The species-specific egg receptor for sea urchin sperm adhesion is EBR1,a novel ADAMTS protein

Intraspecific variation and functional study of VERL polymorphism in Pacific abalone (Haliotis discus hannai Ino) and giant abalone (H. gigantea Gmelin)

Sperm and eggs have interacting proteins on their surfaces that affect their compatibility during fertilization. These gamete recognition proteins (GRPs) are typically polymorphic within species, resulting in variations in gamete affinity. This study presents the full-length sequence of VERL, an egg vitelline envelope receptor for sperm lysin, in Pacific abalone (Haliotis discus hannai Ino) and giant abalone (Haliotis gigantea Gmelin). The full-length VERL cDNA sequence in Pacific abalone (11,373 bp) encodes 3790 amino acids, most of which are organized into 21 tandem repeats. In giant abalone, the VERL spans 9405 bp and encodes 3134 amino acids, including 17 tandem repeats. By resolving the nucleotide polymorphisms from resequencing data into haplotypes, high allelic diversity was found in VERL domain repeats 1-2 in both species. Further analysis of intraspecific variation in this region revealed that the alleles of VERL repeats 1-2 in Pacific abalone clustered into two clades, yet no significant divergence was observed among the three Pacific abalone geographic populations. In contrast, alleles in giant abalone were subdivided into two distinct branches, separated by a maximum of 12 amino acid substitutions. Notably, this study provides the first evidence of assortative mating between male and female gametes based on VERL genotypes in giant abalone. Overall, this study revealed the amino acid polymorphism of VERL in two abalone species and its functional effects on fertilization potential. These findings provide a foundation for future research on gamete recognition mechanisms and support selective breeding strategies to enhance abalone reproductive efficiency.

Identification of Six Novel Proteins Containing a ZP Module from Nemertean Species

During fertilization, a series of reactions between the eggs and spermatozoa proceed predominantly in a species-specific manner. The molecules mediating these species-specific reactions remain unknown except in a few organisms. In this study, we focused on two species belonging to the phylum Nemertea, Kulikovia alborostrata and K. fulva, and explored molecules involved in species-specific interactions between gametes. Orthologs of molecules known to be involved in species-specific reactions were not expressed in the ovaries of these two species. In contrast, we identified six novel proteins, named NeZPL1-NeZPL6, containing a ZP module. Among these, we found that NeZPL6 is located on the surface of an unfertilized egg and is suggested to be involved in its interaction with spermatozoa. Furthermore, we found an indel of three amino acids in the EGF-like domain of NeZPL6, which possibly confers species specificity to this interaction. Our results suggested the existence of a novel system for species recognition in animal gametes.

The Effect of Synthetic Polyamine BPA-C8 on the Fertilization Process of Intact and Denuded Sea Urchin Eggs

Sea urchin eggs are covered with layers of extracellular matrix, namely, the vitelline layer (VL) and jelly coat (JC). It has been shown that sea urchin eggs' JC components serve as chemoattractants or ligands for the receptor on the fertilizing sperm to promote the acrosome reaction. Moreover, the egg's VL provides receptors for conspecific sperm to bind, and, to date, at least two sperm receptors have been identified on the surface of sea urchin eggs. Interestingly, however, according to our previous work, denuded sea urchin eggs devoid of the JC and VL do not fail to become fertilized by sperm. Instead, they are bound and penetratedby multiple sperm, raising the possibility that an alternative pathway independent of the VL-residing sperm receptor may be at work. In this research, we studied the roles of the JC and VL using intact and denuded eggs and the synthetic polyamine BPA-C8. BPA-C8 is known to bind to the negatively charged macromolecular complexes in the cells, such as the JC, VL, and the plasma membrane of echinoderm eggs, as well as to the actin filaments in fibroblasts. Our results showed that, when added to seawater, BPA-C8 significantly repressed the Ca2+ wave in the intact P. lividus eggs at fertilization. In eggs deprived of the VL and JC, BPA-C8 binds to the plasma membrane and increases fibrous structures connecting microvilli, thereby allowing the denuded eggs to revert towards monospermy at fertilization. However, the reduced Ca2+ signal in denuded eggs was nullified compared to the intact eggs because removing the JC and VL already decreased the Ca2+ wave. BPA-C8 does not cross the VL and the cell membrane of unfertilized sea urchin eggs to diffuse into the cytoplasm at variance with the fibroblasts. Indeed, the jasplakinolide-induced polymerization of subplasmalemmal actin filaments was inhibited in the eggs microinjected with BPA-C8, but not in the ones bath-incubated with the same dose of BPA-C8.

SPE-51, a sperm-secreted protein with an immunoglobulin-like domain, is required for fertilization in C. elegans

Fertilization is a fundamental process in sexual reproduction during which gametes fuse to combine their genetic material and start the next generation in their life cycle. Fertilization involves species-specific recognition, adhesion, and fusion between the gametes.1,2 In mammals and other model species, some proteins are known to be required for gamete interactions and have been validated with loss-of-function fertility phenotypes.3,4 Yet, the molecular basis of sperm-egg interaction is not well understood. In a forward genetic screen for fertility mutants in Caenorhabditis elegans, we identified spe-51. Mutant worms make sperm that are unable to fertilize the oocyte but otherwise normal by all available measurements. The spe-51 gene encodes a secreted protein that includes an immunoglobulin (Ig)-like domain and a hydrophobic sequence of amino acids. The SPE-51 protein acts cell autonomously and localizes to the surface of the spermatozoa. We further show that the gene product of the mammalian sperm function gene Sof1 is likewise secreted. This is the first example of a secreted protein required for the interactions between the sperm and egg with genetic validation for a specific function in fertilization in C. elegans (also see spe-365). This is also the first experimental evidence that mammalian SOF1 is secreted. Our analyses of these genes begin to build a paradigm for sperm-secreted or reproductive-tract-secreted proteins that coat the sperm surface and influence their survival, motility, and/or the ability to fertilize the egg.

Divergent molecular signatures in fish Bouncer proteins define cross-fertilization boundaries

Molecular compatibility between gametes is a prerequisite for successful fertilization. As long as a sperm and egg can recognize and bind each other via their surface proteins, gamete fusion may occur even between members of separate species, resulting in hybrids that can impact speciation. The egg membrane protein Bouncer confers species specificity to gamete interactions between medaka and zebrafish, preventing their cross-fertilization. Here, we leverage this specificity to uncover distinct amino acid residues and N-glycosylation patterns that differentially influence the function of medaka and zebrafish Bouncer and contribute to cross-species incompatibility. Curiously, in contrast to the specificity observed for medaka and zebrafish Bouncer, seahorse and fugu Bouncer are compatible with both zebrafish and medaka sperm, in line with the pervasive purifying selection that dominates Bouncer's evolution. The Bouncer-sperm interaction is therefore the product of seemingly opposing evolutionary forces that, for some species, restrict fertilization to closely related fish, and for others, allow broad gamete compatibility that enables hybridization.

Eukaryotic fertilization and gamete fusion at a glance

In sexually reproducing organisms, the genetic information is transmitted from one generation to the next via the merger of male and female gametes. Gamete fusion is a two-step process involving membrane recognition and apposition through ligand-receptor interactions and lipid mixing mediated by fusion proteins. HAP2 (also known as GCS1) is a bona fide gamete fusogen in flowering plants and protists. In vertebrates, a multitude of surface proteins have been demonstrated to be pivotal for sperm-egg fusion, yet none of them exhibit typical fusogenic features. In this Cell Science at a Glance article and the accompanying poster, we summarize recent advances in the mechanistic understanding of gamete fusion in eukaryotes, with a particular focus on mammalian species.

A novel role for ATP2B in ascidians: Ascidian-specific mutations in ATP2B contribute to sperm chemotaxis

Sperm chemotaxis, in which sperms are attracted to conspecific eggs via species-specific attractants, plays an important role in fertilization. This phenomenon has been observed in various animals and species-specific sperm attractants have been reported in some species. However, the mechanisms involved in the reception and recognition of the species-specific attractant by the sperms is poorly studied. Previously, we found that the plasma membrane-type Ca²⁺ /ATPase (PMCA) is the receptor for the sperm-activating and -attracting factor (SAAF) in the ascidian Ciona intestinalis. To determine the role of PMCA in species-specific sperm chemotaxis, we identified the amino acid sequences of PMCAs derived from six Phlebobranchia species. The testis-specific splice variant of PMCA was found to be present in all the species investigated and the ascidian-specific sequence was detected near the 3'-terminus. Moreover, dN/dS analysis revealed that the extracellular loops 1, 2, and 4 in ascidian PMCA underwent a positive selection. These findings suggest that PMCA recognizes the species-specific structure of SAAF at the extracellular loops 1, 2, and 4, and its testis-specific C-terminal region is involved in the activation and chemotaxis of ascidian sperms.

Twinkle twinkle brittle star: the draft genome of Ophioderma brevispinum (Echinodermata: Ophiuroidea) as a resource for regeneration research

Background

Echinoderms are established models in experimental and developmental biology, however genomic resources are still lacking for many species. Here, we present the draft genome of Ophioderma brevispinum, an emerging model organism in the field of regenerative biology. This new genomic resource provides a reference for experimental studies of regenerative mechanisms.

Results

We report a de novo nuclear genome assembly for the brittle star O. brevispinum and annotation facilitated by the transcriptome assembly. The final assembly is 2.68 Gb in length and contains 146,703 predicted protein-coding gene models. We also report a mitochondrial genome for this species, which is 15,831 bp in length, and contains 13 protein-coding, 22 tRNAs, and 2 rRNAs genes, respectively. In addition, 29 genes of the Notch signaling pathway are identified to illustrate the practical utility of the assembly for studies of regeneration.

Conclusions

The sequenced and annotated genome of O. brevispinum presented here provides the first such resource for an ophiuroid model species. Considering the remarkable regenerative capacity of this species, this genome will be an essential resource in future research efforts on molecular mechanisms regulating regeneration.

Supplementary Information

The online version contains supplementary material available at (10.1186/s12864-022-08750-y).

Mechanisms of Sperm-Egg Interactions: What Ascidian Fertilization Research Has Taught Us

Fertilization is an essential process in terrestrial organisms for creating a new organism with genetic diversity. Before gamete fusion, several steps are required to achieve successful fertilization. Animal spermatozoa are first activated and attracted to the eggs by egg-derived chemoattractants. During the sperm passage of the egg's extracellular matrix or upon the sperm binding to the proteinaceous egg coat, the sperm undergoes an acrosome reaction, an exocytosis of acrosome. In hermaphrodites such as ascidians, the self/nonself recognition process occurs when the sperm binds to the egg coat. The activated or acrosome-reacted spermatozoa penetrate through the proteinaceous egg coat. The extracellular ubiquitin-proteasome system, the astacin-like metalloproteases, and the trypsin-like proteases play key roles in this process in ascidians. In the present review, we summarize our current understanding and perspectives on gamete recognition and egg coat lysins in ascidians and consider the general mechanisms of fertilization in animals and plants.

An approach to quantitate maternal transcripts localized in sea urchin egg cortex using RT-qPCR with accurate normalization

The sea urchin egg cortex is a peripheral region of eggs comprising a cell membrane and adjacent cytoplasm, which contains actin and tubulin cytoskeleton, cortical granules and some proteins required for early development. Method for isolation of cortices from sea urchin eggs and early embryos was developed in 1970s. Since then, this method has been reliable tool to study protein localization and cytoskeletal organization in cortex of unfertilized eggs and embryos during first cleavages. This study was aimed to estimate the reliability of RT-qPCR to analyze levels of maternal transcripts that are localized in egg cortex. Firstly, we selected seven potential reference genes, 28S, Cycb, Ebr1, GAPDH, Hmg1, Smtnl1 and Ubb, the transcripts of which are maternally deposited in sea urchin eggs. The candidate reference genes were ranked by five different algorithms (BestKeeper, CV, ΔCt, geNorm and NormFinder) based on calculated level of stability in both eggs as well as isolated cortices. Our results showed that gene ranking differs in total RNA and mRNA samples, though Ubb is most suitable reference gene in both cases. To validate feasibility of comparative analysis of eggs and isolated egg cortices, we selected Daglb-2 as a gene of interest, which transcripts are potentially localized in cortex according to transcriptome analysis, and observed increased level of Daglb-2 in egg cortices by RT-qPCR. This suggests that proposed RNA isolation method with subsequent quantitative RT-qPCR analysis can be used to determine cortical association of transcripts in sea urchin eggs.

Responses of sea urchins (Strongylocentrotus intermedius) with different sexes to CO2-induced seawater acidification: Histology, physiology, and metabolomics

Responses of different sexes of farmed Strongylocentrotus intermedius to chronic CO₂-induced seawater acidification were investigated in 120-day lab-based experiments. Four experimental groups were set up as one control group and three seawater acidification groups. The results showed that 1) Specific growth rate and the numbers of mature gamete cells declined in a pH-dependent way in both sexes of adult S. intermedius. 2) There were differences in SDMs identified in females and males reared in acidified seawater reflecting sex-specific response variation in adult S. intermedius. 3) The number of altered metabolic pathways exhibited a linear increasing trend as seawater pH declined in both sexes of adult S. intermedius. Meanwhile seawater acidification might affect metabolic processes via changing the relative expression and activity of key enzymes controlling the corresponding metabolic pathways of adult S. intermedius.

Regulation of the Actin Cytoskeleton-Linked Ca2+ Signaling by Intracellular pH in Fertilized Eggs of Sea Urchin

In sea urchin, the immediate contact of the acrosome-reacted sperm with the egg surface triggers a series of structural and ionic changes in the egg cortex. Within one minute after sperm fuses with the egg plasma membrane, the cell membrane potential changes with the concurrent increases in intracellular Ca2+ levels. The consequent exocytosis of the cortical granules induces separation of the vitelline layer from the egg plasma membrane. While these cortical changes are presumed to prevent the fusion of additional sperm, the subsequent late phase (between 1 and 4 min after fertilization) is characterized by reorganization of the egg cortex and microvilli (elongation) and by the metabolic shift to activate de novo protein and DNA syntheses. The latter biosynthetic events are crucial for embryonic development. Previous studies suggested that the early phase of fertilization was not a prerequisite for these changes in the second phase since the increase in the intracellular pH induced by the exposure of unfertilized sea urchin eggs to ammonia seawater could start metabolic egg activation in the absence of the cortical granule exocytosis. In the present study, we have demonstrated that the incubation of unfertilized eggs in ammonia seawater induced considerable elongations of microvilli (containing actin filaments) as a consequence of the intracellular pH increase, which increased the egg's receptivity to sperm and made the eggs polyspermic at fertilization despite the elevation of the fertilization envelope (FE). These eggs also displayed compromised Ca2+ signals at fertilization, as the amplitude of the cortical flash was significantly reduced and the elevated intracellular Ca2+ level declined much faster. These results have also highlighted the importance of the increased internal pH in regulating Ca2+ signaling and the microvillar actin cytoskeleton during the late phase of the fertilization process.

Recurrent Duplication and Diversification of Acrosomal Fertilization Proteins in Abalone

Reproductive proteins mediating fertilization commonly exhibit rapid sequence diversification driven by positive selection. This pattern has been observed among nearly all taxonomic groups, including mammals, invertebrates, and plants, and is remarkable given the essential nature of the molecular interactions mediating fertilization. Gene duplication is another important mechanism that facilitates the generation of molecular novelty through functional divergence. Following duplication, paralogs may partition ancestral gene function (subfunctionalization) or acquire new roles (neofunctionalization). However, the contributions of duplication followed by sequence diversification to the molecular diversity of gamete recognition genes has been understudied in many models of fertilization. The marine gastropod mollusk abalone is a classic model for fertilization. Its two acrosomal proteins (lysin and sp18) are ancient gene duplicates with unique gamete recognition functions. Through detailed genomic and bioinformatic analyses we show how duplication events followed by sequence diversification has played an ongoing role in the evolution of abalone acrosomal proteins. The common ancestor of abalone had four members of its acrosomal protein family in a tandem gene array that repeatedly experienced positive selection. We find that both sp18 paralogs contain positively selected sites located in different regions of the paralogs, suggestive of functional divergence where selection acted upon distinct binding interfaces in each paralog. Further, a more recent species-specific duplication of both lysin and sp18 in the European abalone H. tuberculata is described. Despite clade-specific acrosomal protein paralogs, there are no concomitant duplications of egg coat proteins in H. tuberculata, indicating that duplication of egg proteins per se is not responsible for retention of duplicated acrosomal proteins. We hypothesize that, in a manner analogous to host/pathogen evolution, sperm proteins are selected for increased diversity through extensive sequence divergence and recurrent duplication driven by conflict mechanisms.

The Importance of Gene Duplication and Domain Repeat Expansion for the Function and Evolution of Fertilization Proteins

The process of gene duplication followed by gene loss or evolution of new functions has been studied extensively, yet the role gene duplication plays in the function and evolution of fertilization proteins is underappreciated. Gene duplication is observed in many fertilization protein families including Izumo, DCST, ZP, and the TFP superfamily. Molecules mediating fertilization are part of larger gene families expressed in a variety of tissues, but gene duplication followed by structural modifications has often facilitated their cooption into a fertilization function. Repeat expansions of functional domains within a gene also provide opportunities for the evolution of novel fertilization protein. ZP proteins with domain repeat expansions are linked to species-specificity in fertilization and TFP proteins that experienced domain duplications were coopted into a novel sperm function. This review outlines the importance of gene duplications and repeat domain expansions in the evolution of fertilization proteins.

Cell-Cell Mating Interactions: Overview and Potential of Single-Cell Force Spectroscopy

It is an understatement that mating and DNA transfer are key events for living organisms. Among the traits needed to facilitate mating, cell adhesion between gametes is a universal requirement. Thus, there should be specific properties for the adhesion proteins involved in mating. Biochemical and biophysical studies have revealed structural information about mating adhesins, as well as their specificities and affinities, leading to some ideas about these specialized adhesion proteins. Recently, single-cell force spectroscopy (SCFS) has added important findings. In SCFS, mating cells are brought into contact in an atomic force microscope (AFM), and the adhesive forces are monitored through the course of mating. The results have shown some remarkable characteristics of mating adhesins and add knowledge about the design and evolution of mating adhesins.

Effects of Dithiothreitol on Fertilization and Early Development in Sea Urchin

The vitelline layer (VL) of a sea urchin egg is an intricate meshwork of glycoproteins that intimately ensheathes the plasma membrane. During fertilization, the VL plays important roles. Firstly, the receptors for sperm reside on the VL. Secondly, following cortical granule exocytosis, the VL is elevated and transformed into the fertilization envelope (FE), owing to the assembly and crosslinking of the extruded materials. As these two crucial stages involve the VL, its alteration was expected to affect the fertilization process. In the present study, we addressed this question by mildly treating the eggs with a reducing agent, dithiothreitol (DTT). A brief pretreatment with DTT resulted in partial disruption of the VL, as judged by electron microscopy and by a novel fluorescent polyamine probe that selectively labelled the VL. The DTT-pretreated eggs did not elevate the FE but were mostly monospermic at fertilization. These eggs also manifested certain anomalies at fertilization: (i) compromised Ca2+ signaling, (ii) blocked translocation of cortical actin filaments, and (iii) impaired cleavage. Some of these phenotypic changes were reversed by restoring the DTT-exposed eggs in normal seawater prior to fertilization. Our findings suggest that the FE is not the decisive factor preventing polyspermy and that the integrity of the VL is nonetheless crucial to the egg's fertilization response.

Selection on genes associated with the evolution of divergent life histories: Gamete recognition or something else?

Gamete compatibility, and fertilization success, is mediated by gamete-recognition genes (GRGs) that are expected to show genetic evidence of a response to sexual selection associated with mating system traits. Changes in the strength of sexual selection can arise from the resolution of sperm competition among males, sexual conflicts of interest between males and females, or other mechanisms of sexual selection. To assess these expectations, we compared patterns of episodic diversifying selection among genes expressed in the gonads of Cryptasterina pentagona and C. hystera, which recently speciated and have evolved different mating systems (gonochoric or hermaphroditic), modes of fertilization (outcrossing or selfing), and dispersal (planktonic larvae or internal brooding). Cryptasterina spp. inhabit the upper intertidal of the coast of Queensland and coral islands of the Great Barrier Reef. We found some evidence for positive selection on a GRG in the outcrossing C. pentagona, and we found evidence of loss of gene function in a GRG of the self-fertilizing C. hystera. The modification or loss of gene functionality may be evidence of relaxed selection on some aspects of gamete interaction in C. hystera. In addition to these genes involved in gamete interactions, we also found genes under selection linked to abiotic stress, chromosomal regulation, polyspermy, and egg-laying. We interpret those results as possible evidence that Cryptasterina spp. with different mating systems may have been adapting in divergent ways to oxidative stress or other factors associated with reproduction in the physiologically challenging environment of the high intertidal. RESEARCH HIGHLIGHTS: Recent speciation between two sea stars was unlikely the result of selection on gamete-recognition genes annotated in this study. Instead, our results point to selection on genes linked to the intertidal environment and reproduction.

Bindin is essential for fertilization in the sea urchin

Species-specific sperm−egg interactions are essential for sexual reproduction. Broadcast spawning of marine organisms is under particularly stringent conditions, since eggs released into the water column can be exposed to multiple different sperm. Bindin isolated from the sperm acrosome results in insoluble particles that cause homospecific eggs to aggregate, whereas no aggregation occurs with heterospecific eggs. Therefore, Bindin is concluded to play a critical role in fertilization, yet its function has never been tested. Here we report that Cas9-mediated inactivation of the bindin gene in a sea urchin results in perfectly normal-looking embryos, larvae, adults, and gametes in both males and females. What differed between the genotypes was that the bindin^−/− sperm never fertilized an egg, functionally validating Bindin as an essential gamete interaction protein at the level of sperm–egg cell surface binding.

Phylogeography, colouration, and cryptic speciation across the Indo-Pacific in the sea urchin genus Echinothrix

The sea urchins Echinothrix calamaris and Echinothrix diadema have sympatric distributions throughout the Indo-Pacific. Diverse colour variation is reported in both species. To reconstruct the phylogeny of the genus and assess gene flow across the Indo-Pacific we sequenced mitochondrial 16S rDNA, ATPase-6, and ATPase-8, and nuclear 28S rDNA and the Calpain-7 intron. Our analyses revealed that E. diadema formed a single trans-Indo-Pacific clade, but E. calamaris contained three discrete clades. One clade was endemic to the Red Sea and the Gulf of Oman. A second clade occurred from Malaysia in the West to Moorea in the East. A third clade of E. calamaris was distributed across the entire Indo-Pacific biogeographic region. A fossil calibrated phylogeny revealed that the ancestor of E. diadema diverged from the ancestor of E. calamaris ~ 16.8 million years ago (Ma), and that the ancestor of the trans-Indo-Pacific clade and Red Sea and Gulf of Oman clade split from the western and central Pacific clade ~ 9.8 Ma. Time since divergence and genetic distances suggested species level differentiation among clades of E. calamaris. Colour variation was extensive in E. calamaris, but not clade or locality specific. There was little colour polymorphism in E. diadema.

Update on Novel Hormonal and Nonhormonal Male Contraceptive Development

BACKGROUND

The advent of new methods of male contraception would increase contraceptive options for men and women and advance male contraceptive agency. Pharmaceutical R&D for male contraception has been dormant since the 1990s. The Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) has supported a contraceptive development program since 1969 and supports most ongoing hormonal male contraceptive development. Nonhormonal methods are in earlier stages of development.

CONTENT

Several hormonal male contraceptive agents have entered clinical trials. Novel single agent products being evaluated include dimethandrolone undecanoate, 11β-methyl-nortestosterone dodecylcarbonate, and 7α-methyl-19-nortestosterone. A contraceptive efficacy trial of Nestorone®/testosterone gel is underway. Potential nonhormonal methods are at preclinical stages of development. Many nonhormonal male contraceptive targets that affect sperm production, sperm function, or sperm transport have been identified.

SUMMARY

NICHD supports development of reversible male contraceptive agents. Other organizations such as the World Health Organization, the Population Council, and the Male Contraception Initiative are pursuing male contraceptive development, but industry involvement remains limited.

Ovarian fluid proteome variation associates with sperm swimming speed in an externally fertilizing fish

Sperm velocity is a key trait that predicts the outcome of sperm competition. By promoting or impeding sperm velocity, females can control fertilization via postcopulatory cryptic female choice. In Chinook salmon, ovarian fluid (OF), which surrounds the ova, mediates sperm velocity according to male and female identity, biasing the outcome of sperm competition towards males with faster sperm. Past investigations have revealed proteome variation in OF, but the specific components of OF that differentially mediate sperm velocity have yet to be characterized. Here we use quantitative proteomics to investigate whether OF protein composition explains variation in sperm velocity and fertilization success. We found that OF proteomes from six females robustly clustered into two groups and that these groups are distinguished by the abundance of a restricted set of proteins significantly associated with sperm velocity. Exposure of sperm to OF from females in group I had faster sperm compared to sperm exposed to the OF of group II females. Overall, OF proteins that distinguished between these groups were enriched for vitellogenin and calcium ion interactions. Our findings suggest that these proteins may form the functional basis for cryptic female choice via the biochemical and physiological mediation of sperm velocity.

Molecular mechanisms and evolution of fertilization proteins

Sexual reproduction involves a cascade of molecular interactions between the sperm and the egg culminating in cell-cell fusion. Vital steps mediating fertilization include chemoattraction of the sperm to the egg, induction of the sperm acrosome reaction, dissolution of the egg coat, and sperm-egg plasma membrane binding and fusion. Despite decades of research, only a handful of interacting gamete recognition proteins (GRPs) have been identified across taxa mediating each of these steps, most notably in abalone, sea urchins, and mammals. This review outlines and compares notable GRP pairs mediating sperm-egg recognition in these three significant model systems and discusses the molecular basis of species-specific fertilization driven by GRP function. In addition, we explore the evolutionary theory behind the rapid diversification of GRPs between species. In particular, we focus on how the coevolution between interacting sperm and egg proteins may contribute to the formation of boundaries to hybridization. Finally, we discuss how pairing structural information with evolutionary insights can improve our understanding of mechanisms of fertilization and their origins.

Nonspecific expression of fertilization genes in the crown-of-thorns Acanthaster cf. solaris: Unexpected evidence of hermaphroditism in a coral reef predator

The characterization of gene expression in gametes has advanced our understanding of the molecular basis for ecological variation in reproductive success and the evolution of reproductive isolation. These advances are especially significant for ecologically important keystone predators such as the coral-eating crown-of-thorns sea stars (COTS, Acanthaster) which are the most influential predator species in Indo-Pacific coral reef ecosystems and the focus of intensive management efforts. We used RNA-seq and transcriptome assemblies to characterize the expression of genes in mature COTS gonads. We described the sequence and domain organization of eight genes with sex-specific expression and well known functions in fertilization in other echinoderms. We found unexpected expression of genes in one ovary transcriptome that are characteristic of males and sperm, including genes that encode the sperm-specific guanylate cyclase receptor for an egg pheromone, and the sperm acrosomal protein bindin. In a reassembly of previously published RNA-seq data from COTS testes, we found a complementary pattern: strong expression of four genes that are otherwise well known to encode egg-specific fertilization proteins, including the egg receptor for bindin (EBR1) and the acrosome reaction-inducing substance in the egg coat (ARIS1, ARIS2, ARIS3). We also found histological evidence of both eggs and sperm developing in the same gonad in several COTS individuals from a parallel study. These results suggest the occurrence of hermaphrodites, and the potential for reproductive assurance via self-fertilization. Our findings have implications for management of COTS populations, especially in consideration of the large size and massive fecundity of these sea stars.

Species-specific mechanisms during fertilization

The perpetuation and preservation of distinct species rely on mechanisms that ensure that only interactions between gametes of the same species can give rise to viable and fertile offspring. Species-specificity can act at various stages, ranging from physical/behavioral pre-copulatory mechanisms, to pre-zygotic incompatibility during fertilization, to post-zygotic hybrid incompatibility. Herein, we focus on our current knowledge of the molecular mechanisms responsible for species-specificity during fertilization. While still poorly understood, decades of research have led to the discovery of molecules implicated in species-specific gamete interactions, starting from initial sperm-egg attraction to the binding of sperm and egg. While many of these molecules have been described as species-specific in their mode of action, relatively few have been demonstrated as such with definitive evidence. Thus, we also raise remaining questions that need to be addressed in order to characterize gamete interaction molecules as species-specific.

Male Contraceptive Development: Update on Novel Hormonal and Nonhormonal Methods

BACKGROUND

Development of new methods of male contraception would address an unmet need for men to control their fertility and could increase contraceptive options for women. Pharmaceutical research and development for male contraception was active in the 1990s but has been virtually abandoned. The Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) has supported a contraceptive development program since 1969 and supports the majority of hormonal male contraceptive development. Nonhormonal methods are also in development but are at earlier stages.

CONTENT

Several hormonal male contraceptive agents have entered clinical trials. Single-agent products being evaluated include dimethandrolone undecanoate, 11β-methyl-nortestosterone dodecyl carbonate, and 7α-methyl-19-nortestosterone. A contraceptive efficacy trial of Nestorone^® gel and testosterone gel in a single application will begin in 2018. Potential nonhormonal methods are at preclinical stages of development. Many nonhormonal male contraceptive targets that affect either sperm production or sperm function have been identified. Targeted pathways include the retinoic acid pathway, bromodomain and extraterminal proteins, and pathways for Sertoli cell-germ cell adhesion or sperm motility. Druggable targets include CatSper, the sperm Na+/K+-exchanger, TSSK, HIPK4, EPPIN, and ADAMs family proteins. Development of a procedure to reversibly block the vas deferens (initially developed in India in the 1980s) is undergoing early stage research in the US under the trade name Vasalgel™.

SUMMARY

NICHD has supported the development of reversible male contraceptive agents. Other organizations such as the World Health Organization and the Population Council are pursuing male contraceptive development, but industry involvement remains dormant.

Finding genes and lineages under selection in speciation

What are the genes and traits that respond to selection and cause prezygotic reproductive isolation between species? This question has been hard to answer because genomes are large, the targets of selection may be scattered across the genome (Sabeti et al., ) and different genes may respond to the same selective pressure in different populations (Scheinfeldt et al., ). In this issue of Molecular Ecology, Weber et al. () use a clever comparative approach and leading-edge transcriptomic methods to identify the species and genes under positive selection for divergence between brittle stars (the echinoderm class Ophiuroidea) in the Ophioderma longicauda species complex. They found convincing evidence of positive or diversifying selection acting on two genes encoding ion channels that form part of the signal transduction cascade within the sperm in response to pheromones. Evidence for selection was concentrated in genes from one species (called C5, with internal fertilization and female parental care of brooded juveniles and not in the other species (called C3, with more conventional broadcast spawning and planktonic development of embryos and larvae). That analysis greatly extends the range of taxa, life history traits and molecules that are associated with positive selection in speciation. It also illustrates some of the current limitations on the application of RNAseq methods in the search for the targets of selection in nonmodel organisms like brittle stars. From both points of view, the new work by Weber et al. () has important implications for our understanding of speciation in the ocean.

Interspecific gamete compatibility and hybrid larval fitness in reef-building corals: Implications for coral reef restoration

Climate warming is a major cause of the global decline of coral reefs. Active reef restoration, although still in its infancy, is one of several possible ways to help restore coral cover and reef ecosystem function. The deployment of mature coral larvae onto depauperate reef substratum has been shown to significantly increase larval recruitment, providing a novel option for the delivery of ex situ bred coral stock to the reef for restoration purposes. The success of such reef restoration approaches may be improved by the use of coral larval stock augmented for climate resilience. Here we explore whether coral climate resilience can be enhanced via interspecific hybridization through hybrid vigour. Firstly, we assessed cross-fertility of four pairs of Acropora species from the Great Barrier Reef. Temporal isolation in gamete release between the Acropora species was limited, but gametic incompatibility was present with varying strength between species pairs and depending on the direction of the hybrid crosses. We subsequently examined the fitness of hybrid and purebred larvae under heat stress by comparing their survival and settlement success throughout 10 days of exposure to 28 °C, 29.5 °C and 31 °C. Fitness of the majority of Acropora hybrid larvae was similar to that of the purebred larvae of both parental species, and in some instances it was higher than that of the purebred larvae of one of the parental species. Lower hybrid fertilization success did not affect larval fitness. These findings indicate that high hybrid fitness can be achieved after overcoming partial prezygotic barriers, and that interspecific hybridization may be a tool to enhance coral recruitment and climate resilience.

The role of metalloproteases in fertilisation in the ascidian Ciona robusta

In the ascidian Ciona robusta (formerly C. intestinalis type A), the mechanism underlying sperm penetration through the egg investment remains unknown. We previously reported that proteins containing both an astacin metalloprotease domain and thrombospondin type 1 repeats are abundant in the sperm surface protein-enriched fraction of C. robusta. Here we investigated the involvement of those proteins in fertilisation. We refined the sequences of astacin metalloproteases, confirmed that five of them are present in the sperm, and labelled them as tunicate astacin and thrombospondin type 1 repeat-containing (Tast) proteins. Fertilisation of C. robusta eggs was potently inhibited by a metalloprotease inhibitor GM6001. The eggs cleaved normally when they were vitelline coat-free or the inhibitor was added after insemination. Furthermore, vitelline coat proteins were degraded after incubation with intact sperm. These results suggest that sperm metalloproteases are indispensable for fertilisation, probably owing to direct or indirect mediation of vitelline-coat digestion during sperm penetration. TALEN-mediated knockout of Tast genes and the presence of GM6001 impaired larval development at the metamorphic stage, suggesting that Tast gene products play a key role in late development.

The Ly6/uPAR protein Bouncer is necessary and sufficient for species-specific fertilization

Fertilization is fundamental for sexual reproduction, yet its molecular mechanisms are poorly understood. We found that an oocyte-expressed Ly6/uPAR protein, which we call Bouncer, is a crucial fertilization factor in zebrafish. Membrane-bound Bouncer mediates sperm-egg binding and is thus essential for sperm entry into the egg. Remarkably, Bouncer not only is required for sperm-egg interaction but is also sufficient to allow cross-species fertilization between zebrafish and medaka, two fish species that diverged more than 200 million years ago. Our study thus identifies Bouncer as a key determinant of species-specific fertilization in fish. Bouncer's closest homolog in tetrapods, SPACA4, is restricted to the male germline in internally fertilizing vertebrates, which suggests that our findings in fish have relevance to human biology.

Egg Coat Proteins Across Metazoan Evolution

All animal oocytes are surrounded by a glycoproteinaceous egg coat, a specialized extracellular matrix that serves both structural and species-specific roles during fertilization. Egg coat glycoproteins polymerize into the extracellular matrix of the egg coat using a conserved protein-protein interaction module-the zona pellucida (ZP) domain-common to both vertebrates and invertebrates, suggesting that the basic structural features of egg coats have been conserved across hundreds of millions of years of evolution. Egg coat proteins, as with other proteins involved in reproduction, are frequently found to be rapidly evolving. Given that gamete compatibility must be maintained for the fitness of sexually reproducing organisms, this finding is somewhat paradoxical and suggests a role for adaptive diversification in reproductive protein evolution. Here we review the structure and function of metazoan egg coat proteins, with an emphasis on the potential role their evolution has played in the creation and maintenance of species boundaries.

Starfish as a Model System for Analyzing Signal Transduction During Fertilization

The starfish oocyte and egg offer advantages for use as a model system for signal transduction research. Some of these have been recognized for over a century, including the ease of procuring gametes, in vitro fertilization, and culturing the embryos. New advances, particularly in genomics, have also opened up opportunities for the use of these animals. In this chapter, we give a few examples of the historical use of the starfish for research in cell biology and then describe some new areas in which we believe the starfish can contribute to our understanding of signal transduction-particularly in fertilization.

Fertilization in Starfish and Sea Urchin: Roles of Actin

Marine animals relying on "external fertilization" provide advantageous opportunities to study the mechanisms of gamete activation and fusion, as well as the subsequent embryonic development. Owing to the large number of eggs that are easily available and handled, starfish and sea urchins have been chosen as favorable animal models in this line of research for over 150 years. Indeed, much of our knowledge on fertilization came from studies in the echinoderms. Fertilization involves mutual stimulation between eggs and sperm, which leads to morphological, biochemical, and physiological changes on both sides to ensure successful gamete fusion. In this chapter, we review the roles of actin in the fertilization of starfish and sea urchin eggs. As fertilization is essentially an event that takes place on the egg surface, it has been predicted that suboolemmal actin filaments would make significant contributions to sperm entry. A growing body of evidence from starfish and sea urchin eggs suggests that the prompt reorganization of the actin pools around the time of fertilization plays crucial regulatory roles not only in guiding sperm entry but also in modulating intracellular Ca²⁺ signaling and egg activation.

Genomic Signatures of Sexual Conflict

Sexual conflict is a specific class of intergenomic conflict that describes the reciprocal sex-specific fitness costs generated by antagonistic reproductive interactions. The potential for sexual conflict is an inherent property of having a shared genome between the sexes and, therefore, is an extreme form of an environment-dependent fitness effect. In this way, many of the predictions from environment-dependent selection can be used to formulate expected patterns of genome evolution under sexual conflict. However, the pleiotropic and transmission constraints inherent to having alleles move across sex-specific backgrounds from generation to generation further modulate the anticipated signatures of selection. We outline methods for detecting candidate sexual conflict loci both across and within populations. Additionally, we consider the ability of genome scans to identify sexually antagonistic loci by modeling allele frequency changes within males and females due to a single generation of selection. In particular, we highlight the need to integrate genotype, phenotype, and functional information to truly distinguish sexual conflict from other forms of sexual differentiation.

Postmating Female Control: 20 Years of Cryptic Female Choice

Cryptic female choice (CFC) represents postmating intersexual selection arising from female-driven mechanisms at or after mating that bias sperm use and impact male paternity share. Although biologists began to study CFC relatively late, largely spurred by Eberhard's book published 20 years ago, the field has grown rapidly since then. Here, we review empirical progress to show that numerous female processes offer potential for CFC, from mating through to fertilization, although seldom has CFC been clearly demonstrated. We then evaluate functional implications, and argue that, under some conditions, CFC might have repercussions for female fitness, sexual conflict, and intersexual coevolution, with ramifications for related evolutionary phenomena, such as speciation. We conclude by identifying directions for future research in this rapidly growing field.

Positive selection on sperm ion channels in a brooding brittle star: consequence of life-history traits evolution

Closely related species are key models to investigate mechanisms leading to reproductive isolation and early stages of diversification, also at the genomic level. The brittle star cryptic species complex Ophioderma longicauda encompasses the sympatric broadcast-spawning species C3 and the internal brooding species C5. Here, we used de novo transcriptome sequencing and assembly in two closely related species displaying contrasting reproductive modes to compare their genetic diversity and to investigate the role of natural selection in reproductive isolation. We reconstructed 20 146 and 22 123 genes for C3 and C5, respectively, and characterized a set of 12 229 orthologs. Genetic diversity was 1.5-2 times higher in C3 compared to C5, confirming that species with low parental investment display higher levels of genetic diversity. Forty-eight genes were the targets of positive diversifying selection during the evolution of the two species. Notably, two genes (NHE and TetraKCNG) are sperm-specific ion channels involved in sperm motility. Ancestral sequence reconstructions show that natural selection targeted the two genes in the brooding species. This may result from an adaptation to the novel environmental conditions surrounding sperm in the brooding species, either directly affecting sperm or via an increase in male/female conflict. This phenomenon could have promoted prezygotic reproductive isolation between C3 and C5. Finally, the sperm receptors to egg chemoattractants differed between C3 and C5 in the ligand-binding region. We propose that mechanisms of species-specific gamete recognition in brittle stars occur during sperm chemotaxis (sperm attraction towards the eggs), contrary to other marine invertebrates where prezygotic barriers to interspecific hybridization typically occur before sperm-egg fusion.

The challenges involved in elucidating the molecular basis of sperm-egg recognition in mammals and approaches to overcome them

Sexual reproduction is used by many different organisms to create a new generation of genetically distinct progeny. Cells originating from separate sexes or mating types segregate their genetic material into haploid gametes which must then recognize and fuse with each other in a process known as fertilization to form a diploid zygote. Despite the central importance of fertilization, we know remarkably little about the molecular mechanisms that are involved in how gametes recognize each other, particularly in mammals, although the proteins that are displayed on their surfaces are almost certainly involved. This paucity of knowledge is largely due to both the unique biological properties of mammalian gametes (sperm and egg) which make them experimentally difficult to manipulate, and the technical challenges of identifying interactions between membrane-embedded cell surface receptor proteins. In this review, we will discuss our current knowledge of animal gamete recognition, highlighting where important contributions to our understanding were made, why particular model systems were helpful, and why progress in mammals has been particularly challenging. We discuss how the development of mammalian in vitro fertilization and targeted gene disruption in mice were important technological advances that triggered progress. We argue that approaches employed to discover novel interactions between cell surface gamete recognition proteins should account for the unusual biochemical properties of membrane proteins and the typically highly transient nature of their interactions. Finally, we describe how these principles were applied to identify Juno as the egg receptor for sperm Izumo1, an interaction that is essential for mammalian fertilization.

From molecules to mating: Rapid evolution and biochemical studies of reproductive proteins

Sexual reproduction and the exchange of genetic information are essential biological processes for species across all branches of the tree of life. Over the last four decades, biochemists have continued to identify many of the factors that facilitate reproduction, but the molecular mechanisms that mediate this process continue to elude us. However, a recurring observation in this research has been the rapid evolution of reproductive proteins. In animals, the competing interests of males and females often result in arms race dynamics between pairs of interacting proteins. This phenomenon has been observed in all stages of reproduction, including pheromones, seminal fluid components, and gamete recognition proteins. In this article, we review how the integration of evolutionary theory with biochemical experiments can be used to study interacting reproductive proteins. Examples are included from both model and non-model organisms, and recent studies are highlighted for their use of state-of-the-art genomic and proteomic techniques.

SIGNIFICANCE

Despite decades of research, our understanding of the molecular mechanisms that mediate fertilization remain poorly characterized. To date, molecular evolutionary studies on both model and non-model organisms have provided some of the best inferences to elucidating the molecular underpinnings of animal reproduction. This review article details how biochemical and evolutionary experiments have jointly enhanced the field for 40 years, and how recent work using high-throughput genomic and proteomic techniques have shed additional insights into this crucial biological process.

Evolution of gamete attraction molecules: evidence for purifying selection in speract and its receptor, in the pantropical sea urchin Diadema

Many free-spawning marine invertebrates, such as sea urchins, lack any courtship or assortative mating behavior. Mate recognition in such cases occur at the gametic level, and molecules present on the sperm and egg are major determinants of species-specific fertilization. These molecules must also coevolve in relation to each other in order to preserve functional integrity. When sea urchins release their gametes in seawater, diffusible molecules from the egg, termed sperm-activating peptides, activate and attract the sperm to swim toward the egg, initiating a series of interactions between the gametes. Although the compositions and diversity of such sperm-activating peptides have been characterized in a variety of sea urchins, little is known about the evolution of their genes. Here we characterize the genes encoding the sperm-activating peptide of the egg (speract) and its receptor on the sperm, and examine their evolutionary dynamics in the sea urchin genus Diadema, in the interest of determining whether they are involved in reproductive isolation between the species. We found evidence of purifying selection on several codon sites in both molecules and of selectively neutral evolution in others. The diffusible speract peptide that activates sperm is invariant across species, indicating that Diadema egg peptides do not discriminate between con- and hetero-specific sperm at this stage of the process. Speract and its receptor do not contribute to reproductive isolation in Diadema.

ADAMTS proteases in fertility

The reproductive organs are unique among adult organs in that they must undergo continual tissue remodelling as a key aspect of their normal function. The processes for persistent maturation and release of new gametes, as well as fertilisation, implantation, placentation, gestation and parturition involve cyclic development and regression of tissues that must continually regenerate to support fertility. The ADAMTS family of proteases has been shown to contribute to many aspects of the tissue morphogenesis required for development and function of each of the reproductive organs. Dysregulation or functional changes in ADAMTS family proteases have been associated with reproductive disorders such as polycystic ovarian syndrome (PCOS) and premature ovarian failure (POF). Likewise, proteolytic substrates of ADAMTS enzymes have also been linked to reproductive function. New insight into the roles of ADAMTS proteases has yielded a deeper understanding of the molecular mechanisms behind fertility with clinical potential to generate therapeutic targets to resolve infertility, develop biomarkers that predict dysfunction of the reproductive organs and potentially offer targets for development of non-hormonal male and female contraceptives.

Assortative mating drives linkage disequilibrium between sperm and egg recognition protein loci in the sea urchin Strongylocentrotus purpuratus

Sperm and eggs have interacting proteins on their surfaces that influence their compatibility during fertilization. These proteins are often polymorphic within species, producing variation in gamete affinities. We first demonstrate the fitness consequences of various sperm bindin protein (Bindin) variants in the sea urchin Strongylocentrotus purpuratus, and assortative mating between males and females based on their sperm Bindin genotype. This empirical finding of assortative mating based on sperm Bindin genotype could arise by linkage disequilibrium (LD) between interacting sperm and egg recognition loci. We then examine sequence variation in eight exons of the sea urchin egg receptor for sperm Bindin (EBR1). We find little evidence of LD among the eight exons of EBR1, yet strong evidence for LD between sperm Bindin and EBR1 overall, and varying degrees of LD between sperm Bindin among the eight exons. We reject the alternate hypotheses of LD driven by shared evolutionary histories, population structure, or close physical linkage between these interacting loci on the genome. The most parsimonious explanation for this pattern of LD is that it represents selection driven by assortative mating based on interactions among these sperm and egg loci. These findings indicate the importance of ongoing sexual selection in the maintenance of protein polymorphisms and LD, and more generally highlight how LD can be used as an indication of current mate choice, as opposed to historic selection.

The biological networks in studying cell signal transduction complexity: The examples of sperm capacitation and of endocannabinoid system

Cellular signal transduction is a complex phenomenon, which plays a central role in cell surviving and adaptation. The great amount of molecular data to date present in literature, together with the adoption of high throughput technologies, on the one hand, made available to scientists an enormous quantity of information, on the other hand, failed to provide a parallel increase in the understanding of biological events. In this context, a new discipline arose, the systems biology, aimed to manage the information with a computational modeling-based approach. In particular, the use of biological networks has allowed the making of huge progress in this field.

Here we discuss two possible application of the use of biological networks to explore cell signaling: the study of the architecture of signaling systems that cooperate in determining the acquisition of a complex cellular function (as it is the case of the process of activation of spermatozoa) and the organization of a single specific signaling systems expressed by different cells in different tissues (i.e. the endocannabinoid system). In both the cases we have found that the networks follow a scale free and small world topology, likely due to the evolutionary advantage of robustness against random damages, fastness and specific of information processing, and easy navigability.

Incipient speciation of sea star populations by adaptive gamete recognition coevolution

Reproductive isolation--the key event in speciation--can evolve when sexual conflict causes selection favoring different combinations of male and female adaptations in different populations. Likely targets of such selection include genes that encode proteins on the surfaces of sperm and eggs, but no previous study has demonstrated intraspecific coevolution of interacting gamete recognition genes under selection. Here, we show that selection drives coevolution between an egg receptor for sperm (OBi1) and a sperm acrosomal protein (bindin) in diverging populations of a sea star (Patiria miniata). We found positive selection on OBi1 in an exon encoding part of its predicted substrate-binding protein domain, the ligand for which is found in bindin. Gene flow was zero for the parts of bindin and OBi1 in which selection for high rates of amino acid substitution was detected; higher gene flow for other parts of the genome indicated selection against immigrant alleles at bindin and OBi1. Populations differed in allele frequencies at two key positively selected sites (one in each gene), and differences at those sites predicted fertilization rate variation among male-female pairs. These patterns suggest adaptively evolving loci that influence reproductive isolation between populations.

Reproductive isolation in Caenorhabditis briggsae: Dysgenic interactions between maternal- and zygotic-effect loci result in a delayed development phenotype

In sexual species, speciation occurs through the accumulation of genetic barriers to gene flow. In Caenorhabditis briggsae, one such barrier impedes gene flow between temperate strains and the tropical AF16 strain. Up to 20% of F2 progeny derived from crosses of AF16 to strains from the temperate clade exhibit a delayed development phenotype. This phenotype, which results from dysgenic interactions between maternal- and zygyotic-effect loci, causes a ~21% decrease in the intrinsic growth rate. The maternal-effect requires contributions from both parental genotypes. The dysgenic maternal-effect allele appears to be fixed in the temperate clade of C. briggsae and appears to have arisen between 700 and 15,000 y ago. The dysgenic zygotic allele appears to be present only in AF16 and also may be of recent origin.

Adaptive evolution of M3 lysin--a candidate gamete recognition protein in the Mytilus edulis species complex

Marine invertebrate gamete recognition proteins (GRPs) are classic examples of rapid adaptive evolution of reproductive proteins, and hybridizing Mytilus blue mussels allow us to study the evolution of GRPs during speciation following secondary contact. Even with frequent hybridization, positive selection drives divergence of M7 lysin, one of the three Mytilus egg vitelline envelope (VE) lysins. Mytilus trossulus and M. edulis form a broad hybrid zone in the Canadian Maritimes and eastern Maine, isolated by strong (but partial) gamete incompatibility. M7 lysin, however, is an unlikely GRP controlling this gametic incompativility, as earlier studies showed either weak or no positive selection and extensive introgression between the two species. We used reverse transcriptase-polymerase chain reaction and cloned several alleles of M3 lysin, a potent VE lysin encoded by a nonhomologous gene whose evolution has not been studied. McDonald-Kreitman and HKA tests reveal strong positive selection, which PAML branch-site models detect in 19.7% of the codons. Protein structure predictions show that replacements map exclusively to one face of the carbohydrate recognition domain (CRD) of this C-type lectin, with codons under positive selection localizing to CRD regions known to control ligand specificity. Polymorphism/divergence analyses show that selective sweep has purged M. edulis but not M. trossulus of polymorphism, and unique to M3 is an absence of fixed substitutions and broad haplotype sharing between M. edulis and Mediterranean M. galloprovincialis. Taken together, these results suggest that different lysins serve as GRPs in different Mytilus hybrid zones, with M3 likely co-opted to play this role in the western Atlantic.