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.

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.

The role of jelly coats in sperm-egg encounters, fertilization success, and selection on egg size in broadcast spawners.

Sperm limitation may be an important selective force influencing gamete traits such as egg size. The relatively inexpensive extracellular structures surrounding many marine invertebrate eggs might serve to enhance collision rates without the added cost of increasing the egg cell. However, despite decades of research, the effects of extracellular structures on fertilization have not been conclusively documented. Here, using the sea urchin Lytechinus variegatus, we remove jelly coats from eggs, and we quantify sperm collisions to eggs with jelly coats, eggs without jelly coats, and inert plastic beads. We also quantify fertilization success in both egg treatment groups. We find that sperm-egg collision rates increase as a function of sperm concentration and target size and that sperm are not chemotactically attracted to eggs nor to jelly coats in this species. In fertilization assays, the presence of the jelly coat is correlated with a significant but smaller-than-expected improvement in fertilization success. A pair of optimality models predict that, despite the large difference in the energetic value of egg contents and jelly material, the presence of the jelly coat does not diminish selection for larger egg cell size when sperm are limiting.

Fertilization selection on egg and jelly-coat size in the sand dollar Dendraster excentricus.

Organisms with external fertilization are often sperm limited, and in echinoids, larger eggs have a higher probability of fertilization than smaller eggs. This difference is thought to be a result of the more frequent sperm-egg collisions experienced by larger targets. Here we report how two components of egg target size, the egg cell and jelly coat, contributed to fertilization success in a selection experiment. We used a cross-sectional analysis of correlated characters to estimate the selection gradients on egg and jelly-coat size in five replicate male pairs of the sand dollar Dendraster excentricus. Results indicated that eggs with larger cells and jelly coats were preferentially fertilized under sperm limitation in the laboratory. The selection gradients were an average of 922% steeper for egg than for jelly-coat size. The standardized selection gradients for egg and jelly-coat size were similar. Our results suggest that fertilization selection can act on both egg-cell and jelly-coat size but that an increase in egg-cell volume is much more likely to increase fertilization success than an equal change in jelly-coat volume. The strengths of the selection gradients were inversely related to the correlation of egg traits across replicate egg clutches. This result suggests the importance of replication in studies of selection of correlated characters.

Sperm velocity and longevity trade off each other and influence fertilization in the sea urchin Lytechinus variegatus.

The theoretical prediction that fast sperm should be more effective at fertilizing eggs has never been documented empirically. Interspecific comparisons suggest an inverse relationship between sperm velocity and sperm longevity but this trade-off has never been demonstrated within a species. Here I investigate how sperm velocity and sperm longevity influence the patterns of fertilization in the sea urchin Lytechinus variegatus. In the laboratory I examined 11 male female pairs of sea urchins for variation in sperm velocity and sperm longevity, and determined the correlations of these traits with the percentage of eggs fertilized with serially diluted sperm. Males with faster sperm had higher rates of fertilization than males with slower sperm. Within individual males, as sperm aged they slowed down and showed a reduced percentage activity and lower rates of fertilization. Across males, the average velocity of freshly spawned sperm was inversely related to sperm longevity. These results establish the possibility that sperm traits are adapted for varying conditions along a continuum from sperm limitation to sperm competition.

Sperm limitation in the sea.

Because sperm outnumber eggs, it is often assumed that variation in female reproductive success has little to do with male or sperm availability. Similarly for males, access to viable eggs and sperm competition are thought to drive variation in male fertilization success. These assumptions result from empirical studies on organisms with internal fertilization. However, recent evidence from free-spawning organisms suggests that sperm can often be limiting. This finding may alter our perspective on mating-system evolution, especially in externally fertilizing organisms.

The analysis of paternity and maternity in the marine hydrozoan Hydractinia symbiolongicarpus using randomly amplified polymorphic DNA (RAPD) markers.

For organisms in which direct observation of mating and subsequent dispersal of offspring and relatives is impossible, patterns of reproductive success and genealogical relationship can only be established using genetic markers. The ideal genetic assay would (1) employ highly polymorphic genetic markers for distinguishing among individuals; (2) use little tissue for analysing early life-history stages; and (3) require minimal investment in time and money for population level studies. From this perspective, DNA polymorphisms revealed by PCR amplification using random ten-base primers [Randomly Amplified Polymorphic DNA (PCR-RAPD) or Arbitrarily Primed DNA (AP-PCR)] have great potential. However, the evidence that RAPD/AP markers are both heritable and can be repeatably amplified remains controversial. This study characterizes patterns of inheritance and polymorphism of RAPD markers in the free-spawning, colonial marine hydrozoan Hydractinia symbiolongicarpus. In all cases, the amplification products were identical among extractions from the same clone. Of 56 primers screened, 13 had sufficient polymorphism and scoreability for an analysis of parentage and higher-order genetic relationships in three matings. These primers generated 156 unique amplification products (putative loci), of which 133 were polymorphic. All but four of these loci were inherited as dominant mendelian markers. Our study suggests that the presence of a marker represents a single allele at a locus; however, what appear to be single null alleles may actually comprise several segregating alleles. When the identity of neither parent was known a priori, inclusion (unique markers present in offspring and only one of the potential parents) proved to be more efficient than exclusion for assigning offspring to parents. The most powerful approach, however, was cluster analysis of all presence/absence information for the marker bands. Clustering avoided the pitfalls caused by the appearance of occasional nonparental bands, and constructed a hierarchical framework that correctly reflected all genealogical relationships.

The importance of sperm limitation to the evolution of egg size in marine invertebrates.

Interspecific variation in egg size of marine invertebrates has been previously explained by a trade-off between gamete quality and quantity: the production of many small eggs with high mortality or fewer large eggs that develop quickly and experience reduced planktonic mortality. This theory assumes 100% fertilization of eggs and predicts that either strategy results in a similar number of settling offspring per unit of energy invested in reproduction. Empirical support for the theory has been equivocal. Here I offer an alternative hypothesis: larger eggs present a larger target for sperm and thus are fertilized at a higher rate. This theory suggests a trade-off between the production of many small eggs with a low probability of fertilization or fewer large eggs with a higher probability of fertilization. This hypothesis is tested with three congeneric sea urchins, Strongylocentrotus purpuratus, Strongylocentrotus franciscanus, and Strongylocentrotus droebachiensis, with a fivefold difference in egg volume. Species with larger eggs are fertilized at a higher rate and, if one assumes an equal allocation of resources, produce at least as many zygotes as species with smaller, more numerous eggs. This alternate hypothesis can explain continuous variation in egg size between species and provides a strong link between larval and adult life histories.

For adults only? Supply-side ecology and the history of larval biology.

When ecologists study organisms with multiphasic life cycles, they must often confront the problem of which phase to scrutinize. In principle, the dynamics and interactions of all stages could play a major role in the regulation of adult populations and species assemblages. In practice, however, the roles of larger and more sedentary phases - being easier to count and manipulate than motile propagules - have been emphasized. Nonetheless, several recent studies on the small, dispersing larval phase of marine invertebrate life cycles reach the conclusion that the spatial distribution and supply of propagules can control the distribution and abundance of populations of benthic adults. To some, the present emphasis on planktonic propagules amounts to a resurrection of ideas developed during a long and rich history of larval biology. To others, studies of demographic and ecological connections between larval and adult populations represent a substantial revision of ecological paradigms.

Influence of Body Size and Population Density on Fertilization Success and Reproductive Output in a Free-Spawning Invertebrate.

Gamete production and fertilization influence zygote production. While gamete production is correlated positively with body size, individual fertilization success may be a function of population density. Usually it is assumed that when high population density leads to reduced body size and gamete production, per capita zygote production is diminished. This field study of the sea urchin Diadema antillarum Philippi provides a test of this assumption. Three experiments were conducted to determine the effect of male spawning on fertilization success. In the first experiment, unfertilized eggs were placed in Nitex bags at three distances up and downstream from a spawning male. In the second experiment, unfertilized eggs were released and captured at three distances downstream from a sperm source. In both experiments, fertilization decreased with distance from the sperm source. The final experiment tested the influences of male size and population density on fertilization success; the effect of density was significant, but size was not. A simple model estimates the average number of zygotes produced by females of average size under different density regimes: at high population density, increased fertilization success can compensate for decreased gamete production. Small individuals at high population density may have similar per capita zygote production as large individuals at low population density. Thus, estimates of reproductive output based on body size or gamete production alone can be misleading.

Kinetics of Fertilization in the Sea Urchin Strongylocentrotus franciscanus: Interaction of Gamete Dilution, Age, and Contact Time.

Determining fertilization success of free spawning organisms in the field requires knowledge of how eggs and sperm interact under varying encounter frequencies and durations. In the laboratory, we investigated the relative influence of sperm concentration, egg concentration, sperm-egg contact time, and sperm age on fertilization in the sea urchin Strongylocentrotus franciscanus. Our results indicated that sperm concentration, sperm-egg contact time, sperm age, and individual variability were sequentially the most important factors influencing fertilization success. Egg concentration was not significant over the range tested. A theoretical model of fertilization (Vogel-Czihak-Chang-Wolf model) was used to estimate the two rate constants of fertilization kinetics: the rate constant of sperm-egg encounter and rate constant of fertilization. This model explained 91% of the variation in fertilization success, provided estimates of the rate constants involved in fertilization, and indicated the proportion (3%) of sperm-egg contacts that result in fertilization. Estimates of sperm swimming velocity and egg diameter were used to independently calculate the rate of sperm-egg encounter and confirm the predictions of the model. This model also predicts the non-significant effect of egg concentration on fertilization success found empirically.