Sperm Competition in Fishes: The Evolution of Testis Size and Ejaculate Characteristics

Sperm competition in a fish with external fertilization: the contribution of sperm number, speed and length

The role of sperm number and quality in male competitiveness was investigated using in vitro fertilization experiments with bluegill (Lepomis macrochirus). Bluegill males use one of three mating tactics: 'sneakers', which streak spawn; 'satellites', which mimic females; and 'parentals', which are territorial. The in vitro experiments mimicked natural spawning by incorporating these males' mean proximity to eggs and timing of sperm release. Using a maximum-likelihood algorithm, raffle equations were fit to paternity data, which revealed a strong effect of sperm number on male competitiveness. There was no difference in sperm flagellum length, curvilinear swim speed or path linearity among the three male mating types, and these traits did not explain any additional variation in male competitiveness. It was estimated that, given closer proximity to eggs, satellites need release only 0.34 times as many sperm as parentals to obtain equal paternity. Despite being farther from the eggs and releasing sperm about half a second after parentals, sneakers need only release 0.58 times as many sperm as parentals to obtain equal paternity. Thus, the increased competitiveness of sneakers' sperm must come from a component of sperm quality other than speed or length.

Evolutionary trade-off between weapons and testes

It has long been recognized that male mating competition is responsible for the evolution of weaponry for mate acquisition. However, when females mate with more than one male, competition between males can continue after mating in the form of sperm competition. Theory predicts that males should increase their investment in sperm production as sperm competition is increased, but it assumes that males face a trade-off between sperm production and other life-history traits such as mate acquisition. Here, we use a genus of horned beetle, Onthophagus, to examine the trade-off between investment in testes required for fertilizations and investment in weapons used to obtain matings. In a within-species study, we prevented males from developing horns and found that these males grew larger and invested relatively more in testes growth than did males allowed to grow horns. Among species, there was no general relationship between the relative sizes of horns and testes. However, the allometric slope of horn size on body size was negatively associated with the allometric slope of testes size on body size. We suggest that this reflects meaningful evolutionary changes in the developmental mechanisms regulating trait growth, specifically in the degree of nutrition-dependent phenotypic plasticity versus canalization of traits. Finally, we show how this resource allocation trade-off has influenced the evolutionary diversification of weapons, revealing a rich interplay between developmental trade-offs and both pre- and postmating mechanisms of sexual competition.

Sperm competition enhances functional capacity of mammalian spermatozoa

When females mate promiscuously, sperm from rival males compete within the female reproductive tract to fertilize ova. Sperm competition is a powerful selective force that has shaped sexual behavior, sperm production, and sperm morphology. However, nothing is known about the influence of sperm competition on fertilization-related processes, because it has been assumed that sperm competition only involves a race to reach the site of fertilization. We compared four closely related rodent species with different levels of sperm competition to examine whether there are differences in the proportion of spermatozoa that become ready to interact with the ovum ("capacitated") and in the proportion of spermatozoa that experience the acrosome reaction in response to a natural stimulant. Our results show that differences between species in levels of sperm competition were associated with the proportion of spermatozoa that undergo capacitation and with the proportion of spermatozoa that respond to progesterone, an ovum-associated signal. Sperm competition thus favors a larger population of spermatozoa that are competent to fertilize, and spermatozoa that are more sensitive to the signals emitted by the ovum and that may penetrate the ova vestments more rapidly. These results suggest that, contrary to previous assumptions, competition between spermatozoa from rival males continues at the site of fertilization. These findings may have further evolutionary implications because the enhanced competitiveness of spermatozoa during fertilization may increase the risk of polyspermy to females. This could lead to antagonistic coevolution between the sexes and may contribute to the explanation of the rapid divergence observed in fertilization-related traits.

Theoretical influence of female mating status and remating propensity on male sperm allocation patterns

Theoretical models predict that males should allocate more sperm in matings where the immediate risk of sperm competition is high. It has therefore often been argued that males should invest less sperm in matings with virgin females compared with matings with already mated females. However, with relatively polyandrous females, high sperm competition risk will covary with high sperm competition intensity leading to more unpredictable conditions, as high competition intensity should favour smaller ejaculates. With the use of a genetic algorithm, we found that males should allocate more sperm in matings with virgin females when female mating frequency is relatively high, whereas low remating rates will select for higher effort in matings with nonvirgin females. At higher remating rates, first male sperm precedence favours larger ejaculates in matings with virgin females and second male precedence favours the reverse. These results shed some light on several findings that have been difficult to explain adaptively by the hitherto developed theory on sperm allocation.

Genetic mating patterns studied in pools with manipulated nest site availability in two populations of Pomatoschistus minutus

Pomatoschistus minutus show paternal care in a resource defence mating system. We investigated the effect of nest-site availability on parasitic spawning. Each experimental pool contained four potentially nest-building males, four females and nests-sites in shortage (2) or excess (6). Both treatments were conducted in two populations; one with natural nest-sites in excess, one with a nest-site shortage. Microsatellite-DNA revealed that all nest-holders had fertilized most of the eggs they tended. Yet, 35% of the nests contained eggs fertilized by another male and 14.4% of the males had performed parasitic spawning. There was no site or treatment effect. Several females spawned in two nests, which coincided with parasitic spawnings, suggesting a cost to the nest-holder in terms of lost mating success. Nest-holders with and without eggs and non-nesting males all spawned parasitically, generating a significantly lower opportunity for sexual selection compared to if there had been no parasitic spawning.

Slow fertilization of stickleback eggs: the result of sexual conflict?

Background

The fertilization success in sperm competition in externally fertilizing fish depends on number and quality of sperm. The time delay between sequential ejaculations may further influence the outcome of sperm competition. Such a time interval can load the raffle over fertilization if fertilization takes place very fast. Short fertilization times are generally assumed for externally fertilizing fish such as the three-spined stickleback (Gasterosteus aculeatus). In this pair-spawning fish, territorial males often try to steal fertilizations in nests of neighbouring males. This sneaking behaviour causes sperm competition. Sneakers will only get a share of paternity when eggs are not fertilized immediately after sperm release. Contrary to males, females may be interested in multiple paternity of their clutch of eggs. There thus may be a sexual conflict over the speed of fertilization.

Results

In this study we used two different in vitro fertilization experiments to assess how fast eggs are fertilized in sticklebacks. We show that complete fertilization takes more than 5 min which is atypically long for externally fertilizing fishes.

Conclusion

This result suggests that the time difference does not imply high costs to the second stickleback male to ejaculate. Slow fertilization (and concomitant prolonged longevity of sperm) may be the result of sexual conflict in which females aimed at complete fertilization and/or multiple paternity.

Have male and female genitalia coevolved? A phylogenetic analysis of genitalic morphology and sexual size dimorphism in web-building spiders (Araneae: Araneoidea)

Sexual size dimorphism (SSD) can strongly influence the evolution of reproductive strategies and life history. If SSD is extreme, and other characters (e.g., genitalic size) also increase with size, then functional conflicts may arise between the sexes. Spiders offer an excellent opportunity to investigate this issue because of their wide range of SSD. By using modern phylogenetic methods with 16 species of orb-weaving spiders, we provide strong evidence for the "positive genitalic divergence" model, implying that sexual genitalic dimorphism (SGD) increases as SSD increases. This pattern is supported by an evolutionary mismatch between the absolute sizes of male and female genitalia across species. Indeed, our findings reveal a dramatic reversal from male genitalia that are up to 87x larger than female genitalia in size-monomorphic species to female genitalia that are up to 2.8x larger in extremely size-dimorphic species. We infer that divergence in SGD could limit SSD both in spiders, and potentially in other taxa as well. Further, male and female body size, as well as male and female genitalia size, are decoupled evolutionarily. Finally, we show a negative scaling (hypoallometry) of male and female genitalic morphology within sexes. Evolutionary forces specific to each sex, such as larger female size (increased fecundity) or smaller male size (enhanced mate-searching ability), may be balanced by stabilizing selection on relative genitalic size.

Heritability of sperm length in the bumblebee Bombus terrestris

Sperm length is highly variable, both between and within species, but the evolutionary significance of this variation is poorly understood. Sexual selection on sperm length requires a significant additive genetic variance, but few studies have actually measured this. Here we present the first estimates of narrow sense heritability of sperm length in a social insect, the bumblebee Bombus terrestris. In spite of a balanced and straightforward rearing design of colonies, and the possibility to replicate measurements of sperm within single males nested within colonies, the analysis proved to be complex. Several appropriate statistical models were derived, each depending on different assumptions. The heritability estimates obtained ranged from h (2) = 0.197 +/- 0.091 to h (2) = 0.429 +/- 0.154. All our estimates were substantially lower than previous estimates of sperm length heritability in non-social insects and vertebrates.

Coevolution of male and female reproductive traits in a simultaneously hermaphroditic land snail

Inter- and intraspecific studies in gonochoristic animals reveal a covariation between sperm characteristics and the size of the female reproductive tract, indicating a rapid evolutionary divergence, which is consistent with the theory of post-copulatory sexual selection. Simultaneous hermaphrodites differ from species with separate sexes (gonochorists) in that they possess both functional male and female reproductive organs at the same time. We investigated whether in hermaphroditic animals intraspecific variation in reproductive traits results from divergent coevolution, by quantifying the variation in male and female traits among six natural populations of the snail Arianta arbustorum and examining the covariation in interacting traits. There was a significant among-population variation in spermatophore volume, number of sperm transferred and sperm length, as well as in volume of the sperm storage organ (spermatheca) and number of tubules, but not in spermatheca length. We found a positive association between sperm number transferred and spermatheca volume. This result suggests that the same post-copulatory mechanisms as in gonochorists drive the correlated evolution of reproductive characters in hermaphrodites.

Fertilization dynamics of sperm from different male mating tactics in bluegill (Lepomis macrochirus)

Sperm competition results in the evolution of ejaculate characteristics such as high sperm density, high motility, and fast sperm swimming speed. A fundamental assumption of sperm competition theory is that ejaculates with high motility and fast-swimming sperm have an advantage with respect to fertilization success. We tested this assumption by studying the fertilization dynamics of alternative mating tactics (cuckolders and parentals) of male bluegill (Lepomis macrochirus Rafinesque, 1819). Sneakers (cuckolders) have faster swimming sperm and a higher proportion of motile sperm immediately following sperm activation than do parentals; however, these variables decline more quickly over time in sneaker sperm than in the sperm of parental males. We used a controlled fertilization experiment to test the prediction that parental males will have higher fertilization success than sneakers late in the sperm activation cycle because of the reduced rate of decline in ejaculate quality over time. We found that as the time from sperm activation increases parental sperm fertilizes more eggs than the sperm of sneakers. Our results support the idea that fertilization success is higher when ejaculates contain a higher proportion of either motile sperm or faster swimming sperm, all else being equal. In addition, after controlling for time from sperm activation, we found a significant bias in fertilization success toward parental males, suggesting that cryptic female choice might play a role in fertilization dynamics.

Spermatozoal competition in common carp (Cyprinus carpio): what is the primary determinant of competition success?

The percentage of sperm motility (92–100%), spermatozoan velocity (112–163 μm·s−1) and control hatching rates (83–96%) were evaluated for each of six gold and five green male common carp (Cyprinus carpio). In all 30 possible paired combinations of sperm-competition tests, hatching rates of 90–97% were achieved. The mean percentage of offspring sired was strongly influenced by the male used (P < 0.001, R 2 = 0.91). The best male sired an average of 88% of the offspring in its competition tests, and the worst male sired only 5%. Spermatozoan-quality parameters could explain only part of the variation in male competitive ability. The male effects alone explained 91.4% of the observed variance, consisting of 17.1% explained by spermatozoan motility and 32.5% by control hatching rates in single fertilizations. Undetermined male effects explained 41.8%. The velocity of spermatozoa had no effect on the outcome of sperm competition. Neither was there any link between spermatozoan velocity and hatching rate in a control hatching test, whereas there was an effect of motility on hatching rate in this same test.

Alternative male mating tactics of the substrate brooding cichlid Telmatochromis temporalis in Lake Tanganyika

Telmatochromis temporalis is a bi-parental substrate brooding cichlid endemic to Lake Tanganyika. Paired males were always larger than their mates and had territories around nests against conspecific males. However, males smaller than the paired females were found in 18% of the nests. Here we report a reproductive tactic of these small males. The small males had as heavy gonads as paired males, and the gonad somatic index (GSI) of the small males was much higher than that of the latter. The examinations of the paternity and maternity using microsatellite-DNA as a genetic marker revealed that the small males were not genetically related to the pair members, and sired some young in 3 of 5 nests. These small males did not guard the broods, suggesting that they are likely to perform reproductive parasitism as sneakers. Paired males could not enter their spawning nests due to their large size, which made it difficult to chase out sneakers once they entered the nest. Some males as small as the sneakers were found outside the territories of paired males, and their gonads were quite small. Circumstantial evidence suggests that small males have two alternative investment patterns: investing in gonad to be sneakers, and investing in growth to probably be territorial males.

Mating-induced reduction in accessory reproductive organ size in the stalk-eyed fly Cyrtodiopsis dalmanni

Background

Internal reproductive organ size is an important determinant of male reproductive success. While the response of testis length to variation in the intensity of sperm competition is well documented across many taxa, few studies address the importance of testis size in determining other components of male reproductive success (such as mating frequency) or the significance of size variation in accessory reproductive organs. Accessory gland length, but not testis length, is both phenotypically and genetically correlated with male mating frequency in the stalk-eyed fly Cyrtodiopsis dalmanni. Here we directly manipulate male mating status to investigate the effect of copulation on the size of both the testes and the accessory glands of C. dalmanni.

Results

Accessory gland length was positively correlated with male mating frequency. Copulation induced a significant decrease in accessory gland size. The size of the accessory glands then recovered slowly over the next 8–48 hours. Neither testis length nor testis area was altered by copulation.

Conclusion

These results reveal that the time course of accessory gland recovery corresponds to field observations of mating behaviour and suggest that accessory gland size may limit male mating frequency in C. dalmanni.

Giant spermatozoon coiled in small egg: fertilization mechanisms and their implications for evolutionary studies on Ostracoda (Crustacea)

Ostracods of the superfamily Cypridoidea have giant spermatozoa. However, little data exist on the sperm-egg interaction in this group: only two publications have so far given the most ambiguous indication that the entire sperm enters the egg on fertilization. These assumptions have not yet been tested with modern techniques, nor has their putative value for developmental and evolutionary investigations been realized. The present paper gives the first, clear, light- and scanning electron microscopical evidence of the entire giant ostracod spermatozoon being incorporated into the egg. Coiling of the sperm underneath the egg shell is shown in the early zygotes of the species Mytilocypris praenuncia and Pseudocandona marchica. Additionally, data on the morphology of female and male reproductive tracts are given for M. praenuncia. Hypotheses on the evolution of giant filiform sperm in the Animal Kingdom are reviewed, and their applicability to ostracods is discussed. The demonstrated ingression of the entire sperm implies the entry of the two giant paternal mitochondrial derivates into the zygote in Cypridoidea and potentially casts doubt upon the dogma of strict maternal inheritance of mitochondrial DNA. Evidence of paternal inheritance of mtDNA in several organisms has recently given rise to a controversial debate on this issue; the possible significance of this phenomenon for molecular studies on ostracod phylogeny and evolution is discussed.

Antlers honestly advertise sperm production and quality

Evolutionary theory proposes that exaggerated male traits have evolved via sexual selection, either through female mate choice or male-male competition. While female preferences for ornamented males have been amply demonstrated in other taxa, among mammals sexual characters are commonly regarded as weapons whose main function is to enhance male competitiveness in agonistic encounters. One particularly controversial hypothesis to explain the function of male sexual characters proposes that they advertise male fertility. We test this hypothesis in red deer (Cervus elaphus), a species where sexual characters (antlers) reach an extreme degree of elaboration. We find that a global measure of relative antler size and complexity is associated with relative testes size and sperm velocity. Our results exclude the possibility that condition dependence, age or time of culling, drive these associations. Red deer antlers could signal male fertility to females, the ability to avoid sperm depletion throughout the reproductive season and/or the competitive ability of ejaculates. By contrast, male antlers could also signal to other males not only their competitive ability at the behavioural level (fighting ability) but also at the physiological level (sperm competition).

Co-evolution of male and female reproductive characters across the Scathophagidae (Diptera)

Sperm morphometry is extremely variable across species, but a general adaptive explanation for this diversity is lacking. As sperm must function within the female, variation in sperm form may be associated with variation in female reproductive tract morphology. We investigated this and other potential evolutionary associations between male and female reproductive characters across the Scathophagidae. Sperm length was positively associated with the length of the spermathecal (sperm store) ducts, indicating correlated evolution between the two. No association was found between sperm length and spermathecal size. However, the size of the spermathecae was positively associated with testis size indicating co-evolution between male investment in sperm production and female sperm storage capacity. Furthermore, species with a higher degree of polyandry (larger testes) had longer spermathecal ducts. However, no associations between sperm length or length variation and testis size were found which suggests greater sperm competition sensu stricto does not select for longer sperm.

Sperm competition and the evolution of testes size in birds

Comparative analyses suggest that a variety of ecological and behavioural factors contribute to the tremendous variability in extrapair mating among birds. In an analysis of 1010 species of birds, we examined several ecological and behavioural factors in relation to testes size; an index of sperm competition and the extent of extrapair mating. In univariate and multivariate analyses, testes size was significantly larger in species that breed colonially than in species that breed solitarily, suggesting that higher breeding density is associated with greater sperm competition. After controlling for phylogenetic effects and other ecological variables, testes size was also larger in taxa that did not participate in feeding their offspring. In analyses of both the raw species data and phylogenetically independent contrasts, monogamous taxa had smaller testes than taxa with multiple social mates, and testes size tended to increase with clutch size, which suggests that sperm depletion may play a role in the evolution of testes size. Our results suggest that traditional ecological and behavioural variables, such as social mating system, breeding density and male parental care can account for a significant portion of the variation in sperm competition in birds.

Potential fitness benefits from mate selection in the Atlantic cod (Gadus morhua)

Little evidence of benefits from female mate choice has been found when males provide no parental care or resources. Yet, good genes models of sexual selection suggest that elaborated male sexual characters are reliable signals of mate quality and that the offspring of males with elaborate sexual ornaments will perform better than those of males with less elaborate ornaments. We used cod (Gadus morhua L.), an externally fertilizing species where males provide nothing but sperm, to examine the potential of optimal mate selection with respect to offspring survival. By applying in vitro fertilizations, we crossed eight females with nine males in all possible combinations and reared each of the 72 sib groups. We found that offspring survival was dependent on which female was mated with which male and that optimal mate selection has the potential to increase mean offspring survival from 31.9 to 55.6% (a 74% increase). However, the size of the male sexual ornaments and sperm quality (i.e. sperm velocity and sperm density) could not predict offspring survival. Thus, even if there may be large fitness benefits of mate selection, we might not yet have identified the male characteristics generating high offspring survival.

Pitfalls in experiments testing predictions from sperm competition theory

As females of many species mate with more than one male, ejaculates often face competition from the sperm of other males. In recent years, numerous papers have been published on theoretical predictions of evolutionary, behavioural and physiological responses to variation in the strength of sperm competition (SC). These theoretical predictions have also been extensively tested. However, although predictions from SC theory are relatively straightforward, extra caution has to be paid in the design of experiments testing them. One difficulty is for example to disentangle immediate and mean SC risk and intensity. Without carefully designed experiments, it is also very easy to simultaneously increase SC risk and the probability of intense SC--a situation for which we currently have no clear predictions, as the theoretical models to date only assume variation in either SC risk or intensity. In this paper, we discuss these and some other pitfalls related to manipulations of SC risk and intensity and suggest how to avoid them.

Sperm competitive ability in Drosophila melanogaster associated with variation in male reproductive proteins

Multiple mating by females establishes the opportunity for postcopulatory sexual selection favoring males whose sperm is preferentially employed in fertilizations. Here we use natural variation in a wild population of Drosophila melanogaster to investigate the genetic basis of sperm competitive ability. Approximately 101 chromosome 2 substitution lines were scored for components of sperm competitive ability (P1', P2', fecundity, remating rate, and refractoriness), genotyped at 70 polymorphic markers in 10 male reproductive genes, and measured for transcript abundance of those genes. Permutation tests were applied to quantify the statistical significance of associations between genotype and phenotype. Nine significant associations were identified between polymorphisms in the male reproductive genes and sperm competitive ability and 13 were identified between genotype and transcript abundance, but no significant associations were found between transcript abundance and sperm competitive ability. Pleiotropy was evident in two genes: a polymorphism in Acp33A associated with both P1' and P2' and a polymorphism in CG17331 associated with both elevated P2' and reduced refractoriness. The latter case is consistent with antagonistic pleiotropy and may serve as a mechanism maintaining genetic variation.

How the sperm lost its tail: the evolution of aflagellate sperm

The typical sperm is comprised of a head, midpiece and flagellum. Around this theme there is an enormous diversity of form--giant sperm, multi-flagellate sperm and also sperm that lack flagella entirely. Explaining this diversity in sperm morphology is a challenging question that evolutionary biologists have only recently engaged in. Nonetheless, one of the selective forces identified as being an important factor in the evolution of sperm form is sperm competition, which occurs when the sperm of two or more males compete to fertilize a female's ova. In species with a truly monandrous mating system, the absence of sperm competition means that the selection pressure on males to produce motile sperm may be relaxed. Potentially aflagellate sperm are less costly to produce, both in terms of energy and time. Thus, selection may therefore favour the loss of the sperm flagellum and any other motile mechanisms in monandrous taxa. A review of the literature revealed that 36 taxonomic groups, from red algae to fish, were found independently to have evolved aflagellate sperm. I review what is known about the mating systems of each of these taxa and their nearest sister taxa. A sister-group analysis using this information provided weak evidence suggesting that the evolution of aflagellate sperm could be linked to the removal of selective pressures generated by sperm competition.

Sperm competition and male ejaculate investment in Nauphoeta cinerea: effects of social environment during development

Selective pressure arising from sperm competition has been predicted to influence evolutionary and behavioural adjustment of ejaculate investment, but also may influence developmental adjustment of ejaculate investment. Immature males able to target resources strategically based on the competitive environment they will experience when they become sexually mature should be at a selective advantage. In our study we investigated how the presence of potential competitors or mates affects ejaculate and testes investment during development in the cockroach Nauphoeta cinerea, a species where males control female remating via their ejaculate size (large spermatophores prevent females from remating and therefore function to avoid sperm competition for males) and females store sperm. Our aim was to determine whether the social environment influences developmental adjustment of ejaculate investment and the relative importance of ejaculate components with different functions; avoidance of or engagement in sperm competition. We conclude that the social environment can influence developmental and behavioural flexibility in specific ejaculate components that may function to avoid or engage in sperm competition.

Social group size, potential sperm competition and reproductive investment in a hermaphroditic leech, Helobdella papillornata (Euhirudinea: Glossiphoniidae)

Social group size may affect the potential for sperm competition, and this in turn may favour ontogenetic adjustments in testicular mass according to the likely requirements for sperm and spermatophore production. In a number of comparative analyses of testis mass among vertebrate species that differ in mating system or social organization, increasing potential for sperm competition is associated with larger testis size. Intraspecific phenotypic plasticity should be able to produce the same pattern if social group size is heterogenous and reflects differing degrees of average sperm competition, but this intraspecific effect is less well studied. We tested the effect of social groups on both male and female investment in the simultaneously hermaphroditic leech, Helobdella papillornata. Leeches were placed in groups of one, two, four or eight. Sexual investment at the onset of reproductive maturity was quantified as the total testisac volume for male function and total egg volume for female function. We found that testisac volume (statistically adjusted for body size) showed a significant increase with increasing group size. Total egg volume (also adjusted for body size) was unaffected by group size. Our findings indicate adaptive developmental plasticity in male gonad investment in response to the potential for sperm competition.

Concepts in sperm heterogeneity, sperm selection and sperm competition as biological foundations for laboratory tests of semen quality

Stringent selection mechanisms, in both internal and external fertilisation systems, reject all but a significant minority of the spermatozoa released at ejaculation. Sperm competition theory provides circumstantial evidence that the selection process involves mechanisms by which the quality of the fertilising spermatozoon is controlled, thereby ensuring that females and their offspring receive high quality genetic material. In this review we examine some of these selection processes to see whether they could be exploited for the improvement of laboratory tests of sperm quality. Such tests are not only required for clinical and agricultural purposes, but are increasingly needed in fields such as reproductive and environmental toxicology where the species requirement is much broader. Despite many years of research, sperm quality assessment methods continue to provide imprecise data about fertility; here we suggest that this may be a consequence of using tests that focus on the spermatozoa that would normally be unable to fertilise under natural conditions.

To achieve fertilisation a spermatozoon must be capable of responding appropriately to external signalling stimuli; those involving protein kinase-regulated flagellar function seem especially influential in governing effects ranging from non-Mendelian inheritance in mammals to sperm chemotaxis in sea urchins. Examination of the elicited responses reveals considerable heterogeneity in all species. Here we propose that this level of heterogeneity is meaningful both in terms of understanding how spermatozoa from some individuals possess fertility advantages over spermatozoa from their rivals in sperm competition, and in that the heterogeneity should be exploitable in the development of more accurate laboratory tests.

From sneaker to parental male: Change of reproductive traits in the black goby,Gobius niger (Teleostei, Gobiidae)

This study focuses on the consequences of the switch of tactic from parasitic to parental male in the black goby, Gobius niger (Teleostei: Gobiidae), a species showing two alternative male mating tactics. Older and larger males defend nests, court, and perform parental care on eggs, while younger and smaller ones behave as parasites, sneaking into nests while spawning occurs. Males adopting different tactics are known to present differences in primary and secondary sex traits. The social context of sneaker males was manipulated to induce a tactic switch. Sneakers were kept under two different experimental treatments with or without a female, and under exclusion of male-male competition. Males changed tactics, courting females, spawning, and performing parental care. All males showed substantial changes in primary sexual traits, such as a reduction in gonadal development and an increase in the investment in accessory structures. The experimental groups differed in the functionality of gonads and accessory organs and in the development of the secondary sex traits. These results demonstrate that the moment of switching is not genetically fixed in the black goby. Sneaker males are able to quickly reallocate energy in primary and secondary sex traits, in accordance with the adopted tactic. Several aspects of this flexible reproductive pattern resemble the socially controlled sex change found in sequential hermaphrodites.