Crystal structure and subunit dynamics of the abalone sperm lysin dimer: egg envelopes dissociate dimers, the monomer is the active species

Solution structure of sperm lysin yields novel insights into molecular dynamics of rapid protein evolution

Protein evolution is driven by the sum of different physiochemical and genetic processes that usually results in strong purifying selection to maintain biochemical functions. However, proteins that are part of systems under arms race dynamics often evolve at unparalleled rates that can produce atypical biochemical properties. In the marine mollusk abalone, lysin and vitelline envelope receptor for lysin (VERL) are a pair of rapidly coevolving proteins that are essential for species-specific interactions between sperm and egg. Despite extensive biochemical characterization of lysin-including crystal structures of multiple orthologs-it was unclear how sites under positive selection may facilitate recognition of VERL. Using a combination of targeted mutagenesis and multidimensional NMR, we present a high-definition solution structure of sperm lysin from red abalone (Haliotis rufescens). Unapparent from the crystallography data, multiple NMR-based analyses conducted in solution reveal clustering of the N and C termini to form a nexus of 13 positively selected sites that constitute a VERL binding interface. Evolutionary rate was found to be a significant predictor of backbone flexibility, which may be critical for lysin bioactivity and/or accelerated evolution. Flexible, rapidly evolving segments that constitute the VERL binding interface were also the most distorted regions of the crystal structure relative to what was observed in solution. While lysin has been the subject of extensive biochemical and evolutionary analyses for more than 30 years, this study highlights the enhanced insights gained from applying NMR approaches to rapidly evolving proteins.

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.

Roles of mutation and selection in speciation: from Hugo de Vries to the modern genomic era

One of the most important problems in evolutionary biology is to understand how new species are generated in nature. In the past, it was difficult to study this problem because our lifetime is too short to observe the entire process of speciation. In recent years, however, molecular and genomic techniques have been developed for identifying and studying the genes involved in speciation. Using these techniques, many investigators have already obtained new findings. At present, however, the results obtained are complex and quite confusing. We have therefore attempted to understand these findings coherently with a historical perspective and clarify the roles of mutation and natural selection in speciation. We have first indicated that the root of the currently burgeoning field of plant genomics goes back to Hugo de Vries, who proposed the mutation theory of evolution more than a century ago and that he unknowingly found the importance of polyploidy and chromosomal rearrangements in plant speciation. We have then shown that the currently popular Dobzhansky–Muller model of evolution of reproductive isolation is only one of many possible mechanisms. Some of them are Oka’s model of duplicate gene mutations, multiallelic speciation, mutation-rescue model, segregation-distorter gene model, heterochromatin-associated speciation, single-locus model, etc. The occurrence of speciation also depends on the reproductive system, population size, bottleneck effects, and environmental factors, such as temperature and day length. Some authors emphasized the importance of natural selection to speed up speciation, but mutation is crucial in speciation because reproductive barriers cannot be generated without mutations.

Selection in the rapid evolution of gamete recognition proteins in marine invertebrates

Animal fertilization is governed by the interaction (binding) of proteins on the surfaces of sperm and egg. In many examples presented herein, fertilization proteins evolve rapidly and show the signature of positive selection (adaptive evolution). This review describes the molecular evolution of fertilization proteins in sea urchins, abalone, and oysters, animals with external fertilization that broadcast their gametes into seawater. Theories regarding the selective forces responsible for the rapid evolution driven by positive selection seen in many fertilization proteins are discussed. This strong selection acting on divergence of interacting fertilization proteins might lead to prezygotic reproductive isolation and be a significant factor in the speciation process. Since only a fraction of all eggs are fertilized and only an infinitesimal fraction of male gametes succeed in fertilizing an egg, gametes are obviously a category of entities subjected to intense selection. It is curious that this is never mentioned in the literature dealing with selection, perhaps because we know so little about fitness differences among gametes. (Ernst Mayr, 1997).

DiMoVo: a Voronoi tessellation-based method for discriminating crystallographic and biological protein-protein interactions

MOTIVATION

Knowledge of the oligomeric state of a protein is often essential for understanding its function and mechanism. Within a protein crystal, each protein monomer is in contact with many others, forming many small interfaces and a few larger ones that are biologically significant if the protein is a homodimer in solution, but not if the protein is monomeric. Telling such 'crystal dimers' from real ones remains a difficult task.

RESULTS

It has already been demonstrated that the interfaces of native and non-native protein-protein complexes can be distinguished using a combination of parameters computed with a method on the Voronoi tessellation. We show in this article that the same parameters highlight significant differences between the interfaces of biological and crystal dimers. Using these parameters as descriptors in machine learning methods leads to accurate classification of specific and non-specific protein-protein interfaces.

AVAILABILITY

Software is available at http://fifi.ibbmc.u-psud.fr/DiMoVo.

A combinatorial score to distinguish biological and nonbiological protein-protein interfaces

With the large amount of protein-protein complex structural data available, to understand the key features governing the specificity of protein-protein recognition and to define a suitable scoring function for protein-protein interaction predictions, we have analyzed the protein interfaces from geometric and energetic points of view. Atom-based potential of mean force (PMFScore), packing density, contact size, and geometric complementarity are calculated for crystal contacts in 74 homodimers and 91 monomers, which include real biological interactions in dimers and nonbiological contacts in monomers and dimers. Simple cutoffs were developed for single and combinatorial parameters to distinguish biological and nonbiological contacts. The results show that PMFScore is a better discriminator between biological and nonbiological interfaces comparable in size. The combination of PMFScore and contact size is the most powerful pairwise discriminator. A combinatorial score (CFPScore) based on the four parameters was developed, which gives the success rate of the homodimer discrimination of 96.6% and error rate of the monomer discrimination of 6.0% and 19.8% according to Valdar's and our definition, respectively. Compared with other statistical learning models, the cutoffs for the four parameters and their combinations are directly based on physical models, simple, and can be easily applied to protein-protein interface analysis and docking studies.

Selectionism and neutralism in molecular evolution

Charles Darwin proposed that evolution occurs primarily by natural selection, but this view has been controversial from the beginning. Two of the major opposing views have been mutationism and neutralism. Early molecular studies suggested that most amino acid substitutions in proteins are neutral or nearly neutral and the functional change of proteins occurs by a few key amino acid substitutions. This suggestion generated an intense controversy over selectionism and neutralism. This controversy is partially caused by Kimura's definition of neutrality, which was too strict (|2Ns|< or =1). If we define neutral mutations as the mutations that do not change the function of gene products appreciably, many controversies disappear because slightly deleterious and slightly advantageous mutations are engulfed by neutral mutations. The ratio of the rate of nonsynonymous nucleotide substitution to that of synonymous substitution is a useful quantity to study positive Darwinian selection operating at highly variable genetic loci, but it does not necessarily detect adaptively important codons. Previously, multigene families were thought to evolve following the model of concerted evolution, but new evidence indicates that most of them evolve by a birth-and-death process of duplicate genes. It is now clear that most phenotypic characters or genetic systems such as the adaptive immune system in vertebrates are controlled by the interaction of a number of multigene families, which are often evolutionarily related and are subject to birth-and-death evolution. Therefore, it is important to study the mechanisms of gene family interaction for understanding phenotypic evolution. Because gene duplication occurs more or less at random, phenotypic evolution contains some fortuitous elements, though the environmental factors also play an important role. The randomness of phenotypic evolution is qualitatively different from allele frequency changes by random genetic drift. However, there is some similarity between phenotypic and molecular evolution with respect to functional or environmental constraints and evolutionary rate. It appears that mutation (including gene duplication and other DNA changes) is the driving force of evolution at both the genic and the phenotypic levels.

Implications of the Effects of Viscosity, Macromolecular Crowding, and Temperature for the Transient Interaction between Cytochrome f and Plastocyanin from the Cyanobacterium Phormidium laminosum

The reaction between cytochrome f and plastocyanin is a central feature of the photosynthetic electron-transport system of all oxygenic organisms. We have studied the reaction in solution to understand how the very weak binding between the two proteins from Phormidium laminosum can nevertheless lead to fast rates of electron transfer. In a previous publication [Schlarb-Ridley, B. G., et al. (2003) Biochemistry 42, 4057-4063], we suggested that the reaction is diffusion-controlled because of a strong effect of viscosity of the medium. The effects of viscosity and temperature have now been examined in detail. High molecular mass viscogens (Ficoll 70 and Dextran 70), which might mimic in vivo conditions, had little effect up to a relative viscosity of 4. Low molecular mass viscogens (ethane diol, glycerol, and sucrose) strongly decreased the bimolecular rate constant (k(2)) over a similar viscosity range. The effects correlated well with the viscosities of the solutions of the three reagents but not with their dielectric constants or molalities. A power law dependence of k(2) on viscosity suggested that k(2) depends on two viscosity-sensitive reactions in series, while the reverse reactions are little affected by viscosity. The results were incompatible with diffusion control of the overall reaction. Determination of the effect of temperature on k(2) gave an activation enthalpy, DeltaH(++) = 45 kJ mol(-)(1), which is also incompatible with diffusion control. The results were interpreted in terms of a model in which the stable form of the protein-protein complex requires further thermal activation to be competent for electron transfer.

Structural characterisation and functional significance of transient protein-protein interactions

Protein-protein complexes that dissociate and associate readily, often depending on the physiological condition or environment, play an important role in many biological processes. In order to characterise these "transient" protein-protein interactions, two sets of complexes were collected and analysed. The first set consists of 16 experimentally validated "weak" transient homodimers, which are known to exist as monomers and dimers at physiological concentration, with dissociation constants in the micromolar range. A set of 23 functionally validated transient (i.e. intracellular signalling) heterodimers comprise the second set. This set includes complexes that are more stable, with nanomolar binding affinities, and require a molecular trigger to form and break the interaction. In comparison to more stable homodimeric complexes, the weak homodimers demonstrate smaller contact areas between protomers and the interfaces are more planar and polar on average. The physicochemical and geometrical properties of these weak homodimers more closely resemble those of non-obligate hetero-oligomeric complexes, whose components can exist either as monomers or as complexes in vivo. In contrast to the weak transient dimers, "strong" transient dimers often undergo large conformational changes upon association/dissociation and are characterised with larger, less planar and sometimes more hydrophobic interfaces. From sequence alignments we find that the interface residues of the weak transient homodimers are generally more conserved than surface residues, consistent with being constrained to maintain the protein-protein interaction during evolution. Protein families that include members with different oligomeric states or structures are identified, and found to exhibit a lower sequence conservation at the interface. The results are discussed in terms of the physiological function and evolution of protein-protein interactions.

Full-length sequence of VERL, the egg vitelline envelope receptor for abalone sperm lysin

Abalone sperm use 16 kDa lysin to create a hole in the egg vitelline envelope (VE) by a species-specific, nonenzymatic mechanism. To create the hole, lysin binds tightly to VERL (the VE receptor for lysin), a giant, unbranched glycoprotein comprising 30% of the VE. Binding of lysin to VERL causes the VERL molecules to lose cohesion and splay apart creating the hole. Lysin and VERL represent a cognate pair of gamete recognition proteins, one male the other female, which mediate fertilization. The coevolution of such cognate pairs may underlie the establishment of species-specific fertilization which could be a component of the mechanism to achieve reproductive isolation and hence new species. Here we present the full-length cDNA sequence (11,166 bp) of VERL from the red abalone (Haliotis rufescens). There are 42 amino acids from the start Met residue to the beginning of the first 'VERL repeat'. Most of VERL (9981 bp; 89.4%) consists of 22 tandem repeats of a approximately 153 amino acid sequence that is predicted to be beta-sheet. The last VERL repeat is followed by 353 non-repeat amino acid residues containing a furin cleavage site (RTRR), a ZP domain and a hydrophobic COOH-terminus with a 3' UTR of only 10 nucleotides. VERL repeats 3-22 have been subjected to concerted evolution and consequently have almost identical sequences. Curiously, comparisons of repeats from other species shows that repeats 1 and 2 of red abalone VERL have not been subjected to concerted evolution since the divergence of the red species from the other six California species.

Codon-substitution models to detect adaptive evolution that account for heterogeneous selective pressures among site classes

The nonsynonymous to synonymous substitution rate ratio (omega = d(N)/d(S)) provides a sensitive measure of selective pressure at the protein level, with omega values <1, =1, and >1 indicating purifying selection, neutral evolution, and diversifying selection, respectively. Maximum likelihood models of codon substitution developed recently account for variable selective pressures among amino acid sites by employing a statistical distribution for the omega ratio among sites. Those models, called random-sites models, are suitable when we do not know a priori which sites are under what kind of selective pressure. Sometimes prior information (such as the tertiary structure of the protein) might be available to partition sites in the protein into different classes, which are expected to be under different selective pressures. It is then sensible to use such information in the model. In this paper, we implement maximum likelihood models for prepartitioned data sets, which account for the heterogeneity among site partitions by using different omega parameters for the partitions. The models, referred to as fixed-sites models, are also useful for combined analysis of multiple genes from the same set of species. We apply the models to data sets of the major histocompatibility complex (MHC) class I alleles from human populations and of the abalone sperm lysin genes. Structural information is used to partition sites in MHC into two classes: those in the antigen recognition site (ARS) and those outside. Positive selection is detected in the ARS by the fixed-sites models. Similarly, sites in lysin are classified into the buried and solvent-exposed classes according to the tertiary structure, and positive selection was detected at the solvent-exposed sites. The random-sites models identified a number of sites under positive selection in each data set, confirming and elaborating the results of the fixed-sites models. The analysis demonstrates the utility of the fixed-sites models, as well as the power of previous random-sites models, which do not use the prior information to partition sites.

The crystal structure of a fusagenic sperm protein reveals extreme surface properties

Sp18 is an 18 kDa protein that is released from abalone sperm during the acrosome reaction. It coats the acrosomal process where it is thought to mediate fusion between sperm and egg cell membranes. Sp18 is evolutionarily related to lysin, a 16 kDa abalone sperm protein that dissolves the vitelline envelope surrounding the egg. The two proteins were generated by gene duplication followed by rapid divergence by positive selection. Here, we present the crystal structure of green abalone sp18 resolved to 1.86 A. Sp18 is composed of a bundle of five alpha-helices with surface clusters of basic and hydrophobic residues, giving it a large dipole moment and making it extremely amphipathic. The large clusters of hydrophobic surface residues and domains of high positive electrostatic surface charge explain sp18's ability as a potent fusagen of liposomes. The overall fold of sp18 is similar to that of green abalone lysin; however, the surface features of the proteins are quite different, accounting for their different roles in fertilization. This is the first crystal structure of a protein implicated in sperm-egg fusion during animal fertilization.

Abalone lysin: the dissolving and evolving sperm protein

Abalone sperm lysin is a non-enzymatic protein that creates a hole for sperm passage in the envelope surrounding the egg. Lysin exhibits species-specificity in making the hole and it evolves rapidly by positive selection. Our studies have focused on combining structural, biochemical, and evolutionary data to understand the mechanism of action and evolution of this remarkable protein. Currently, more is known about lysin than about any other protein involved in animal fertilization. We present an hypothesis to explain lysin's rapid evolution and the evolution of species-specific fertilization in this order of mollusks. We also propose a two-step model for lysin's action in which a dimer of lysin binds species-specifically to its glycoprotein receptor, and then monomerizes and binds the receptor in a non-species-specific manner. This experimental system yields data relevant to the general problem of molecular recognition between cell surfaces, and is also important to our thinking about how new species arise in the sea. BioEssays 23:95-103, 2001.

Maximum-likelihood analysis of molecular adaptation in abalone sperm lysin reveals variable selective pressures among lineages and sites

Maximum-likelihood models of codon substitution were used to analyze sperm lysin genes of 25 abalone (HALIOTIS:) species to identify lineages and amino acid sites under diversifying selection. The models used the nonsynonymous/synonymous rate ratio (omega = d(N)/d(S)) as an indicator of selective pressure and allowed the ratio to vary among lineages or sites. Likelihood ratio tests suggested significant variation in selective pressure among lineages. The variable selective pressure provided an explanation for the previous observation that the omega ratio is >1 in comparisons of closely related species and <1 in comparisons of distantly related species. Computer simulations demonstrated that saturation of nonsynonymous substitutions and constraint on lysin structure were unlikely to account for the observed pattern. Lineages linking closely related sympatric species appeared to be under diversifying selection, while lineages separating distantly related species from different geographic locations were associated with low evolutionary rates. The selective pressure indicated by the omega ratio was found to vary greatly among amino acid sites in lysin. Sites under potential diversifying selection were identified. Ancestral lysins were inferred to trace the route of evolution at individual sites and to provide lysin sequences for future laboratory studies.

The high resolution crystal structure of green abalone sperm lysin: implications for species-specific binding of the egg receptor

Abalone sperm lysin is a 16 kDa acrosomal protein used by sperm to create a hole in the egg vitelline envelope. Lysins from seven California abalone exhibit species-specificity in binding to their egg receptor, and range in sequence identity from 63 % to 90 %. The crystal structure of the sperm lysin dimer from Haliotis fulgens (green abalone) has been determined to 1.71 A by multiple isomorphous replacement. Comparisons with the structure of the lysin dimer from Haliotis rufescens (red abalone) reveal a similar overall fold and conservation of features contributing to lysin's amphipathic character. The two structures do, however, exhibit differences in surface residues and electrostatics. A large clustering of non-conserved surface residues around the waist and clefts of the dimer, and differences in charged residues around these regions, indicate areas of the molecule which may be involved in species-specific egg recognition.

Interspecies chimeric sperm lysins identify regions mediating species-specific recognition of the abalone egg vitelline envelope

Abalone sperm lysin is a nonenzymatic, 16-kDa protein that creates a hole in the egg vitelline envelope (VE) through which the sperm swims to fuse with the egg. The dissolution of isolated VE by lysin is species specific. Interspecies comparisons show that the most divergent region of lysin is the N-terminal segment of residues 1-12 which is always species-unique. The C-terminus and three internal segments are moderately variable between species, but not species unique. Analysis of nucleotide substitutions shows that lysin evolves rapidly by positive Darwinian selection, suggesting that there is adaptive value in altering its amino acid sequence. The results reported here, in which segments of lysin were exchanged between two species, prove by direct experimentation that the interspecies variable termini play major roles in the species-specific recognition between sperm lysin and the egg VE.

Evolution of gamete recognition proteins

REVIEW Although fertilization has been studied for more than a century, the cell surface proteins mediating the process are only now becoming known. Gamete interaction in animals appears to be molecularly complex. Although it is difficult to generalize at present, diversity of structure may be a recurring theme in the evolution of fertilization proteins. Examples of rapid evolution of fertilization proteins by positive selection are known, and concerted evolution can influence the differentiation of gamete recognition proteins between closely related species.

Mechanism and evolution of protein dimerization

We have investigated the mechanism and the evolutionary pathway of protein dimerization through analysis of experimental structures of dimers. We propose that the evolution of dimers may have multiple pathways, including (1) formation of a functional dimer directly without going through an ancestor monomer, (2) formation of a stable monomer as an intermediate followed by mutations of its surface residues, and (3), a domain swapping mechanism, replacing one segment in a monomer by an equivalent segment from an identical chain in the dimer. Some of the dimers which are governed by a domain swapping mechanism may have evolved at an earlier stage of evolution via the second mechanism. Here, we follow the theory that the kinetic pathway reflects the evolutionary pathway. We analyze the structure-kinetics-evolution relationship for a collection of symmetric homodimers classified into three groups: (1) 14 dimers, which were referred to as domain swapping dimers in the literature; (2) nine 2-state dimers, which have no measurable intermediates in equilibrium denaturation; and (3), eight 3-state dimers, which have stable intermediates in equilibrium denaturation. The analysis consists of the following stages: (i) The dimer is divided into two structural units, which have twofold symmetry. Each unit contains a contiguous segment from one polypeptide chain of the dimer, and its complementary contiguous segment from the other chain. (ii) The division is repeated progressively, with different combinations of the two segments in each unit. (iii) The coefficient of compactness is calculated for the units in all divisions. The coefficients obtained for different cuttings of a dimer form a compactness profile. The profile probes the structural organization of the two chains in a dimer and the stability of the monomeric state. We describe the features of the compactness profiles in each of the three dimer groups. The profiles identify the swapping segments in domain swapping dimers, and can usually predict whether a dimer has domain swapping. The kinetics of dimerization indicates that some dimers which have been assigned in the literature as domain swapping cases, dimerize through the 2-state kinetics, rather than through swapping segments of performed monomers. The compactness profiles indicate a wide spectrum in the kinetics of dimerization: dimers having no intermediate stable monomers; dimers having an intermediate with a stable monomer structure; and dimers having an intermediate with a stable structure in part of the monomer. These correspond to the multiple evolutionary pathways for dimer formation. The evolutionary mechanisms proposed here for dimers are applicable to other oligomers as well.

Secondary structure, stability and tetramerisation of recombinant K(V)1.1 potassium channel cytoplasmic N-terminal fragment

The recombinant N-terminal fragment (amino acids 14-162) of a tetrameric voltage-gated potassium channel (K(V)1.1) has been studied using spectroscopic techniques. Evidence is presented that it forms a tetramer in aqueous solution, whereas when solubilised in 1% Triton X-100 it remains monomeric. The secondary structure content of both monomeric and tetrameric K(V)1.1 N-terminal fragment has been estimated from FTIR and CD spectroscopy to be 20-25% alpha-helix, 20-25% beta-sheet, 20% turns and 30-40% random coil. Solubilisation of the protein in detergent is shown by hydrogen-deuterium exchange analysis to alter tertiary structure rather than secondary structure and this may be the determining factor in tetramerisation ability. Using molecular modelling we propose a supersecondary structure consisting of two structural domains.

The abalone egg vitelline envelope receptor for sperm lysin is a giant multivalent molecule

Abalone sperm lysin is a 16-kDa acrosomal protein, which nonenzymatically and species selectively creates a hole in the egg vitelline envelope (VE) through which the sperm passes to reach the egg cell membrane. The crystal structures of both monomeric and dimeric lysins have been solved and the sequences of lysins from 20 abalone species have been determined. As a first step in understanding the molecular mechanism by which lysin creates a hole in the VE, its VE receptor was isolated. The VE receptor for lysin (VERL) is an unbranched, rod-like molecule with an approximate relative molecular mass of 2 million; half the mass being carbohydrate. Fluorescence polarization studies showed positive cooperativity in the binding of lysin to VERL (EC50 approximately 9 nM) and were consistent with the species selectivity of lysin in dissolving VEs. Each molecule of VERL bound between 126 and 142 molecules of monomeric lysin (two independent assays), showing that VERL possesses repetitive lysin-binding motifs.

Positive Darwinian selection on two homologous fertilization proteins: what is the selective pressure driving their divergence?

Most examples of positive selection inferred from nucleotide sequence data involve hostpathogen interactions. However, positive selection also promotes the divergence of proteins mediating sperm-egg recognition in marine invertebrates. The abalone spermatozoon has a large acrosomal vesicle containing two proteins of 16 kDa and 18 kDa. Lysin, the 16-kDa protein, exhibits species-specificity in dissolving a hole in the egg vitelline envelope through which the sperm swims to reach the egg plasma membrane. The 18-kDa protein coats the sperm acrosomal process and probably mediates fusion of the two gametes. In this review, we compare sequences of both proteins from five species of California abalones. Both proteins show extensive divergence which has been promoted by positive Darwinian selection. The ratios of nonsynonymous to synonymous nucleotide substitutions may be the highest yet discovered for full-length sequences. Although extensive divergence has occurred, there is conservation of the shape and polarity of residues in both proteins. The two acrosomal proteins arose by a gene duplication followed by their extensive divergence. Five hypotheses are presented which attempt to explain the nature of the unknown selective force responsible for the robust positive selection. The positive selection may, in some unknown way, be related to the establishment of prezygotic barriers to reproduction. Because positive selection promotes the divergence of unrelated, species-specific gamete recognition proteins in both abalones and sea urchins, we predict that positive selection may be a general phenomenon in the evolution of gamete recognition systems in marine invertebrates.