plants, emphasizing common patterns. Although we are just beginning to understand these patterns, by making comparisons among stages of reproduction for diverse organisms we can begin to understand the selective forces driving reproductive protein diversity and the functional consequences of reproductive protein evolution.

SEA URCHIN BINDIN DIVERGENCE PREDICTS GAMETE COMPATIBILITY

Studies on the evolution of reproductive proteins have shown that they tend to evolve more rapidly than other proteins, frequently under positive selection. Progress on understanding the implications of these patterns is possible for marine invertebrates, where molecular evolution can be linked to gamete compatibility. In this study, we surveyed data from the literature from five genera of sea urchins for which there was information on gamete compatibility, divergence of the sperm-egg recognition protein bindin, and mitochondrial divergence. We draw three conclusions: (1) bindin divergence at nonsynonymous sites predicts gamete compatibility, whereas (2) bindin divergence at synonymous sites and mitochondrial DNA divergence do not, and (3) as few as 10 amino acid changes in bindin can lead to complete gamete incompatibility between species. Using mitochondrial divergence as a proxy for time, we find that complete gamete incompatibility can evolve in approximately one and a half million years, whereas sister species can maintain complete gamete compatibility for as long as five million years.

SEA URCHIN BINDIN DIVERGENCE PREDICTS GAMETE COMPATIBILITY

Studies on the evolution of reproductive proteins have shown that they tend to evolve more rapidly than other proteins, frequently under positive selection. Progress on understanding the implications of these patterns is possible for marine invertebrates, where molecular evolution can be linked to gamete compatibility. In this study, we surveyed data from the literature from five genera of sea urchins for which there was information on gamete compatibility, divergence of the sperm-egg recognition protein bindin, and mitochondrial divergence. We draw three conclusions: (1) bindin divergence at nonsynonymous sites predicts gamete compatibility, whereas (2) bindin divergence at synonymous sites and mitochondrial DNA divergence do not, and (3) as few as 10 amino acid changes in bindin can lead to complete gamete incompatibility between species. Using mitochondrial divergence as a proxy for time, we find that complete gamete incompatibility can evolve in approximately one and a half million years, whereas sister species can maintain complete gamete compatibility for as long as five million years.

Reproductive Protein Evolution

▪ Abstract The evolution of proteins involved in reproduction is only now beginning to be studied. A reoccurring observation is the rapid evolution of the molecules mediating reproductive events following the release of gametes. We review the examples where rapid evolution of reproductive proteins has been documented, covering taxa ranging from diatoms to humans. The selective pressures causing this divergence remain unknown, but several hypotheses are presented. The functional consequences of this rapid divergence could be involved in speciation.

The rapid evolution of reproductive proteins.

Many genes that mediate sexual reproduction, such as those involved in gamete recognition, diverge rapidly, often as a result of adaptive evolution. This widespread phenomenon might have important consequences, such as the establishment of barriers to fertilization that might lead to speciation. Sequence comparisons and functional studies are beginning to show the extent to which the rapid divergence of reproductive proteins is involved in the speciation process.

Positive Darwinian Selection Drives the Evolution of Several Female Reproductive Proteins in Mammals

Rapid evolution driven by positive Darwinian selection is a recurrent theme in male reproductive protein evolution. In contrast, positive selection has never been demonstrated for female reproductive proteins. Here, we perform phylogeny-based tests on three female mammalian fertilization proteins and demonstrate positive selection promoting their divergence. Two of these female fertilization proteins, the zona pellucida glycoproteins ZP2 and ZP3, are part of the mammalian egg coat. Several sites identified in ZP3 as likely to be under positive selection are located in a region previously demonstrated to be involved in species-specific sperm-egg interaction, suggesting the selective pressure is related to male-female interaction. The results provide long-sought evidence for two evolutionary hypotheses: sperm competition and sexual conflict.

Positive Darwinian selection drives the evolution of several female reproductive proteins in mammals.

Rapid evolution driven by positive Darwinian selection is a recurrent theme in male reproductive protein evolution. In contrast, positive selection has never been demonstrated for female reproductive proteins. Here, we perform phylogeny-based tests on three female mammalian fertilization proteins and demonstrate positive selection promoting their divergence. Two of these female fertilization proteins, the zona pellucida glycoproteins ZP2 and ZP3, are part of the mammalian egg coat. Several sites identified in ZP3 as likely to be under positive selection are located in a region previously demonstrated to be involved in species-specific sperm-egg interaction, suggesting the selective pressure is related to male-female interaction. The results provide long-sought evidence for two evolutionary hypotheses: sperm competition and sexual conflict.