Role In Sperm Competition Research Articles

Bundle formation of sperm: Influence of environmental factors

Cooperative behaviour of sperm is one of the mechanisms that plays a role in sperm competition. It has been observed in several species that spermatozoa interact with each other to form agglomerates or bundles. In this study, we investigate the effect of physical and biochemical factors that will most likely promote bundle formation in bull sperm. These factors include fluid viscosity, swim-up process, post-thaw incubation time and media additives which promote capacitation. While viscosity does not seem to influence the degree of sperm bundling, swim-up, post-thaw migration time and suppressed capacitation increase the occurrence of sperm bundles. This leads to the conclusion that sperm bundling is a result of hydrodynamic and adhesive interactions between the cells which occurs frequently during prolonged incubation times.

Number and Relative Proportions of Eusperm and Parasperm in Various Lepidoptera

Lepidoptera produce two kinds of sperm, eusperm, which are conventional nucleated spermatozoa, and parasperm, which lack nuclei and are usually smaller. Both travel in the female to the sperm storage organ, the spermatheca, but there the parasperm become inactive or disappear without taking any direct role in fertilization of the eggs. There are widespread accounts in the literature of an excess of parasperm over eusperm, though relatively few actual counts, and those counts are from various locations in the lifetime of the sperm: from testes, seminal vesicles, ejaculates, or spermathecae. With phylogenetic comparisons in mind, we counted sperm in the final sperm storage organs of males, the seminal vesicles (ductus ejaculatorius duplex), to explore the prevalence of these two types of sperm over a wide range of species and sizes. We found parasperm in all species, including the most phylogenetically basal (non-ditrysian) ones, and a preponderance of parasperm over eusperm in almost all, including a majority of the smaller species with small total numbers of sperm. Total sperm number of both types increased with wingspan (our proxy for size) as did the ratio of parasperm to eusperm. The ubiquity of parasperm and their preponderance even in very small species indicates an important role in lepidopteran reproduction. Further, the large numbers of both sperm types in larger species of Lepidoptera, apparently vastly more than are needed to fertilize eggs, suggests a role in sperm competition.

Expression of Mst89B and CG31287 is Needed for Effective Sperm Storage and Egg Fertilization in Drosophila

In Drosophila, male reproductive fitness can be affected by any number of processes, ranging from development of gametes, transfer to and storage of mature sperm within the female sperm storage organs, and utilization of sperm for fertilization. We have previously identified the 89B cytogenetic map position of D. melanogaster as a hub for genes that effect male paternity success when disturbed. Here, we used RNA interference to test 11 genes that are highly expressed in the testes and located within the 89B region for their role in sperm competition and male fecundity when their expression is perturbed. Testes-specific knockdown (KD) of bor and CSN5 resulted in complete sterility, whereas KD of CG31287, Manf and Mst89B, showed a breakdown in sperm competitive success when second to mate (P2 < 0.5) and reduced fecundity in single matings. The low fecundity of Manf KD is explained by a significant reduction in the amount of mature sperm produced. KD of Mst89B and CG31287 does not affect sperm production, sperm transfer into the female bursa or storage within 30 min after mating. Instead, a significant reduction of sperm in female storage is observed 24 h after mating. Egg hatchability 24 h after mating is also drastically reduced for females mated to Mst89B or CG31287 KD males, and this reduction parallels the decrease in fecundity. We show that normal germ-line expression of Mst89B and CG31287 is needed for effective sperm usage and egg fertilization.

The development, evaluation, and illustration of a timeline procedure for testing the role of sperm competition in the evolution of sexual traits using paternity data

The empirical study of the role of sperm competition in the evolution of sexual traits has historically been problematic through the inability either to measure sperm competition levels directly in the present or to reconstruct changes in the evolutionary past. Here, we develop and test a procedure based on paternity data that potentially permits both. For our pilot study, we use the rate of change of the seminal protein gene SEMG2 for catarrhine primates published by Dorus et al. (Nat Genet 36:1326–1329, 2004). From their data, Dorus et al. proposed a two-part hypothesis: (1) sperm competition plays a role in the evolution of the SEMG2 gene and (2) higher levels of sperm competition generate more positive selection for change in SEMG2 than lower levels. Dorus et al. were limited, however, by being able to use only proxy measures of sperm competition and only seven “recent” segments of catarrhine primate phylogeny. Here, we develop a “timeline procedure” that permits the Dorus hypothesis to be tested using data from across the whole of catarrhine phylogeny. Our analysis supports part (1) of the Dorus hypothesis but questions part (2), suggesting instead that changes in level of sperm competition have a more powerful influence on the rate of evolution of traits than the level of sperm competition itself. We conclude that the timeline procedure developed here could be a valuable investigative tool in the role of sperm competition in the evolution of sexual traits measured over evolutionary time such as SEMG2. The empirical study of the role of sperm competition in the evolution of sexual traits has historically been problematic through the inability to measure sperm competition levels directly in the present and to reconstruct changes in the evolutionary past. Here, we test a “timeline procedure” based on paternity data that potentially permits both. For our pilot study, we use the rate of change of the seminal protein gene SEMG2 for catarrhine primates published by Dorus et al. (Nat Genet 36:1326–1329, 2004). Whereas Dorus et al. were limited to using proxy measures for only “recent” segments of catarrhine primate phylogeny, our method permits direct measures of sperm competition to be applied across the whole of Catarrhine phylogeny. We conclude that this new procedure could be valuable for investigation of the role of sperm competition in the evolution of a wide range of sexual traits.

Female Genetic Contributions to Sperm Competition in Drosophila melanogaster.

In many species, sperm can remain viable in the reproductive tract of a female well beyond the typical interval to remating. This creates an opportunity for sperm from different males to compete for oocyte fertilization inside the female's reproductive tract. In Drosophila melanogaster, sperm characteristics and seminal fluid content affect male success in sperm competition. On the other hand, although genome-wide association studies (GWAS) have demonstrated that female genotype plays a role in sperm competition outcome as well, the biochemical, sensory, and physiological processes by which females detect and selectively use sperm from different males remain elusive. Here, we functionally tested 26 candidate genes implicated via a GWAS for their contribution to the female's role in sperm competition, measured as changes in the relative success of the first male to mate (P1). Of these 26 candidates, we identified eight genes that affect P1 when knocked down in females, and showed that five of them do so when knocked down in the female nervous system. In particular, Rim knockdown in sensory pickpocket (ppk)+ neurons lowered P1, confirming previously published results, and a novel candidate, caup, lowered P1 when knocked down in octopaminergic Tdc2+ neurons. These results demonstrate that specific neurons in the female's nervous system play a functional role in sperm competition and expand our understanding of the genetic, neuronal, and mechanistic basis of female responses to multiple matings. We propose that these neurons in females are used to sense, and integrate, signals from courtship or ejaculates, to modulate sperm competition outcome accordingly.

Male reproductive system of the arrow crab Stenorhynchus seticornis (Inachoididae)

AbstractOur aim was to describe the reproductive system of males and the formation of sperm packages in the seminal receptacle (SR) of recently mated females of the arrow crab Stenorhynchus seticornis. The male reproductive system was analyzed, and was described using light microscopy and histological and histochemical methods. The first pair of gonopods was described by means of scanning electron microscopy. Additionally, the dehiscence of spermatophores was tested using samples obtained from the vas deferens of males and from the seminal receptacle of recently mated females. Testes were tubular type, and each vas deferens consisted of three regions: the anterior vas deferens (AVD), including a proximal portion that was filled with free spermatozoa and a distal portion contained developing spermatophores; the median vas deferens (MVD) that contained completely formed spermatophores; and the posterior vas deferens (PVD), which contained only granular secretions. The accessory gland, which was filled with secretions, was located in the transition region between the MVD and the PVD. The spermatophores from the MVD were of different sizes, and none of them showed dehiscence in seawater, whereas those spermatophores in contact with the seminal receptacle were immediately broken. The ultrastructure of the gonopods revealed the presence of denticles at the distal portion, which contribute to the mechanical rupture of the spermatophore wall during the transfer of sperm. The contents of the PVD and accessory gland of males are transferred together with the spermatophores, and are responsible for the secretions observed among the sperm packets in the SR of the female. We suggest that these secretions formed the layers found in the SR of recently mated females, and may play a role in sperm competition in arrow crabs.

Sperm Storage Patterns in Doubly Mated Female Anastrepha suspensa (Diptera: Tephritidae)

We examined sperm storage patterns of doubly mated females in a species of tephritid fly (Anastrepha suspensa (Loew) [Diptera: Tephritidae]) with four sperm storage organs and an unusually complex reproductive system to elucidate possible reproductive arenas in which sexual selection can or has played a role in sperm competition and cryptic female choice and to predict effects on paternity outcomes. The duration of copulation with each male of doubly mated female flies was recorded and the sperm storage organs were dissected to determine the location and identity of each male’s sperm (by microsatellite polymerase chain reaction) and its relative quantity. Short copulations with the first male generally resulted in little transfer of sperm and predicted the length of time spent in second copulations, resulting in disproportionate sperm storage from second copulations. For doubly mated females, most storage organs contained sperm from second males only, although the fertilization chamber often contained sperm from both males (allowing for possible sperm competition). Long-term storage organs (spermathecae) always contained sperm from single males exclusively possibly limiting opportunities for sperm competition. Our data provide a mechanism for second male precedence without invoking sperm competition, and caution that paternity patterns must be critically interpreted with inclusion of information on sperm storage patterns.

The morphology of the male and female reproductive system in two species of spider crabs (Decapoda: Brachyura: Majoidea) and the issue of the velum in majoid reproduction

The reproductive system of spider crabs (Majoidea) has raised considerable interest due to the complexity of female sperm storage organs. In several majoid species, the seminal receptacle has been described as being divided into a dorsal storage chamber and a ventral fertilization chamber separated by a muscular velum. The velum is supposed to control the amount of sperm used for fertilization and to play an important role in sperm competition. Here, we present a study on the reproductive systems of the two majoid species, Mithraculus sculptus (Lamarck, 1818) and Stenorhynchus seticornis (Herbst, 1788) using various morphological techniques such as μCT scans and 3D-reconstructions, complemented by paraffin histology. The male gonopods of the herein investigated species are similar in their general morphology and in the presence and distribution of setae. The tubular first gonopod holding the ejaculatory canal is much longer than the short and stout second gonopod, which is supposed to function as a piston in the transport of sperm into the female ducts. The female reproductive system of M. sculptus and S. seticornis conforms to that of other Eubrachyura in possessing paired ovaries, oviducts, seminal receptacles, vaginae, and vulvae. Based on our 3D-reconstructions we demonstrate that there is no division of the seminal receptacle into two chambers separated by a velum. In contrast to this, we observed a spatially restricted invagination of the seminal receptacle. A comparison of our data with those of previous studies, allows for the conclusion that the invagination of the seminal receptacle may have been misinterpreted and mistaken for a velum by other authors. Thus, the division of the seminal receptacle into two chambers separated by a velum is a character which needs to be re-evaluated.

Proteomic characterization of seminal plasma from alternative reproductive tactics of Chinook salmon (Oncorhynchus tswatchysha)

Chinook salmon (Oncorhynchus tshawytscha) are external fertilizers that display sneak-guard alternative reproductive tactics. The larger hooknose males dominate mating positions, while the smaller jack males utilize sneak tactics to achieve fertilization. Although poorly understood, previous studies have suggested that differences in spermatozoa quality may play a critical role in sperm competition. Considering that the seminal plasma strongly regulates spermatozoa quality and other processes critical for fertilization success, we employed label free quantitative mass spectrometry utilizing ion mobility separation coupled to cross-species bioinformatics to examine the seminal plasma proteome of Chinook salmon. A total of 345 proteins were identified in all biological replicates analyzed, including many established seminal plasma proteins that may serve as future biomarkers for Chinook salmon fertility and sperm competition. Moreover, we elucidated statistically significant protein abundance differences between hooknose and jack male tactics. Proteins involved in membrane remodeling, proteolysis, hormonal transport, redox regulation, immunomodulation, and ATP metabolism were among the proteins reproducibly identified at different levels and represent putative factors influencing sperm competition between jack and hooknose males. This study represents the largest seminal plasma proteome from teleost fish and the first reported for Chinook salmon. SignificanceChinook salmon (Oncorhynchus tshawytscha) males represent an example of male alternative reproductive tactics where diverse reproductive strategies are thought to increase sexual selection. While seminal plasma has been shown to play an important regulatory role in sperm competition in many species, little is known about the protein composition of the seminal plasma of salmon. Therefore, seminal plasma isolated from the two alternative reproductive tactics of Chinook salmon (small sneaky jacks and large dominant hooknoses) were analyzed by label free quantitative mass spectrometry employing data independent acquisition and ion mobility separation. This yielded the largest proteome data set of the seminal plasma from salmon and the first to examine protein abundance differences between male alternative reproductive tactics. The quantitative proteomic data provides insight into possible unique mechanistic aspects of Chinook salmon alternative reproductive tactics utilized for sperm competition and fertilization success.

Sperm competition risk drives plasticity in seminal fluid composition.

BackgroundEjaculates contain a diverse mixture of sperm and seminal fluid proteins, the combination of which is crucial to male reproductive success under competitive conditions. Males should therefore tailor the production of different ejaculate components according to their social environment, with particular sensitivity to cues of sperm competition risk (i.e. how likely it is that females will mate promiscuously). Here we test this hypothesis using an established vertebrate model system, the house mouse (Mus musculus domesticus), combining experimental data with a quantitative proteomics analysis of seminal fluid composition. Our study tests for the first time how both sperm and seminal fluid components of the ejaculate are tailored to the social environment.ResultsOur quantitative proteomics analysis reveals that the relative production of different proteins found in seminal fluid – i.e. seminal fluid proteome composition – differs significantly according to cues of sperm competition risk. Using a conservative analytical approach to identify differential expression of individual seminal fluid components, at least seven of 31 secreted seminal fluid proteins examined showed consistent differences in relative abundance under high versus low sperm competition conditions. Notably three important proteins with potential roles in sperm competition – SVS 6, SVS 5 and CEACAM 10 – were more abundant in the high competition treatment groups. Total investment in both sperm and seminal fluid production also increased with cues of heightened sperm competition risk in the social environment. By contrast, relative investment in different ejaculate components was unaffected by cues of mating opportunities.ConclusionsOur study reveals significant plasticity in different ejaculate components, with the production of both sperm and non-sperm fractions of the ejaculate strongly influenced by the social environment. Sperm competition risk is thus shown to be a key factor in male ejaculate production decisions, including driving plasticity in seminal fluid composition.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-015-0197-2) contains supplementary material, which is available to authorized users.

Post-Coital Sperm Competence in Polygamous Animals: The Role of Sperm Traits in Species?Specific Strategies

In some species, the females may mate with more than one male. In these polygamous animals, both pre-coital and post-coital strategies are required for reproductive success. This mini review aims at providing an update of the role of sperm traits in species–specific strategies for post-coital sperm competence. According to the spermcompetition theory, in such species the reproductive success of the males then depends upon both the magnitude of inter-male rivalry and covert sperm competition. The number of inseminated sperm, the percentage of sperm without morphological abnormalities in the ejaculates, and the sperm velocity variables play a significant role in sperm competition. Therefore, males that transfer more sperm per ejaculate or who copulate more often, are more likely to sire offspring. The transfer of large numbers of sperm is also guaranteed by ejaculates with high sperm concentrations. Usually, males of polygamous species have large testes and produce ejaculates with high sperm concentrations. Sperm competition acts as a selective force driving evolutionary change in sperm morphology and function. The degree of sperm pleiomorphy shown by a species varies according to the degree of sperm competition its behaviour entails. For example, the ejaculates of very polygamous species have much more homogenous sperm with lower sperm abnormalities. Finally, recent studies in birds have shown that the straight-line velocity (VSL) value, the most accurate estimate of sperm cell velocity, is of great importance in competition scenarios. Thus, sperm with a higher VSL affords a male fertilization advantages over his rivals.

Large Neurological Component to Genetic Differences Underlying Biased Sperm Use in Drosophila

Sperm competition arises as a result of complex interactions among male and female factors. While the roles of some male factors are known, little is known of the molecules or mechanisms that underlie the female contribution to sperm competition. The genetic tools available for Drosophila allow us to identify, in an unbiased manner, candidate female genes that are critical for mediating sperm competition outcomes. We first screened for differences in female sperm storage and use patterns by characterizing the natural variation in sperm competition in a set of 39 lines from the sequenced Drosophila Genetic Reference Panel (DGRP) of wild-derived inbred lines. We found extensive female variation in sperm competition outcomes. To generate a list of candidate female genes for functional studies, we performed a genome-wide association mapping, utilizing the common single-nucleotide polymorphisms (SNPs) segregating in the DGRP lines. Surprisingly, SNPs within ion channel genes and other genes with roles in the nervous system were among the top associated SNPs. Knockdown studies of three candidate genes (para, Rab2, and Rim) in sensory neurons innervating the female reproductive tract indicate that some of these candidate female genes may affect sperm competition by modulating the neural input of these sensory neurons to the female reproductive tract. More extensive functional studies are needed to elucidate the exact role of all these candidate female genes in sperm competition. Nevertheless, the female nervous system appears to have a previously unappreciated role in sperm competition. Our results indicate that the study of female control of sperm competition should not be limited to female reproductive tract-specific genes, but should focus also on diverse biological pathways.

Sperm competition roles and ejaculate investment in a promiscuous mammal

Theoretical models of sperm competition predict how males should allocate sperm and seminal fluid components to ejaculates according to their mating role (dominant vs. subordinate). Here, we present a detailed analysis of ejaculate expenditure according to male roles in the bank vole (Myodes glareolus). Sperm competition occurs regularly in this species, and dominant males typically achieve higher fertilization success than subordinates. Contrary to theoretical predictions, we found that dominant male bank voles invest more sperm per ejaculate than subordinates, both absolutely and relative to body and testes mass. The testes of dominant males were also absolutely (although not relatively) larger than those of subordinates. However, we found no evidence that subordinate males compensate for lower sperm numbers per ejaculate by increasing ejaculation frequency or sperm velocity. Similarly, we found no evidence for differential investment in copulatory plug size according to male roles in sperm competition, although dominant males had significantly larger seminal vesicles (both absolutely and relative to body mass) compared with subordinates. We conclude that sperm competition roles can have significant but unexpected influences on ejaculate investment in mammals with clearly defined differences in male social status.

Heterogenous Turnover of Sperm and Seminal Vesicle Proteins in the Mouse Revealed by Dynamic Metabolic Labeling

Plasticity in ejaculate composition is predicted as an adaptive response to the evolutionary selective pressure of sperm competition. However, to respond rapidly to local competitive conditions requires dynamic modulation in the production of functionally relevant ejaculate proteins. Here we combine metabolic labeling of proteins with proteomics to explore the opportunity for such modulation within mammalian ejaculates. We assessed the rate at which proteins are synthesized and incorporated in the seminal vesicles of male house mice (Mus musculus domesticus), where major seminal fluid proteins with potential roles in sperm competition are produced. We compared rates of protein turnover in the seminal vesicle with those during spermatogenesis, the timing of which is well known in mice. The subjects were fed a diet containing deuterated valine ([(2)H(8)]valine) for up to 35 days, and the incorporation of dietary-labeled amino acid into seminal vesicle- or sperm-specific proteins was assessed by liquid chromatography-mass spectrometry of samples recovered from the seminal vesicle lumen and cauda epididymis, respectively. Analyses of epididymal contents were consistent with the known duration of spermatogenesis and sperm maturation in this species and in addition revealed evidence for a subset of epididymal proteins subject to rapid turnover. For seminal vesicle proteins, incorporation of the stable isotope was evident from day 2 of labeling, reaching a plateau of labeling by day 24. Hence, even in the absence of copulation, the seminal vesicle proteins and certain epididymal proteins demonstrate considerable turnover, a response that is consonant with the capacity to rapidly modulate protein production. These techniques can now be used to assess the extent of phenotypic plasticity in mammalian ejaculate production and allocation according to social and environmental cues of sperm competition.

Changes in the curvature of sperm apical hooks in murine rodents

Sperm apical hooks in murine rodents play an important role in sperm competition. Apical hooks are more curved and longer in species with relatively larger testes, that is in species with a higher risk of sperm competition. The sperm can form aggregations, ‘trains’, that can move faster than individual sperm, thus reaching the egg earlier as was observed in Apodemus sylvaticus. The apical hook plays an important role for train formation. This study focuses on the changes in the curvature of sperm apical hooks during the final stages of spermiogenesis and stages before fertilization (sperm-life span). Apical hook curvatures of field mice (A. agrarius and A. sylvaticus) vary significantly between dormant and active sperm. In contrast, there are no significant differences among the stages in the eastern house mouse. Since there are high ranges of angle values in all stages, the mean angles of apical hook curvature are not appropriate for evaluating risk of sperm competiton. The ranges of angle values point to a level of flexibility of the apical hooks. The lengths of sperm hooks in individual species do not change during particular stages. The length and flexibility of the sperm apical hooks are important for the formation of sperm aggregations, thus these sperm characters indicate the risk of sperm competition and the sperm strategies in murine rodents.

Male Mating Rate Is Constrained by Seminal Fluid Availability in Bedbugs, Cimex lectularius

Sexual selection, differences in reproductive success between individuals, continues beyond acquiring a mating partner and affects ejaculate size and composition (sperm competition). Sperm and seminal fluid have very different roles in sperm competition but both components encompass production costs for the male. Theoretical models predict that males should spend ejaculate components prudently and differently for sperm and seminal fluid but empirical evidence for independent variation of sperm number and seminal fluid volume is scarce. It is also largely unknown how sperm and seminal fluid variation affect future mating rate. In bedbugs we developed a protocol to examine the role of seminal fluids in ejaculate allocation and its effect on future male mating rate. Using age-related changes in sperm and seminal fluid volume we estimated the lowest capacity at which mating activity started. We then showed that sexually active males allocate 12% of their sperm and 19% of their seminal fluid volume per mating and predicted that males would be depleted of seminal fluid but not of sperm. We tested (and confirmed) this prediction empirically. Finally, the slightly faster replenishment of seminal fluid compared to sperm did not outweigh the faster decrease during mating. Our results suggest that male mating rate can be constrained by the availability of seminal fluids. Our protocol might be applicable to a range of other organisms. We discuss the idea that economic considerations in sexual conflict research might benefit from distinguishing between costs and benefits that are ejaculate dose-dependent and those that are frequency-dependent on the mating rate per se.

NO EVIDENCE FOR POSTCOPULATORY INBREEDING AVOIDANCE IN DROSOPHILA MELANOGASTER

Selection to avoid inbreeding is predicted to vary across species due to differences in population structure and reproductive biology. Over the past decade, there have been numerous investigations of postcopulatory inbreeding avoidance, a phenomenon that first requires discrimination of mate (or sperm) relatedness and then requires mechanisms of male ejaculate tailoring and/or cryptic female choice to avoid kin. The number of studies that have found a negative association between male-female genetic relatedness and competitive fertilization success is roughly equal to the number of studies that have not found such a relationship. In the former case, the underlying mechanisms are largely unknown. The present study was undertaken to verify and expand upon a previous report of postcopulatory inbreeding avoidance in D. melanogaster, as well as to resolve underlying mechanisms of inbreeding avoidance using transgenic flies that express a sperm head-specific fluorescent tag. However, siblings did not have a lower fertilization success as compared to unrelated males in either the first (P(1) ) or second (P(2) ) mate role in sperm competition with a standard unrelated competitor male in our study population of D. melanogaster. Analyses of mating latency, copulation duration, egg production rate, and remating interval further revealed no evidence for inbreeding avoidance.

Tactic-dependent plasticity in ejaculate traits in the swordtail Xiphophorus nigrensis

In species with alternative reproductive tactics, males that sneak copulations often have larger, higher quality ejaculates relative to males that defend females or nest sites. Ejaculate traits can, however, exhibit substantial phenotypic plasticity depending on a male's mating role in sperm competition, which may depend on the tactic of his competitor. We tested whether exposure to males of different tactics affected sperm number and quality in the swordtail Xipophorus nigrensis, a species with small males that sneak copulations and large males that court females. Sperm swimming speed was higher when the perceived competitor was small than when the competitor was large. Plasticity, however, was only exhibited by small males. Sperm number and viability were invariant between social environments. Our results suggest sperm quality is role-dependent and that plastic responses to the social environment can differ between male reproductive tactics.

Inbreeding depresses sperm competitiveness, but not fertilization or mating success in male Tribolium castaneum

As populations decline to levels where reproduction among close genetic relatives becomes more probable, subsequent increases in homozygous recessive deleterious expression and/or loss of heterozygote advantage can lead to inbreeding depression. Here, we measure how inbreeding across replicate lines of the flour beetle Tribolium castaneum impacts on male reproductive fitness in the absence or presence of male-male competition. Effects on male evolution from mating pattern were removed by enforcing monogamous mating throughout. After inbreeding across eight generations, we found that male fertility in the absence of competition was unaffected. However, we found significant inbreeding depression of sperm competitiveness: non-inbred males won 57 per cent of fertilizations in competition, while inbred equivalents only sired 42 per cent. We also found that the P(2) 'offence' role in sperm competition was significantly more depressed under inbreeding than sperm 'defence' (P(1)). Mating behaviour did not explain these differences, and there was no difference in the viability of offspring sired by inbred or non-inbred males. Sperm length variation was significantly greater in the ejaculates of inbred males. Our results show that male ability to achieve normal fertilization success was not depressed under strong inbreeding, but that inbreeding depression in these traits occurred when conditions of sperm competition were generated.

Adult Responses to Larval Population Size in the Almond Moth, Cadra cautella

AbstractJuvenile population size may affect the potential for future mating opportunities and therefore potentially sperm competition; this may favour ontogenetic adjustments in sperm production. Theory predicts that males should optimize their ejaculatory investment in accordance with the risk of sperm competition. Evidence for these theories is typically revealed in males of highly polyandrous species. Whether such responses to environmental cues exist for females, or are maintained in mildly polyandrous species in which most females do not re‐mate, is unknown. Male lepidopterans produce normal, fertilizing sperm (eupyrene) and non‐fertilizing (apyrene) sperm. Apyrene sperm are associated with reduced female receptivity, suggesting a role in sperm competition. We tested the effect of juvenile population size on life‐history parameters and reproductive investment in the mildly polyandrous almond moth, Cadra cautella, a species in which current male ejaculate traits suggest previous selection for paternity protection consistent with a sperm‐competitive environment. Larvae were reared at high (H) or low population sizes (L). We recorded larval development time, adult longevity and male gametic investment. Our results show a response by adults to signals in the juvenile environment. H males transferred more apyrene, but not eupyrene sperm. We also examined potential trade‐offs between somatic characters and reproductive behaviours. Larval duration was longer for H individuals, females and heavier individuals. Further, H females and L males lived longer than L females. Our data are consistent with the theory that males should adjust their reproductive investment in accordance with sperm competition risk.