Selection on sperm size in response to promiscuity and variation in female sperm storage organs

Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulations of high promiscuity (10 mates per female) showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2–3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Furthermore, when promiscuity is low, disruptive selection on male traits will occur under much more limited circumstances (i.e. only with higher among-female variation) than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.     

Examining the potential for resource-dependent female reproductive fluid-sperm interactive effects in a livebearing fish

Sexually selected traits can be costly to produce and maintain. The amount of resources available to an individual is therefore expected to influence investment in costly sexual traits. While resource-dependent expression of sexually selected traits has traditionally been examined in males, resource limitation can also influence how sexual selection operates in females. Female reproductive fluids are thought to be costly to produce and may play an important role in shaping the outcome of postcopulatory sexual selection by influencing sperm performance. However, we know surprisingly little about whether and how female reproductive fluids are influenced by resource limitation. Here, we examine if resource restriction influences female reproductive fluid-sperm interactive effects in the pygmy halfbeak (Dermogenys collettei), a small internally fertilizing freshwater fish where females store sperm. After experimentally altering female diets (high vs. restricted diets), we compared how female reproductive fluids influence two key metrics of sperm quality: sperm viability and velocity. While female reproductive fluids enhanced sperm viability and velocity, we found no evidence that female diet influenced the interactive effect between female reproductive fluids and sperm viability or velocity. Our findings build on the growing evidence that female reproductive fluids influence sperm performance and call for further attention to be devoted to understanding how resource quantity and quality influence how female reproductive fluids affect sperm performance.     

Colourful male guppies produce faster and more viable sperm

In guppies (Poecilia reticulata) precopulatory sexual selection (via female choice) and post-copulatory selection (via sperm competition) both favour males with relatively high levels of carotenoid (orange) pigmentation, suggesting that colourful males produce more competitive ejaculates. Here we test whether there is a positive association between male orange pigmentation and sperm quality. Our analysis of sperm quality focused on sperm swimming speeds (using CASA: computer-assisted sperm analysis to estimate three parameters of sperm velocity in vitro), sperm viability (proportion of live sperm per stripped ejaculate) and sperm lengths. We found that males with relatively large areas of orange pigmentation had significantly faster and more viable sperm than their less ornamented counterparts, suggesting a possible link between dietary carotenoid intake and sperm quality. By contrast, we found no relationship between sperm length (head length and total sperm length) and male phenotype. These findings, in conjunction with previous work showing that highly ornamented male guppies sire higher quality offspring, suggest that female preference for colourful males and sperm competition work in concert to favour intrinsically higher quality males.     

Secondary sexual characters and sperm traits in coho salmon Oncorhynchus kisutch

A study was undertaken to examine secondary sexual characters (spawning colouration and overall body size) in relation to sperm metrics in one alternative reproductive tactic of coho salmon Oncorhynchus kisutch : large hooknose males that spawn in dominance-based hierarchies. Males with less intense red spawning colouration had higher sperm velocities than males with darker red spawning colouration. There was no relationship between male body size and sperm metrics. These results suggest that within an alternative reproductive tactic, variation in sperm competition intensity may select for a trade-off between investment in sexual colouration and sperm quality.     

THE SPERM WHALE'S NOSE: SEXUAL SELECTION ON A GRAND SCALE?1

The world's largest nose belongs to the sperm whale, yet its functional significance remains equivocal. In order to help shed light on its function, the head of a postmortem neonate sperm whale was subjected to CT scanning. Geometric comparisons between homologous cephalic structures in sperm whales and dolphins (normalized for body size) show extreme hypertrophy and size sexual dimorphism in the sperm whale's lipid spermaceti organ. Anatomic geometry, energetics, and behavior suggest that this immense nasal apparatus is a bioacoustical machine. Sexual selection via an acoustic display is suggested as an explanation for the size and continuous (physiologically isolated) energy investment in the construction and maintenance of the male's spermaceti organ.     

THE QUANTITATIVE GENETICS AND COEVOLUTION OF MALE AND FEMALE REPRODUCTIVE TRAITS

Studies of experimental sexual selection have tested the effect of variation in the intensity of sexual selection on male investment in reproduction, particularly sperm. However, in several species, including Drosophila pseudoobscura, no sperm response to experimental evolution has occurred. Here, we take a quantitative genetics approach to examine whether genetic constraints explain the limited evolutionary response. We quantified direct and indirect genetic variation, and genetic correlations within and between the sexes, in experimental populations of D. pseudoobscura. We found that sperm number may be limited by low heritability and evolvability whereas sperm quality (length) has moderate V_A and CV_A but does not evolve. Likewise, the female reproductive tract, suggested to drive the evolution of sperm, did not respond to experimental sexual selection even though there was sufficient genetic variation. The lack of genetic correlations between the sexes supports the opportunity for sexual conflict over investment in sperm by males and their storage by females. Our results suggest no absolute constraint arising from a lack of direct or indirect genetic variation or patterns of genetic covariation. These patterns show why responses to experimental evolution are hard to predict, and why research on genetic variation underlying interacting reproductive traits is needed.     

EXPERIMENTAL MANIPULATION OF SEXUAL SELECTION PROMOTES GREATER MALE MATING CAPACITY BUT DOES NOT ALTER SPERM INVESTMENT

Sexual selection theory makes clear predictions regarding male spermatogenic investment. To test these predictions we used experimental sexual selection in Drosophila pseudoobscura , a sperm heteromorphic species in which males produce both fertile and sterile sperm, the latter of which may function in postmating competition. Specifically, we determined whether the number and size of both sperm types, as well as relative testis mass and accessory gland size, increased with increased sperm competition risk and whether any fitness benefits could accrue from such changes. We found no effect of sexual selection history on either the number or size of either sperm morph, or on relative testis mass. However, males experiencing a greater opportunity for sexual selection evolved the largest accessory glands, had the greatest mating capacity, and sired the most progeny. These findings suggest that sterile sperm are not direct targets of sexual selection and that accessory gland size, rather than testis mass, appears to be an important determinant of male reproductive success. We briefly review the data from experimental sexual selection studies and find that testis mass may not be a frequent target of postcopulatory sexual selection and, even when it is, the resulting changes do not always improve fitness.     

Evolutionary reduction in testes size and competitive fertilization success in response to the experimental removal of sexual selection in dung beetles

Sexual selection is thought to favor the evolution of secondary sexual traits in males that contribute to mating success. In species where females mate with more than one male, sexual selection also continues after copulation in the form of sperm competition and cryptic female choice. Theory suggests that sperm competition should favor traits such as testes size and sperm production that increase a male's competitive fertilization success. Studies of experimental evolution offer a powerful approach for assessing evolutionary responses to variation in sexual selection pressures. Here we removed sexual selection by enforcing monogamy on replicate lines of a naturally polygamous horned beetle, Onthophagus taurus, and monitoring male investment in their testes for 21 generations. Testes size decreased in monogamous lines relative to lines in which sexual selection was allowed to continue. Differences in testes size were dependent on selection history and not breeding regime. Males from polygamous lines also had a competitive fertilization advantage when in sperm competition with males from monogamous lines. Females from polygamous lines produced sons in better condition, and those from monogamous lines increased their sons condition by mating polygamously. Rather than being costly for females, multiple mating appears to provide females with direct and/or indirect benefits. Neither body size nor horn size diverged between our monogamous and polygamous lines. Our data show that sperm competition does drive the evolution of testes size in onthophagine beetles, and provide general support for sperm competition theory.     

Do male secondary sexual characters signal ejaculate quality? A meta‐analysis

There are two reasons why researchers are interested in the phenotypic relationship between the expression of male secondary sexual characters (SSCs) and ‘ejaculate quality’ (defined as sperm/ejaculate traits that are widely assumed to increase female fertility and/or sperm competitiveness). First, if the relationship is positive then females could gain a direct benefit by choosing more attractive males for fertility assurance reasons (‘the phenotype‐linked fertility’ hypothesis). Second, there is much interest in the direction of the correlation between traits favoured by pre‐copulatory sexual selection (i.e. affecting mating success) and those favoured by post‐copulatory sexual selection (i.e. increasing sperm competitiveness). If the relationship is negative this could lead to the two forms of selection counteracting each other. Theory predicts that the direction of the relationship could be either positive or negative depending on the underlying genetic variance and covariance in each trait, the extent of variation among males in condition (resources available to allocate to reproductive traits), and variation among males in the cost or rate of mating. We conducted a meta‐analysis to determine the average relationship between the expression of behavioural and morphological male secondary sexual characters and four assays of ejaculate quality (sperm number, viability, swimming speed and size). Regardless of how the data were partitioned the mean relationship was consistently positive, but always statistically non‐significant. The only exception was that secondary sexual character expression was weakly but significantly positively correlated with sperm viability (r = 0.07, P < 0.05). There was no significant difference in the strength or direction of the relationship between behavioural and morphological SSCs, nor among relationships using the four ejaculate quality assays. The implications of our findings are discussed.     

Males mate indiscriminately in the tropical jumping spider Hasarius adansoni (Audouin, 1826)

Traditionally, sexual selection has been seen as a process in which choosy females select non‐choosy males. Recent studies, however, have challenged this view by showing that males can also be choosy in many species. We assessed the sexual preferences in males of the tropical jumping spider Hasarius adansoni (Audouin, 1826). We measured mating effort in males and determined how female quality influences offspring quality, quantity, and survival. We also estimated total sperm load and how much sperm was invested in a mating with a particular female. We found no evidence of male mating effort in terms of mating frequency nor sperm investment. Similarly, there was no relationship between female quality (i.e., size and condition) and offspring quality (i.e., survival and feeding performance) or quantity. We found strong evidence that the sperm invested in a particular female is a function of the amount of sperm the male had available for usage at that particular mating. The fact that males probably find females sequentially, along with the lack of relationship between female quality and offspring quality/quantity, likely explains the lack of differences in mating effort by males.     

No evidence for a trade‐off between sperm length and male premating weaponry

Male ornaments and armaments that mediate success in mate acquisition and ejaculate traits influencing competitive fertilization success are under intense sexual selection. However, relative investment in these pre‐ and post‐copulatory traits depends on the relative importance of either selection episode and on the energetic costs and fitness gains of investing in these traits. Theoretical and empirical work has improved our understanding of how precopulatory sexual traits and investments in sperm production covary in this context. It has recently also been suggested that male weapon size may trade off with sperm length as another post‐copulatory sexual trait, but the theoretical framework for this suggestion remains unclear. We evaluated the relationship between precopulatory armaments and sperm length, previously reported in ungulates, in five taxa as well as meta‐analytically. Within and between taxa, we found no evidence for a negative or positive relationship between sperm length and male traits that are important in male–male contest competition. It is important to consider pre‐ and post‐copulatory sexual selection together to understand fitness, and to study investments in different reproductive traits jointly rather than separately. A trade‐off between pre‐ and post‐copulatory sexual traits may not manifest itself in sperm length but rather in sperm number or function. Particularly in large‐bodied taxa such as ungulates, sperm number is more variable interspecifically and likely to be under more intense selection than sperm length. We discuss our and the previous results in this context.     

SPERM COMPETITION SELECTS BEYOND RELATIVE TESTES SIZE IN BIRDS

Sperm morphology varies considerably across taxa, and postcopulatory sexual selection is thought to be one of the main forces responsible for this diversity. Several studies have investigated the effects of the variation in sperm design on sperm function, but the consequences of variation in sperm design on testis morphology have been overlooked. Testes size or architecture may determine the size of the sperm they produce, and selection for longer sperm may require concomitant adaptations in the testes. Relative testes size differs greatly between species and is often used as an index of sperm competition, but little is known about whether larger testes have more sperm-producing tissue or produce sperm at a faster rate. Using a comparative approach in New World Blackbirds (Icteridae), we found (1) a strong link between testis histology and sperm length, suggesting selection on testis architecture through selection on sperm size, and (2) that species under intense sperm competition had a greater proportion of sperm-producing tissue within their testes. These results support the prediction that sperm competition fosters adaptations in reproductive organs that extend beyond testes size, and raise questions about the trade-offs influencing reproductive investment.     

Polyandry reduces sperm length variation in social insects

Postcopulatory sexual selection, either in the form of sperm competition or cryptic female choice, is an important selective force that is thought to have generated the enormous variation in sperm morphology observed interspecifically. However, the evolutionary significance of intraspecific variation in sperm morphology, and the role that postcopulatory sexual selection plays in influencing this variation, remains poorly investigated in invertebrates. Here, we tested the hypothesis that postcopulatory sexual selection reduces variation in sperm morphology, both between and within males, in 27 species of eusocial ants and bees. These eusocial species offer an unusual opportunity to assess how selection acts on variance in sperm morphology, as haploid males produce clonal, haploid sperm that does not experience haploid-diploid conflict. We provide solid evidence that males of polyandrous ant and bee species indeed produce less-variable sperm, indicating that sperm competition selected for sperm of superior quality. Our results offer a mechanistic explanation for the evolution of high-quality sperm and provide comprehensive evidence that sperm morphology of social insects is influenced by sexual selection.     

EJACULATE QUALITY AND CONSTRAINTS IN RELATION TO SPERM COMPETITION LEVELS AMONG EUTHERIAN MAMMALS

The outcome of sperm competition is influenced by the relative quantity and quality of sperm among competing ejaculates. Whereas it is well established that individual ejaculate traits evolve rapidly under postcopulatory sexual selection, little is known about other factors that might influence the evolution of ejaculates. For example, the metabolic rate is likely to affect the sperm production rate and the cellular activity or metabolism of sperm, and it has recently been suggested to constrain the evolution of sperm length in large but not small mammals. I thus examined in eutherian mammals how ejaculate quality traits vary with one another and with testis mass, body size, and metabolism. I found all ejaculate traits to covary positively with one another and to increase with relative testis mass. When controlling for testis mass, small‐bodied species showed superior sperm quality (but not sperm number). Furthermore, sperm motility and viability were positively associated with the mass‐corrected metabolic rate, but the percentage of morphologically normal and acrosome‐intact sperm were not. These results indicate that body size and the energy budget may also influence the evolution of ejaculate quality, although these influences appear to vary among traits.     

Positive correlations between pre‐ and post‐copulatory sexual traits in warblers

Theoretical models predict that investment in pre‐copulatory and post‐copulatory sexually selected traits should trade‐off. At the macroevolutionary scale, the majority of studies to date have focused on male weaponry as the target of pre‐copulatory sexual selection, but the trade‐off should equally apply to traits used to attract females, such as bird song and plumage. We studied the Old World leaf warblers (Phylloscopidae), a group of socially monogamous songbirds that experience relatively high levels of sperm competition. We examined the relationships between song duration and number of elements in the song with sperm length across 21 species, and between the same song variables and combined testes mass in a subset of these species (n = 10). Across species, these song variables and testes mass/sperm length were generally positively correlated, albeit not statistically significantly so or with borderline significance. In contrast to theory, we found no evidence for negative associations between pre‐ and post‐copulatory traits. We argue that this is a consequence of males of some species investing more into overall fertilization success (i.e. the sum of pre‐ and post‐copulatory sexual selection) than males of other species, and high fertilization success is achieved through investment into both mate attraction and sperm competition.     

Experimental evolution reveals trade-offs between mating and immunity

Immune system maintenance and upregulation is costly. Sexual selection intensity, which increases male investment into reproductive traits, is expected to create trade-offs with immune function. We assayed phenoloxidase (PO) and lytic activity of individuals from populations of the Indian meal moth, Plodia interpunctella, which had been evolving under different intensities of sexual selection. We found significant divergence among populations, with males from female-biased populations having lower PO activity than males from balanced sex ratio or male-biased populations. There was no divergence in anti-bacterial lytic activity. Our data suggest that it is the increased male mating demands in female-biased populations that trades-off against immunity, and not the increased investment in sperm transfer per mating that characterizes male-biased populations.     

Sperm traits negatively covary with size and asymmetry of a secondary sexual trait in a freshwater crayfish

In species where females mate promiscuously, the reproductive success of males depends both on their ability to acquire mates (pre-copulatory sexual selection) and ability of their ejaculates to outcompete those of other males (post-copulatory sexual selection). Sperm competition theory predicts a negative relationship between investment in body traits favouring mate acquisition (secondary sexual characters, SSCs) and investment in ejaculate size or quality, due to the inherent costs of sperm production. In contrast, the phenotype-linked fertility hypothesis posits that male fertilizing efficiency is reliably reflected by the phenotypic expression of male SSCs, allowing females to obtain direct benefits by selecting more ornamented males as copulation partners. In this study, we investigated the relationships between male SSCs and size and quality (viability and longevity) of ejaculates allocated to females in mating trials of the freshwater crayfish Austropotamobius italicus. We showed that the relative size of male weapons, the chelae, was negatively related to ejaculate size, and that chelae asymmetry, resulting from regeneration of lost chelipeds, negatively covaried with sperm longevity. Moreover, males allocated more viable sperm to mates from their own rather than different stream of origin. Our findings thus suggest that, according to sperm competition theory, pre-copulatory sexual selection for large weapons used in male fighting may counteract post-copulatory sperm competition in this crayfish species, and that investment in cheliped regeneration may impair ejaculate quality.     

The evolution of sperm competition genes: The effect of mating system on levels of genetic variation within and between species

It is widely established that proteins involved in reproduction diverge between species more quickly than other proteins. For male sperm proteins, rapid divergence is believed to be caused by postcopulatory sexual selection and/or sexual conflict. Here, we derive the expected levels of gene diversity within populations and divergence between them for male sperm protein genes evolving by postcopulatory, prezygotic fertility competition, i.e. the function imputed for some sperm and seminal fluid genes. We find that, at the mutation‐selection equilibrium, both gene diversity within species and divergence between them are elevated relative to genes with similar selection coefficients expressed by both sexes. We show that their expected level of diversity is a function of the harmonic mean number of mates per female, which affects the strength of fertility selection stemming from male–male sperm competition. Our predictions provide a null hypothesis for distinguishing between other selective hypotheses accounting for the rapid evolution of male reproductive genes.     

Sexual selection drives the coevolution of male and female reproductive traits in Peromyscus mice

When females mate with multiple partners within a single reproductive cycle, sperm from rival males may compete for fertilization of a limited number of ova, and females may bias the fertilization of their ova by particular sperm. Over evolutionary timescales, these two forms of selection shape both male and female reproductive physiology when females mate multiply, yet in monogamous systems, post-copulatory sexual selection is weak or absent. Here, we examine how divergent mating strategies within a genus of closely related mice, Peromyscus, have shaped the evolution of reproductive traits. We show that in promiscuous species, males exhibit traits associated with increased sperm production and sperm swimming performance, and females exhibit traits that are predicted to limit sperm access to their ova including increased oviduct length and a larger cumulus cell mass surrounding the ova, compared to monogamous species. Importantly, we found that across species, oviduct length and cumulus cell density are significantly correlated with sperm velocity, but not sperm count or relative testes size, suggesting that these female traits may have coevolved with increased sperm quality rather than quantity. Taken together, our results highlight how male and female traits evolve in concert and respond to changes in the level of post-copulatory sexual selection.     

Dynamic resource allocation between pre- and postcopulatory episodes of sexual selection determines competitive fertilization success

In polyandrous mating systems, male reproductive success depends on both mate-acquisition traits (precopulatory) and sperm competitive abilities (postcopulatory). Empirical data on the interaction between these traits are inconsistent; revealing positive, negative or no relationships. It is generally expected that the investment in pre- and postcopulatory traits is mediated by environmental conditions. To test how dietary resource availability affects sexual ornamentation, sperm quality and their interrelationship in three-spined sticklebacks (Gasterosteus aculeatus), full-sibling groups were raised under three conditions differing in food quantity and/or quality (i.e. carotenoid content): (i) high-quantity/high-quality, (ii) high-quantity/low-quality or (iii) low-quantity/low-quality. After 1 year of feeding, food-restricted males developed a more intense breeding coloration and faster sperm compared with their well-fed brothers, indicating that they allocated relatively more in pre- and postcopulatory traits. Moreover, they outcompeted their well-fed, carotenoid-supplemented brothers in sperm competition trials with equal numbers of competing sperm, suggesting that food-restricted males maximize their present reproductive success. This may result in reduced future reproductive opportunities as food-restricted males suffered from a higher mortality, had an overall reduced body size, and sperm number available for fertilization. In accordance with theory, a trade-off between the investment in pre- and postcopulatory traits was observed in food-restricted males, whereas well-fed males were able to allocate to both traits resulting in a significantly positive relationship.