Contemporary evolution of sea urchin gamete-recognition proteins: experimental evidence of density-dependent gamete performance predicts shifts in allele frequencies over time

Population density effects on gamete traits and fertilisation dynamics under varying sperm environments in mussels

Gamete traits can vary widely among species, populations and individuals, influencing fertilisation dynamics and overall reproductive fitness. Sexual selection can play an important role in determining the evolution of gamete traits with local environmental conditions determining the strength and direction of sexual selection. Here, we test for signatures of post-mating selection on gamete traits in relation to population density, and possible interactive effects of population density and sperm concentration on sperm motility and fertilisation rates among natural populations of mussels. Our study shows that males from high-density populations produce smaller sperm compared with males from low-density populations, but we detected no effect of population density on egg size. Our results also reveal that females from low-density populations tended to exhibit lower fertilisation rates across a range of sperm concentrations, although this became less important as sperm concentration increased. Variances in fertilisation success were higher for females than males and the effect of gamete compatibility between males and females increases as sperm concentrations increase. These results suggest that local population density can influence gamete traits and fertilisation dynamics but also highlight the importance of phenotypic plasticity in governing sperm-egg interactions in a highly dynamic selective environment.

Density-dependent patterns of multivariate selection on sperm motility and morphology in a broadcast spawning mussel

Sperm cells exhibit extraordinary phenotypic variation, both among taxa and within individual species, yet our understanding of the adaptive value of sperm trait variation across multiple contexts is incomplete. For species without the opportunity to choose mating partners, such as sessile broadcast spawning invertebrates, fertilization depends on gamete interactions, which in turn can be strongly influenced by local environmental conditions that alter the concentration of sperm and eggs. However, the way in which such environmental factors impact phenotypic selection on functional gamete traits remains unclear in most systems. Here, we analyze patterns of linear and nonlinear multivariate selection under experimentally altered local sperm densities (densities within the capture zone of eggs) on a range of functionally important sperm traits in the broadcast spawning marine mussel, Mytilus galloprovincialis. Specifically, we assay components of sperm motility and morphology across two fertilization environments that simulate either sperm limitation (when there are too few sperm to fertilize all available eggs), or sperm saturation (when there are many more sperm than required for fertilization, and the risk of polyspermy and embryonic failure is heightened). Our findings reveal that the strength, form, and targets of selection on sperm depend on the prevailing fertilization environment. In particular, our analyses revealed multiple significant axes of nonlinear selection on sperm motility traits under sperm limitation, but only significant negative directional selection on flagellum length under sperm saturation. These findings highlight the importance of local sperm densities in driving the adaptation of sperm phenotypes, particularly those related to sperm motility, in broadcast spawning invertebrates.

Sexual selection after gamete release in broadcast spawning invertebrates

Broadcast spawning invertebrates offer highly tractable models for evaluating sperm competition, gamete-level mate choice and sexual conflict. By displaying the ancestral mating strategy of external fertilization, where sexual selection is constrained to act after gamete release, broadcast spawners also offer potential evolutionary insights into the cascade of events that led to sexual reproduction in more 'derived' groups (including humans). Moreover, the dynamic reproductive conditions faced by these animals mean that the strength and direction of sexual selection on both males and females can vary considerably. These attributes make broadcast spawning invertebrate systems uniquely suited to testing, extending, and sometimes challenging classic and contemporary ideas in sperm competition, many of which were first captured in Parker's seminal papers on the topic. Here, we provide a synthesis outlining progress in these fields, and highlight the burgeoning potential for broadcast spawners to provide both evolutionary and mechanistic understanding into gamete-level sexual selection more broadly across the animal kingdom. This article is part of the theme issue 'Fifty years of sperm competition'.

Slow evolution under purifying selection in the gamete recognition protein bindin of the sea urchin Diadema

Bindin is a sperm protein that mediates attachment and membrane fusion of gametes. The mode of bindin evolution varies across sea urchin genera studied to date. In three genera it evolves under positive selection, in four under mostly purifying selection, and in one, results have been mixed. We studied bindin evolution in the pantropical sea urchin Diadema, which split from other studied genera 250 million years ago. We found that Diadema bindin is structurally similar to that of other genera, but much longer (418 amino acids). In seven species of Diadema, bindin evolves under purifying selection, more slowly than in any other sea urchin genus. Only bindin of the recently rediscovered D. clarki shows evidence of positive selection. As D. clarki is sympatric with D. setosum and D. savignyi, positive selection could arise from avoidance of maladaptive hybridization. However, D. setosum and D. savignyi overlap in the Indo-West Pacific, yet their bindins show no evidence of positive selection, possibly because the two species spawn at different times. Bindin in the East Pacific D. mexicanum, the West Atlantic D. antillarum, the East Atlantic D. africanum, and the Indo-Pacific D. paucispinum also evolves slowly under purifying selection.

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 Male Variation in Fertilization Success in Determining the Costs and Benefits of Polyandry in the Broadcast Spawning Urchin Lytechinus variegatus

Although the benefits to males mating with multiple females have been well documented, the benefits to females mating with multiple males (polyandry) are less studied, particularly the mechanism that might drive these potential benefits. Benefits of polyandry might stem from increasing the chance of mating with a high-quality or compatible male or stem from the ability of multiple males to fertilize more eggs than any single male. We examine the fertilization consequences of polyandry in the sea urchin Lytechinus variegatus. This species has variation in spine color, and we conducted matings between individual and pooled sperm from two males that matched or mismatched in color. The results indicate that (1) males with white spines achieved higher fertilization and were more likely to cause polyspermy than males with purple spines, and there was no effect of female spine color on fertilization; (2) when comparing the average success of individual matings with pooled-sperm matings, there was a net benefit to polyandry when purple-spine males were pooled, a net cost when white-spine males were pooled, and no difference when mismatched spine color males were pooled; and (3) the success under pooled-sperm trials, with any of the spine color combinations, never exceeded the success of the more successful male in the individual-male trials. Together these results suggest that the consequences of polyandry depend on the relation between sperm availability and the sensitivity of eggs to sperm limitation and polyspermy with respect to the specific set of available males. The potential fertilization consequences of a female spawning with multiple males might be associated primarily with increasing the amount of sperm available to fertilize her eggs and secondarily with increasing the chances of mating with a higher-quality or more compatible male, as opposed to a diversity of males.

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.

Positive selection on human gamete-recognition genes

Coevolution of genes that encode interacting proteins expressed on the surfaces of sperm and eggs can lead to variation in reproductive compatibility between mates and reproductive isolation between members of different species. Previous studies in mice and other mammals have focused in particular on evidence for positive or diversifying selection that shapes the evolution of genes that encode sperm-binding proteins expressed in the egg coat or zona pellucida (ZP). By fitting phylogenetic models of codon evolution to data from the 1000 Genomes Project, we identified candidate sites evolving under diversifying selection in the human genes ZP3 and ZP2. We also identified one candidate site under positive selection in C4BPA, which encodes a repetitive protein similar to the mouse protein ZP3R that is expressed in the sperm head and binds to the ZP at fertilization. Results from several additional analyses that applied population genetic models to the same data were consistent with the hypothesis of selection on those candidate sites leading to coevolution of sperm- and egg-expressed genes. By contrast, we found no candidate sites under selection in a fourth gene (ZP1) that encodes an egg coat structural protein not directly involved in sperm binding. Finally, we found that two of the candidate sites (in C4BPA and ZP2) were correlated with variation in family size and birth rate among Hutterite couples, and those two candidate sites were also in linkage disequilibrium in the same Hutterite study population. All of these lines of evidence are consistent with predictions from a previously proposed hypothesis of balancing selection on epistatic interactions between C4BPA and ZP3 at fertilization that lead to the evolution of co-adapted allele pairs. Such patterns also suggest specific molecular traits that may be associated with both natural reproductive variation and clinical infertility.

The reef-building coral Acropora conditionally hybridize under sperm limitation

Multi-specific synchronous spawning risks both sperm limitation, which reduces fertilization success, and hybridization with other species. If available sperm of conspecifics are limited, hybridization with heterospecific sperm could be an alternative. Some species of the reef-building coral Acropora produce hybrid offspring in vitro, and therefore hybridization between such species does sometimes occur in nature. Here, we report that the interbreeding species Acropora florida and A. intermedia preferentially bred with conspecifics at optimal gamete concentrations (10(6) cells ml(-1)), but when sperm concentration was low (10(4) cells ml(-1)), A florida eggs displayed an increased incidence of fertilization by sperm of A intermedia However, A intermedia eggs never crossed with heterospecific sperm, regardless of gamete concentrations. It appears that A florida eggs conditionally hybridize with heterospecific sperm; in nature, this would allow A florida to cross with later-spawning species such as A intermedia These results indicate that hybridization between some Acropora species could occur in nature according to the number of available sperm, and the choice of heterospecific sperm for fertilization could be one of the fertilization strategies in the sperm-limited condition.

Do Sperm Really Compete and Do Eggs Ever Have a Choice? Adult Distribution and Gamete Mixing Influence Sexual Selection, Sexual Conflict, and the Evolution of Gamete Recognition Proteins in the Sea

The evolution of gametic compatibility and the effectiveness of compatibility, within and across species, depend on whether sperm from different males directly compete for an egg and whether eggs ever have a choice. Direct sperm competition and egg choice depend on whether sperm from different males arrive at an egg in the brief interval between first sperm contact and fertilization. Although this process may be relevant for all sexually reproducing organisms, it is most easily examined in aquatic external fertilizers. When sperm are released into the sea, packets of seawater at the spatial scale relevant to single eggs might contain sperm from only one male, eliminating the potential for direct sperm competition and egg choice. Field experiments and a simple heuristic model examining the degree of sperm mixing for the sea urchin Strongylocentrotus franciscanus indicate that degree of competitive fertilization depends on density and distribution of competing males and that the nature of this competition influences whether males with high- or low-affinity gamete recognition protein genotypes have higher reproductive success. These results provide a potential explanation for the generation and maintenance of variation in gamete recognition proteins and why effectiveness of conspecific sperm precedence can be density dependent.

Rarity and persistence

Rarity is a population characteristic that is usually associated with a high risk of extinction. We argue here, however, that chronically rare species (those with low population densities over many generations across their entire ranges) may have individual-level traits that make populations more resistant to extinction. The major obstacle to persistence at low density is successful fertilisation (union between egg and sperm), and chronically rare species are more likely to survive when (1) fertilisation occurs inside or close to an adult, (2) mate choice involves long-distance signals, (3) adults or their surrogate gamete dispersers are highly mobile, or (4) the two sexes are combined in a single individual. In contrast, external fertilisation and wind- or water-driven passive dispersal of gametes, or sluggish or sedentary adult life habits in the absence of gamete vectors, appear to be incompatible with sustained rarity. We suggest that the documented increase in frequency of these traits among marine genera over geological time could explain observed secular decreases in rates of background extinction. Unanswered questions remain about how common chronic rarity actually is, which traits are consistently associated with chronic rarity, and how chronically rare species are distributed among taxa, and among the world's ecosystems and regions.

Sensitivity of sea urchin fertilization to pH varies across a natural pH mosaic

In the coastal ocean, temporal fluctuations in pH vary dramatically across biogeographic ranges. How such spatial differences in pH variability regimes might shape ocean acidification resistance in marine species remains unknown. We assessed the pH sensitivity of the sea urchin Strongylocentrotus purpuratus in the context of ocean pH variability. Using unique male-female pairs, originating from three sites with similar mean pH but different variability and frequency of low pH (pHT ≤ 7.8) exposures, fertilization was tested across a range of pH (pHT 7.61-8.03) and sperm concentrations. High fertilization success was maintained at low pH via a slight right shift in the fertilization function across sperm concentration. This pH effect differed by site. Urchins from the site with the narrowest pH variability regime exhibited the greatest pH sensitivity. At this site, mechanistic fertilization dynamics models support a decrease in sperm-egg interaction rate with decreasing pH. The site differences in pH sensitivity build upon recent evidence of local pH adaptation in S. purpuratus and highlight the need to incorporate environmental variability in the study of global change biology.

Individual female differences in chemoattractant production change the scale of sea urchin gamete interactions

Sperm selection by females is an important process influencing fertilization and, particularly in broadcast-spawning organisms, often occurs before sperm reach the egg. Waterborne sperm chemoattractants are one mechanism by which eggs selectively influence conspecific sperm behavior, but it remains an open question whether the eggs from different females produce different amounts of sperm chemoattractant, and how that might influence sperm behavior. Here, we quantify the differences in attractant production between females of the sea urchin species Lytechinus pictus and use computational models and microfluidic sperm chemotaxis assays to determine how differences in chemoattractant production between females affects their ability to attract sperm. Our study demonstrates that there is significant individual female variation in egg chemoattractant production, and that this variation changes the scope and strength of sperm attraction. These results provide evidence for the importance of individual female variability in differential sperm attraction and fertilization success.

Sexual selection in hermaphrodites, sperm and broadcast spawners, plants and fungi

Darwin was the first to recognize that sexual selection is a strong evolutionary force. Exaggerated traits allow same-sex individuals to compete over access to mates and provide a mechanism by which mates are selected. It is relatively easy to appreciate how inter- and intrasexual selection work in organisms with the sensory capabilities to perceive physical or behavioural traits that signal mate quality or mate compatibility, and to assess the relative quality of competitors. It is therefore not surprising that most studies of sexual selection have focused on animals with separate sexes and obvious adaptations that function in the context of reproductive competition. Yet, many sexual organisms are both male and female at the same time, often lack sexual dimorphism and never come into direct contact at mating. How does sexual selection act in such species, and what can we learn from them? Here, we address these questions by exploring the potential for sexual selection in simultaneous hermaphrodites, sperm- and broadcast spawners, plants and fungi. Our review reveals a range of mechanisms of sexual selection, operating primarily after gametes have been released, which are common in many of these groups and also quite possibly in more familiar (internally fertilizing and sexually dimorphic) organisms.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.

Ovarian fluid allows directional cryptic female choice despite external fertilization

In species with internal fertilization, females can favour certain males over others, not only before mating but also within the female's reproductive tract after mating. Here, we ask whether such directional post-mating (that is, cryptic) female mate choice can also occur in species with external fertilization. Using an in vitro sperm competition experiment, we demonstrate that female ovarian fluid (ovarian fluid) changes the outcome of sperm competition by decreasing the importance of sperm number thereby increasing the relative importance of sperm velocity. We further show that ovarian fluid does not differentially affect sperm from alternative male phenotypes, but generally enhances sperm velocity, motility, straightness and chemoattraction. Under natural conditions, female ovarian fluid likely increases the paternity of the preferred parental male phenotype, as these males release fewer but faster sperm. These results imply females have greater control over fertilization and potential to exert selection on males in species with external fertilization than previously thought possible.

Sperm chemotaxis promotes individual fertilization success in sea urchins

Reproductive success fundamentally shapes an organism's ecology and evolution, and gamete traits mediate fertilization, which is a critical juncture in reproduction. Individual male fertilization success is dependent on the ability of sperm from one male to outcompete the sperm of other males when searching for a conspecific egg. Sperm chemotaxis, the ability of sperm to navigate towards eggs using chemical signals, has been studied for over a century, but such studies have long assumed that this phenomenon improves individual male fitness without explicit evidence to support this claim. Here, we assess fertilization changes upon use of a chemoattractant-digesting peptidase and use a microfluidic device coupled with a fertilization assay to determine the effect of sperm chemotaxis on individual male fertilization success in the sea urchin Lytechinus pictus. We show that removing chemoattractant from the gametic environment decreases fertilization success. We further find that individual male differences in chemotaxis to a well-defined gradient of attractant correlate with individual male differences in fertilization success. These results demonstrate that sperm chemotaxis is an important contributor to individual reproductive success.

The more pieces, the better the puzzle: sperm concentration increases gametic compatibility

The genetic benefits individuals receive from mate choice have been the focus of numerous studies, with several showing support for both intrinsic genetic benefits and compatibility effects on fertilization success and offspring viability. However, the robustness of these effects have rarely been tested across an ecologically relevant environmental gradient. In particular, sperm environment is a crucial factor determining fertilization success in many species, especially those with external fertilization. Here, we test the importance of sperm environment in mediating compatibility‐based selection on fertilization using a factorial breeding design. We detected a significant intrinsic male effect on fertilization success at only one of four sperm concentrations. Compatibility effects were significant at the two highest sperm concentrations and, interestingly, the magnitude of the compatibility effect consistently increased with sperm concentration. This suggests that females are able to modify the probability of sperm–egg fusion as the amount of sperm available increases.

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.

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.

Sneaker "jack" males outcompete dominant "hooknose" males under sperm competition in Chinook salmon (Oncorhynchus tshawytscha)

In a variety of taxa, males deploy alternative reproductive tactics to secure fertilizations. In many species, small "sneaker" males attempt to steal fertilizations while avoiding encounters with larger, more aggressive, dominant males. Sneaker males usually face a number of disadvantages, including reduced access to females and the higher likelihood that upon ejaculation, their sperm face competition from other males. Nevertheless, sneaker males represent an evolutionarily stable strategy under a wide range of conditions. Game theory suggests that sneaker males compensate for these disadvantages by investing disproportionately in spermatogenesis, by producing more sperm per unit body mass (the "fair raffle") and/or by producing higher quality sperm (the "loaded raffle"). Here, we test these models by competing sperm from sneaker "jack" males against sperm from dominant "hooknose" males in Chinook salmon. Using two complementary approaches, we reject the fair raffle in favor of the loaded raffle and estimate that jack males were ∼1.35 times as likely as hooknose males to fertilize eggs under controlled competitive conditions. Interestingly, the direction and magnitude of this skew in paternity shifted according to individual female egg donors, suggesting cryptic female choice could moderate the outcomes of sperm competition in this externally fertilizing species.

Effects of ocean warming and acidification on fertilization in the Antarctic echinoid Sterechinus neumayeri across a range of sperm concentrations

The gametes of marine invertebrates are being spawned into an ocean that is simultaneously warming and decreasing in pH. Predicting the potential for interactive effects of these stressors on fertilization is difficult, especially for stenothermal polar invertebrates adapted to fertilization in cold, viscous water and, when decreased sperm availability may be an additional stressor. The impact of increased temperature (2-4 °C above ambient) and decreased pH (0.2-0.4 pH units below ambient) on fertilization in the Antarctic echinoid Sterechinus neumayeri across a range of sperm concentrations was investigated in cross-factorial experiments in context with near future ocean change projections. The high temperature treatment (+4 °C) was also used to assess thermal tolerance. Gametes from multiple males and females in replicate experiments were used to reflect the multiple spawner scenario in nature. For fertilization at low sperm density we tested three hypotheses, 1) increased temperature enhances fertilization success, 2) low pH reduces fertilization and, 3) due to the cold stenothermal physiology of S. neumayeri, temperature would be the more significant stressor. Temperature and sperm levels had a significant effect on fertilization, but decreased pH did not affect fertilization. Warming enhanced fertilization at the lowest sperm concentration tested likely through stimulation of sperm motility and reduced water viscosity. Our results indicate that fertilization in S. neumayeri, even at low sperm levels potentially found in nature, is resilient to near-future ocean warming and acidification.

Asymmetric conspecific sperm precedence in relation to spawning times in the Montastraea annularis species complex (Cnidaria: Scleractinia)

In broadcast spawners, prezygotic reproductive isolation depends on differences in the spatial and temporal patterns of gamete release and gametic incompatibility. Typically, gametic incompatibility is measured in no-choice crosses, but conspecific sperm precedence (CSP) can prevent hybridization in gametes that are compatible in the absence of sperm competition. Broadcast spawning corals in the Montastraea annularis species complex spawn annually on the same few evenings. Montastraea franksi spawns an average of 110 min before M. annularis, with a minimum gap of approximately 40 min. Gametes are compatible in no-choice heterospecific assays, but it is unknown whether eggs exhibit choice when in competition. Hybridization depends on either M. franksi eggs remaining unfertilized and in proximity to M. annularis when the latter species spawns or M. franksi sperm remaining in sufficient viable concentrations when M. annularis spawns. We found that the eggs of the early spawning M. franksi demonstrate strong CSP, whereas CSP appears to be lacking for M. annularis eggs. This study provides evidence of diverging gamete affinities between these recently separated species and suggests for the first time that selection may favour CSP in earlier spawning species when conspecific sperm is diluted and aged and is otherwise at a numeric and viability disadvantage with heterospecific sperm.