Duplicate abalone egg coat proteins bind sperm lysin similarly, but evolve oppositely, consistent with molecular mimicry at fertilization

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.

Faster Rates of Molecular Sequence Evolution in Reproduction-Related Genes and in Species with Hypodermic Sperm Morphologies

Sexual selection drives the evolution of many striking behaviors and morphologies and should leave signatures of selection at loci underlying these phenotypes. However, although loci thought to be under sexual selection often evolve rapidly, few studies have contrasted rates of molecular sequence evolution at such loci across lineages with different sexual selection contexts. Furthermore, work has focused on separate sexed animals, neglecting alternative sexual systems. We investigate rates of molecular sequence evolution in hermaphroditic flatworms of the genus Macrostomum. Specifically, we compare species that exhibit contrasting sperm morphologies, strongly associated with multiple convergent shifts in the mating strategy, reflecting different sexual selection contexts. Species donating and receiving sperm in every mating have sperm with bristles, likely to prevent sperm removal. Meanwhile, species that hypodermically inject sperm lack bristles, potentially as an adaptation to the environment experienced by hypodermic sperm. Combining functional annotations from the model, Macrostomum lignano, with transcriptomes from 93 congeners, we find genus-wide faster sequence evolution in reproduction-related versus ubiquitously expressed genes, consistent with stronger sexual selection on the former. Additionally, species with hypodermic sperm morphologies had elevated molecular sequence evolution, regardless of a gene's functional annotation. These genome-wide patterns suggest reduced selection efficiency following shifts to hypodermic mating, possibly due to higher selfing rates in these species. Moreover, we find little evidence for convergent amino acid changes across species. Our work not only shows that reproduction-related genes evolve rapidly also in hermaphroditic animals, but also that well-replicated contrasts of different sexual selection contexts can reveal underappreciated genome-wide effects.

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.

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.

Parallel Evolution of Sperm Hyper-Activation Ca2+ Channels

Sperm hyper-activation is a dramatic change in sperm behavior where mature sperm burst into a final sprint in the race to the egg. The mechanism of sperm hyper-activation in many metazoans, including humans, consists of a jolt of Ca²⁺ into the sperm flagellum via CatSper ion channels. Surprisingly, all nine CatSper genes have been independently lost in several animal lineages. In Drosophila, sperm hyper-activation is performed through the cooption of the polycystic kidney disease 2 (pkd2) Ca²⁺ channel. The parallels between CatSpers in primates and pkd2 in Drosophila provide a unique opportunity to examine the molecular evolution of the sperm hyper-activation machinery in two independent, nonhomologous calcium channels separated by > 500 million years of divergence. Here, we use a comprehensive phylogenomic approach to investigate the selective pressures on these sperm hyper-activation channels. First, we find that the entire CatSper complex evolves rapidly under recurrent positive selection in primates. Second, we find that pkd2 has parallel patterns of adaptive evolution in Drosophila. Third, we show that this adaptive evolution of pkd2 is driven by its role in sperm hyper-activation. These patterns of selection suggest that the evolution of the sperm hyper-activation machinery is driven by sexual conflict with antagonistic ligands that modulate channel activity. Together, our results add sperm hyper-activation channels to the class of fast evolving reproductive proteins and provide insights into the mechanisms used by the sexes to manipulate sperm behavior.

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.

Sperm competition and the evolution of gametic compatibility in externally fertilizing taxa

Proteins expressed on the surface of sperm and egg mediate gametic compatibility and these proteins can be subject to intense positive selection. In this review, we discuss what is known about the patterns of adaptive evolution of gamete recognition proteins (GRPs). We focus on species that broadcast eggs and sperm into the environment for external fertilization, as the ease of observing and manipulating gamete interactions has allowed for greater advances in the understanding of GRP evolution, uncomplicated by confounding behavioral and physiological components that offer alternative evolutionary targets in internal fertilizers. We discuss whether interspecific mechanisms, such as selection to avoid fertilization between species (reinforcement selection), or intraspecific mechanisms, such as selection to increase (or decrease) the affinity between eggs and sperm based on the intensity of sperm competition, may be responsible for the pattern of GRP evolution observed. Variation in these proteins appears to influence gametic compatibility; GRP divergence among species is a better predictor of hybrid fertilization than neutral genetic markers and GRP variation within species predicts reproductive success among individuals within a population. Evidence suggests that sperm competition may play a large role in the evolution of gametic compatibility.

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.

Functional similarity and molecular divergence of a novel reproductive transcriptome in two male-pregnant Syngnathus pipefish species

Evolutionary studies have revealed that reproductive proteins in animals and plants often evolve more rapidly than the genome-wide average. The causes of this pattern, which may include relaxed purifying selection, sexual selection, sexual conflict, pathogen resistance, reinforcement, or gene duplication, remain elusive. Investigative expansions to additional taxa and reproductive tissues have the potential to shed new light on this unresolved problem. Here, we embark on such an expansion, in a comparison of the brood-pouch transcriptome between two male-pregnant species of the pipefish genus Syngnathus. Male brooding tissues in syngnathid fishes represent a novel, nonurogenital reproductive trait, heretofore mostly uncharacterized from a molecular perspective. We leveraged next-generation sequencing (Roche 454 pyrosequencing) to compare transcript abundance in the male brooding tissues of pregnant with nonpregnant samples from Gulf (S. scovelli) and dusky (S. floridae) pipefish. A core set of protein-coding genes, including multiple members of astacin metalloprotease and c-type lectin gene families, is consistent between species in both the direction and magnitude of expression bias. As predicted, coding DNA sequence analysis of these putative "male pregnancy proteins" suggests rapid evolution relative to nondifferentially expressed genes and reflects signatures of adaptation similar in magnitude to those reported from Drosophila male accessory gland proteins. Although the precise drivers of male pregnancy protein divergence remain unknown, we argue that the male pregnancy transcriptome in syngnathid fishes, a clade diverse with respect to brooding morphology and mating system, represents a unique and promising object of study for understanding the perplexing evolutionary nature of reproductive molecules.