Seminal fluid enhances competitiveness of territorial males' sperm in a fish with alternative male reproductive tactics

Sperm competition experiments reveal low prezygotic postmating isolation between parasitic and nonparasitic lamprey ecotypes

The role of postmating sexual selection as a potential reproductive barrier in speciation is not well understood. Here, we studied the effects of sperm competition and cryptic female choice as putative postmating barriers in two lamprey ecotypes with a partial reproductive isolation. The European river lamprey Lampetra fluviatilis is anadromous and parasitic of other fish species, whereas the brook lamprey Lampetra planeri is freshwater resident and nonparasitic. We measured sperm traits in both ecotypes and designed sperm competition experiments to test the occurrence of cryptic female choice. We also performed sperm competition experiments either at equal semen volume or equal sperm number to investigate the role of sperm velocity on fertilization success. We observed distinct sperm traits between ecotypes with a higher sperm concentration and a lower sperm velocity for L. planeri compared with L. fluviatilis. The outcomes of sperm competition reflected these differences in sperm traits, and there was no evidence for cryptic female choice irrespective of female ecotype. At equal semen volume, L. planeri males had a higher fertilization success than L. fluviatilis and vice versa at equal sperm number. Our results demonstrate that different sperm traits between ecotypes can influence the male reproductive success and thus gene flow between L. planeri and L. fluviatilis. However, postmating prezygotic barriers are absent and thus cannot explain the partial reproductive isolation between ecotypes.

Molecular, behavioural and morphological comparisons of sperm adaptations in a fish with alternative reproductive tactics

In species with alternative reproductive tactics, there is much empirical support that parasitically spawning males have larger testes and greater sperm numbers as an evolved response to a higher degree of sperm competition, but support for higher sperm performance (motility, longevity and speed) by such males is inconsistent. We used the sand goby (Pomatoschistus minutus) to test whether sperm performance differed between breeding-coloured males (small testes, large mucus-filled sperm-duct glands; build nests lined with sperm-containing mucus, provide care) and parasitic sneaker-morph males (no breeding colouration, large testes, rudimentary sperm-duct glands; no nest, no care). We compared motility (per cent motile sperm), velocity, longevity of sperm, gene expression of testes and sperm morphometrics between the two morphs. We also tested if sperm-duct gland contents affected sperm performance. We found a clear difference in gene expression of testes between the male morphs with 109 transcripts differentially expressed between the morphs. Notably, several mucin genes were upregulated in breeding-coloured males and two ATP-related genes were upregulated in sneaker-morph males. There was a partial evidence of higher sperm velocity in sneaker-morph males, but no difference in sperm motility. Presence of sperm-duct gland contents significantly increased sperm velocity, and nonsignificantly tended to increase sperm motility, but equally so for the two morphs. The sand goby has remarkably long-lived sperm, with only small or no decline in motility and velocity over time (5 min vs. 22 h), but again, this was equally true for both morphs. Sperm length (head, flagella, total and flagella-to-head ratio) did not differ between morphs and did not correlate with sperm velocity for either morph. Thus, other than a clear difference in testes gene expression, we found only modest differences between the two male morphs, confirming previous findings that increased sperm performance as an adaptation to sperm competition is not a primary target of evolution.

Behavioural and physiological plasticity in social hierarchies

Individuals occupying dominant and subordinate positions in social hierarchies exhibit divergent behaviours, physiology and neural functioning. Dominant animals express higher levels of dominance behaviours such as aggression, territorial defence and mate-guarding. Dominants also signal their status via auditory, visual or chemical cues. Moreover, dominant animals typically increase reproductive behaviours and show enhanced spatial and social cognition as well as elevated arousal. These biobehavioural changes increase energetic demands that are met via shifting both energy intake and metabolism and are supported by coordinated changes in physiological systems including the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes as well as altered gene expression and sensitivity of neural circuits that regulate these behaviours. Conversely, subordinate animals inhibit dominance and often reproductive behaviours and exhibit physiological changes adapted to socially stressful contexts. Phenotypic changes in both dominant and subordinate individuals may be beneficial in the short-term but lead to long-term challenges to health. Further, rapid changes in social ranks occur as dominant animals socially ascend or descend and are associated with dynamic modulations in the brain and periphery. In this paper, we provide a broad overview of how behavioural and phenotypic changes associated with social dominance and subordination are expressed in neural and physiological plasticity. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Male alternative reproductive tactics and sperm competition: a meta-analysis

In many animal species, males may exhibit one of several discrete, alternative ways of obtaining fertilisations, known as alternative reproductive tactics (ARTs). Males exhibiting ARTs typically differ in the extent to which they invest in traits that improve their mating success, or the extent to which they face sperm competition. This has led to the widespread prediction that males exhibiting ARTs associated with a high sperm competition risk, or lower investment into traits that improve their competitiveness before mating, should invest more heavily into traits that improve their competitiveness after mating, such as large ejaculates and high-quality sperm. However, despite many studies investigating this question since the 1990s, evidence for differences in sperm and ejaculate investment between male ARTs is mixed, and there has been no quantitative summary of this field. Following a systematic review of the literature, we performed a meta-analysis examining how testes size, sperm number and sperm traits differ between males exhibiting ARTs that face either a high or low sperm competition risk, or high or low investment in traits that increase mating success. We obtained data from 92 studies and 67 species from across the animal kingdom. Our analyses showed that male fish exhibiting ARTs facing a high sperm competition risk had significantly larger testes (after controlling for body size) than those exhibiting tactics facing a low sperm competition risk. However, this effect appears to be due to the inappropriate use of the gonadosomatic index as a body-size corrected measure of testes investment, which overestimates the difference in testes investment between male tactics in most cases. We found no significant difference in sperm number between males exhibiting different ARTs, regardless of whether sperm were measured from the male sperm stores or following ejaculation. We also found no significant difference in sperm traits between males exhibiting different ARTs, with the exception of sperm adenosine triphosphate (ATP) content in fish. Finally, the difference in post-mating investment between male ARTs was not influenced by the extent to which tactics were flexible, or by the frequency of sneakers in the population. Overall, our results suggest that, despite clear theoretical predictions, there is little evidence that male ARTs differ substantially in investment into sperm and ejaculates across species. The incongruence between theoretical and empirical results could be explained if (i) theoretical models fail to account for differences in overall resource levels between males exhibiting different ARTs or fundamental trade-offs between investment into different ejaculate and sperm traits, and (ii) studies often use sperm or ejaculate traits that do not reflect overall post-mating investment accurately or affect fertilisation success.

Black goby territorial males adjust their ejaculate's characteristics in response to the presence of sneakers

In many species, males can rapidly adjust their ejaculate performance in response to changing levels of sperm competition, an ability that is probably mediated by seminal fluid adaptive plasticity. In the black goby, Gobius niger, territorial males attach viscous ejaculate trails to the nest roof, from which sperm are slowly released into the water during the long-lasting spawning events. Sneaker males release their sperm in the vicinity of the nest, and territorial males try to keep them at a distance by patrolling their territory. We show here that territorial males' ejaculate trails released a higher proportion of their sperm in the presence of a single sneaker, but this proportion decreased when there were three sneakers, an effect that is most likely mediated by a change in the seminal fluid composition. Field observations showed that when multiple sneaking attempts occurred, territorial males spent more time outside the nest, suggesting that ejaculation rate and territory defence are traded-off. Altogether, these results suggest that the adjustment of sperm release from the ejaculate may be strategic, guaranteeing a more continuous concentration of the territorial male's sperm in the nest, although at a lower level, when he is engaged in prolonged territory defence outside the nest.

Spatial asymmetry of the paternity success in nests of a fish with alternative reproductive tactics

Guard-sneaker tactics are widespread among fish, where territorial males defend a nest and provide parental care while sneakers try to steal fertilizations. Territorials and sneakers adopt diverse pre- and post-mating strategies, adjusting their ejaculate investment and/or behavioural responses to the presence of competitors. The relative distance of competitors from the spawning female plays a major role in influencing male mating strategies and the resulting paternity share. However, territorial male quality and sneaking intensity do not fully account for the variability in the relative siring success occurring among species. An often neglected factor potentially affecting sneakers proximity to females is the nest structure. We conducted a field experiment using the black goby, whose nests show two openings of different size. We found that territorial males defend more and sneaking pressure is higher at the front, larger access of the nest than at the back, smaller one. Moreover, microsatellite paternity analysis shows that territorials sire more offspring at the back of their nest. Such a predictable spatial distribution of the paternity share suggests that nest structure might work as an indirect cue of male relative siring success, potentially influencing the territorial male investment in parental care and/or the female egg deposition strategy.

Sperm and alternative reproductive tactics: a review of existing theory and empirical data

Males that exhibit alternative reproductive tactics (ARTs) often differ in the risk of sperm competition and the energetic trade-offs they experience. The resulting patterns of selection could lead to between-tactic differences in ejaculate traits. Despite extensive research on male ARTs, there is no comprehensive review of whether and what differences in sperm traits exist between male ARTs. We review existing theory on ejaculate evolution relevant to ARTs and then conduct a comprehensive vote-counting review of the empirical data comparing sperm traits between males adopting ARTs. Despite the general expectation that sneaker males should produce sperm that are more competitive (e.g. higher quality or performance), we find that existing theory does not predict explicitly how males adopting ARTs should differ in sperm traits. The majority of studies find no significant difference in sperm performance traits between dominant and sneaker males. However, when there is a difference, sneaker males tend to have higher sperm performance trait values than dominant males. We propose ways that future theoretical and empirical research can improve our understanding of the evolution of ejaculate traits in ARTs. We then highlight how studying ejaculate traits in species with ARTs will improve our broader knowledge of ejaculate evolution. This article is part of the theme issue 'Fifty years of sperm competition'.

Social status and ejaculate composition in the house mouse

Sperm competition theory predicts that males should tailor ejaculates according to their social status. Here, we test this in a model vertebrate, the house mouse (Mus musculus domesticus), combining experimental data with a quantitative proteomics analysis of seminal fluid composition. Our analyses reveal that both sperm production and the composition of proteins found in seminal vesicle secretions differ according to social status. Dominant males invested more in ejaculate production overall. Their epididymides contained more sperm than those of subordinate or control males, despite similar testes size between the groups. Dominant males also had larger seminal vesicle glands than subordinate or control males, despite similar body size. However, the seminal vesicle secretions of subordinate males had a significantly higher protein concentration than those of dominant males. Moreover, detailed proteomic analysis revealed subtle but consistent differences in the composition of secreted seminal vesicle proteins according to social status, involving multiple proteins of potential functional significance in sperm competition. These findings have significant implications for understanding the dynamics and outcome of sperm competition, and highlight the importance of social status as a factor influencing both sperm and seminal fluid investment strategies. This article is part of the theme issue 'Fifty years of sperm competition'.

Seminal fluid and accessory male investment in sperm competition

Sperm production and allocation strategies have been a central concern of sperm competition research for the past 50 years. But during the 'sexual cascade' there may be strong selection for alternative routes to maximizing male fitness. Especially with the evolution of internal fertilization, a common and by now well-studied example is the accessory ejaculate investment represented by seminal fluid, the complex mixture of proteins, peptides and other components transferred to females together with sperm. How seminal fluid investment should covary with sperm investment probably depends on the mechanism of seminal fluid action. If seminal fluid components boost male paternity success by directly enhancing sperm function or use, we might often expect a positive correlation between the two forms of male investment, whereas trade-offs seem more likely if seminal fluid acts independently of sperm. This is largely borne out by a broad taxonomic survey to establish the prevailing patterns of seminal fluid production and allocation during animal evolution, in light of which I discuss the gaps that remain in our understanding of this key ejaculate component and its relationship to sperm investment, before outlining promising approaches for examining seminal fluid-mediated sperm competitiveness in the post-genomic era. This article is part of the theme issue 'Fifty years of sperm competition'.

Alternative reproductive tactics are associated with sperm performance in invasive round goby from two different salinity environments

During male-male competition, evolution can favor alternative reproductive tactics. This often results in a dominant morph that holds a resource, such as a nest for egg laying, which competes with a smaller sneaker morph that reproduces by stealing fertilizations. The salinity environment can influence male growth rates, for example, via osmoregulatory costs, which in turn may influence the use of sneaker tactics for small males competing for mating opportunities. Salinity can also affect sperm directly; however, little is known of how salinity influences sneaker tactics through sperm performance. We sampled males of the invasive round goby (Neogobius melanostomus) from two environments, a freshwater river and a brackish estuary. This fish has two male morphs: nest-holding dark males and non-nest-holding light males. We examined the role of water salinity of 0, 8, and 16 on sperm performance and found that for estuarine males, a salinity of 0 reduced sperm velocity compared to a salinity of 8 and 16. Riverine males had low velocity in all salinities. Sperm viability also decreased by over 30% in 0 salinity, compared to 8 and 16, for fish from both environments. Gobies produce ejaculate contents in specialized glands that could in theory shield sperm in an adverse environment. However, gland contents did not improve sperm performance in our tests. Body mass and age estimates indicate that riverine males invested more in somatic growth compared to estuarine males. Estuarine light morph males had a high enough gonadosomatic index to indicate sneaker tactics. We propose that when sperm performance is low, such as for the riverine males, sneaker tactics are ineffective and will be selected against or phenotypically suppressed. Instead, we interpret the increased investment in somatic growth found in riverine males as a life-history decision that is advantageous when defending a nest in the next reproductive season.

Going beyond conventional parameters to unveil sperm quality in fish: the use of fibre optic technology to assess mitochondrial respiratory performance

Sperm fertilisation success depends on both intrinsic quality and the interactions with the surrounding reproductive fluids. In several fish species, these interactions have a variable effect on sperm performance. Although specific responses to reproductive fluids may depend on intrinsic differences in sperm quality, variations in the traditionally recorded sperm functional traits do not fully account for the observed patterns. New methods to enhance the evaluation of sperm quality may prove to be valuable at both applied and theoretical levels, by improving the breeding protocol of reared species and the understanding of mating success in sperm competition contexts. Here we develop a fibre optic-based technique, also adequate for small ejaculate samples, to test the role of mitochondrial respiratory efficiency in deciphering sperm performance variability. We purposely used as model the grass goby, Zosterisessor ophiocephalus, a fish with guard-sneaker mating tactics where the sperm in each male tactic have similar intrinsic qualities (velocity, viability, ATP content), but sneakers' sperm exploit territorial males' seminal fuid, overall displaying better fertilization ability. We found that sperm differed in their mitochondrial respiratory efficiency, which was higher in sneakers' sperm compared to territorial ones. This result draws the attention to an indicator of sperm quality that might be helpful in disentangling the mechanisms driving sperm-reproductive fluid interactions.

Lectin binding sites on the plasma membranes of Orthoptera Tettigonioidea spermatodesms

In many Vertebrate and invertebrate, the interaction mechanisms among gametes are based on a high affinity and specificity of recognition and link between specific saccharidic residues and receptors present on the surface of gametes. Literature data also suggest that Insects could use this strategy. In particular, Orthoptera Tettigoniidae spermatodesms and sperms undergo radical changes passing through the male to the female genital tracts that may be interpreted as well as a capacitation process. These changes could also concern the presence and distribution of surface glycoconjugates. Our study aims to highlight the presence and distribution of saccharide residues on the spermatozoa surface in five species of Orthoptera Tettigoniidae using a battery of lectins Fluorescein Isothiocyanate Conjugated and Gold Conjugated. The results of this investigation have shown that on the surface of the male gametes are present saccharide residues whose nature and distribution are species-specific, during their transfer to the female genital tract, they significantly undergo a change. These results let us hypothesize that also for this group of Insects, the glycoconjugates play a significant role in the process of interaction between gametes.

Offspring phenotype is shaped by the nonsperm fraction of semen

In a large majority of animal species, the only contribution of males to the next generation has been assumed to be their genes (sperm). However, along with sperm, seminal plasma contains a wide array of extracellular factors that have many important functions in reproduction. Yet, the potential intergenerational effects of these factors are virtually unknown. We investigated these effects in European whitefish (Coregonus lavaretus) by experimentally manipulating the presence and identity of seminal plasma and by fertilizing the eggs of multiple females with the manipulated and unmanipulated semen of several males in a full-factorial breeding design. The presence of both own seminal plasma and foreign seminal plasma inhibited sperm motility, and the removal of own seminal plasma decreased embryo survival. Embryos hatched significantly earlier after both semen manipulations than in control fertilizations; foreign seminal plasma also increased offspring aerobic swimming performance. Given that our experimental design allowed us to control potentially confounding sperm-mediated (sire) effects and maternal effects, our results indicate that seminal plasma may have direct intergenerational consequences for offspring phenotype and performance. This novel source of offspring phenotypic variance may provide new insights into the evolution of polyandry and mechanisms that maintain heritable variation in fitness and associated female mating preferences.

Sperm-duct gland content increases sperm velocity in the sand goby

Sperm performance is often tightly linked to male reproductive success. In many demersal gobiid fishes, the male attaches sperm embedded in a mucus produced by sperm-duct glands to the nest substrate before spawning takes place. Sperm are activated as the mucus and embedded gland content dissolve into the water. To test the importance of gland content on sperm function in Pomatoschistus minutus, a marine fish with external fertilization, we used a paired experimental design, with spermatozoa tested with and without sperm-duct gland content mixed into seawater. We measured sperm velocity, percentage of motile sperm and sperm viability over time. Sperm were found to swim 7.3% faster when gland content was mixed in the seawater. Percentage motile sperm was unaffected by the gland content. Sperm viability in seawater exceeded 24 h, but was unaffected by the gland content. An increase in sperm velocity of similar magnitude as found here has been shown by others to increase fertilization success. Since velocity-boosting properties of sperm-duct gland content have now been found in three distantly related goby species, this trait appears to be conserved across the Gobiidae family and may aid in reproduction across a range of species and environments.

This article has an associated First Person interview with the first author of the paper.

Summary: In sperm, velocity is often traded off for viability. However, we show here that in sand gobies, sperm-duct gland content increase velocity without reducing viability, thus mitigating such a trade-off.