Sperm storage and viability in Photinus fireflies

In many species females mate with and store sperm from multiple males, and some female insects have evolved multiple compartments for sperm storage. Sperm storage and sperm viability were investigated in two firefly species, Photinus greeni and P. ignitus, which differ in the morphology of the female reproductive tract. Although the primary spermatheca is similar in both species, P. greeni females have an additional, conspicuous outpocketing within the bursa copulatrix whose potential role in sperm storage was investigated in this study. An assay that distinguishes between live and dead sperm was used to examine sperm viability in male seminal vesicles and sperm storage sites within the female reproductive tract. For both Photinus species, sperm from male seminal vesicles showed significantly higher viability compared to sperm from the primary spermatheca of single mated females. In single mated P. greeni females, sperm taken from the channel outpocketing (secondary spermatheca) showed significantly higher viability compared to sperm from the primary spermatheca. This sperm viability difference was not evident in double mated females. There were no significant differences between P. greeni and P. ignitus females in the viability of sperm from the primary spermatheca. These studies contribute to our understanding of post-mating processes that may influence paternity success, and suggest that sexual conflict over control of fertilizations may occur in multiply mated firefly females.     

Female remating and sperm competition patterns in a terrestrial crustacean

In most species, both sexes may mate with more than one partner during their life. In terrestrial isopods (woodlice) female remating can occur within a reproductive season (immediate remating) or after a period of sexual rest and sperm storage, that is in a subsequent reproductive season (delayed remating). The pattern of sperm precedence is unknown in both cases. These two female remating patterns may shape male-male competition in different ways. To elucidate both patterns of female remating and sperm precedence, we used an albinism mutation in Armadillidium vulgare to track paternity in laboratory experiments. Males had low remating success after immediate remating attempts, mainly because of the female's refractory behaviour. However, refractory behaviour seemed to be lost after female sexual rest: delayed remating attempts were as successful as first mating attempts with virgin females. In both immediate and delayed remating, competing males had similar fertilization success, but varied in sperm precedence. We hypothesize that males might induce the refractory mating behaviour in females to ensure their paternity. This could be a strategy that evolved in woodlice after the loss of precopulatory mate guarding during adaptation to the terrestrial environment. We discuss the consequences of these findings for woodlice optimal mating strategies. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Seminal proteins but not sperm induce morphological changes in the Drosophila melanogaster female reproductive tract during sperm storage

In most insects, sperm transferred by the male to the female during mating are stored within the female reproductive tract for subsequent use in fertilization. In Drosophila melanogaster, male accessory gland proteins (Acps) within the seminal fluid are required for efficient accumulation of sperm in the female's sperm storage organs. To determine the events within the female reproductive tract that occur during sperm storage, and the role that Acps and sperm play in these events, we identified morphological changes that take place during sperm storage in females mated to wild-type, Acp-deficient or sperm-deficient males. A reproducible set of morphological changes occurs in a wild-type mating. These were categorized into 10 stereotypic stages. Sperm are not needed for progression through these stages in females, but receipt of Acps is essential for progression beyond the first few stages of morphological change. Furthermore, females that received small quantities of Acps reached slightly later stages than females that received no Acps. Our results suggest that timely morphological changes in the female reproductive tract, possibly muscular in nature, may be needed for successful sperm storage, and that Acps from the male are needed in order for these changes to occur.     

Sperm precedence in <Emphasis Type="Italic">Callosobruchus chinensis</Emphasis> estimated using the sterile male technique

P₂, the proportion of offspring sired by the second male to mate, is an indicator of the outcome of postcopulatory sexual selection, which occurs through sperm competition and/or cryptic female choice. We determined the appropriate dose of gamma radiation for sterilization of adult males and, using the sterile male technique, measured P₂ in the adzuki bean beetle, Callosobruchus chinensis. Adult males of C. chinensis were almost completely sterilized when irradiated at 80 Gy. Thus, we obtained sterile males through irradiation at this dose. Neither the probability of female first mating nor the probability of female remating was affected by whether females were paired with normal or sterile males. The P₂ calculated from the hatching success of eggs laid by females that mated both with normal and sterile males did not differ between reciprocal mating sequences, indicating that the sterilization has no effect on sperm fertilizing ability. The P₂ was estimated at 0.25. This study shows that female remating in C. chinensis means the coexistence of sperm from two males and thus the occurrence of postcopulatory sexual selection within the female reproductive tract, resulting in first-male sperm precedence.     

Sperm survival in the female reproductive tract in the fly Scathophaga stercoraria (L.)

While sperm competition risk favours males transferring many sperm to secure fertilizations, females of a variety of species actively reduce sperm numbers reaching their reproductive tract, e.g. by extrusion or killing. Potential benefits of spermicide to females include nutritional gains, influence over sperm storage and paternity, and the elimination of sperm bearing somatic mutations that would lower zygote fitness.We investigated changes in sperm viability after in vivo and in vitro exposure to the female tract in the polyandrous fly, Scathophaga stercoraria. Sperm viability was significantly lower in the females' spermathecae immediately after mating than in the experimental males' testes. Males also varied significantly in the proportion of live sperm found in storage in vivo. However, the exact mechanism of sperm degradation remains to be clarified. In vitro exposure to extracts of the female reproductive tract, including female accessory glands, failed to significantly lower sperm viability compared to controls. These results are consistent either with postcopulatory sperm mortality in vivo depending entirely on the male (with individual differences in sperm viability, motility or longevity) or with postcopulatory sperm mortality being subtly affected by female effects which were not detected by the in vitro experimental conditions. Importantly, we found no evidence in support of the hypothesis that female accessory glands contribute to sexual conflict via spermicide. Therefore, female muscular control remains to date the only ascertained mechanism of female influence on sperm storage in this species.     

Sperm storage and use in polyandrous females of the globally invasive fruitfly, Ceratitis capitata

The medfly, Ceratitis capitata, is an invasive species in which polyandry, associated with sperm precedence, is a common behaviour in the wild. In this species, characterized by internal fertilization, we disclose how the sperm from two males are stored in the female storage organs and how they are used in terms of paternity outcome. The experiments were designed to furnish comparable and unbiased estimates of sperm numbers and progeny in twice-mated females. Results are incorporated in a model through which it is possible to relate the amount of stored sperm with the progeny of twice-mated females. The results show that polyandrous medfly females conserve equal amounts of sperm from the two males to fertilize their eggs. However, we observed a clear advantage of the second male's sperm in siring progeny, which interestingly decreases in favor of the first male as ovipositions progress. The results enable us to exclude differential sperm mortality and suggest that it is the mechanics governing the storage organs which causes the initial, but decreasing second male sperm precedence during the female reproductive life. These outcomes allow us to correlate sperm use in polyandrous females with the mating strategies and invasiveness of this fly.     

Paternity analyses in wild‐caught and laboratory‐reared Caribbean cricket females reveal the influence of mating environment on post‐copulatory sexual selection

Polyandry is ubiquitous in insects and provides the conditions necessary for male‐ and female‐driven forms of post‐copulatory sexual selection to arise. Populations of Amphiacusta sanctaecrucis exhibit significant divergence in portions of the male genitalia that are inserted directly into the female reproductive tract, suggesting that males may exercise some post‐copulatory control over fertilization success. We examine the potential for male–male and male–female post‐copulatory interactions to influence paternity in wild‐caught females of A. sanctaecrucis and contrast our findings with those obtained from females reared in a high‐density laboratory environment. We find that female A. sanctaecrucis exercise control by mating multiple times (females mount males), but that male–male post‐copulatory interactions may influence paternity success. Moreover, post‐copulatory interactions that affect reproductive success of males are not independent of mating environment: clutches of wild‐caught females exhibit higher sire diversity and lower paternity skew than clutches of laboratory‐reared females. There was no strong evidence for last male precedence in either case. Most attempts at disentangling the contributions of male–male and male–female interactions towards post‐copulatory sexual selection have been undertaken in a laboratory setting and may not capture the full context in which they take place – such as the relationship between premating and post‐mating interactions. Our results reinforce the importance of designing studies that can capture the multifaceted nature of sexual selection for elucidating the role of post‐copulatory sexual selection in driving the evolution of male and female reproductive traits, especially when different components (e.g. precopulatory and post‐copulatory interactions) do not exert independent effects on reproductive outcomes.     

Broken Copulatory Organs are Low-Cost Adaptations to Sperm Competition in Redback Spiders

In some spiders, a discrete portion of the male's copulatory organ (the apical sclerite) breaks off during copulation and remains in the female's reproductive tract. Apical sclerites may prevent insemination by rivals (sperm competition), stimulate females to favourably bias paternity (cryptic choice) or breakage may reflect sexual conflict over copulation duration with little direct effect on paternity. It has been assumed that any benefits of organ breakage are balanced by a large cost (male sterility) in species where males could otherwise mate multiply, but this has never been experimentally tested. We examined these ideas in the Australian redback spider (Latrodectus hasselti Thorell 1870, Araneae: Theridiidae), a species where males are functionally sterile after one normal mating. We experimentally removed sclerites and found males were able to mate, had similar copulation durations and transferred similar numbers of sperm as males with intact sclerites. Benefits of organ breakage were examined by forcing intact, rival males to inseminate the same or opposite reproductive tracts (female have paired, independent tracts in this taxon) and assessing paternity as a function of sclerite location. As predicted, apical sclerites were typically deposited at the entrance to the female's sperm storage organ, where they could physically block insemination by rivals. First male precedence was common when males inseminated the same tract and deposited sclerites at the entrance to the spermatheca, but not when sclerites were found elsewhere in the tract, or when rivals inseminated opposite tracts (where physically blocking rivals was impossible). Our data show that, in redbacks, copulatory organ breakage is not a side‐effect of sexual conflict, is unlikely to be a cue for cryptic female choice, but allows males to avoid sperm competition. Moreover, copulatory organ damage can have minimal reproductive cost for males, so assumptions of sterility after organ breakage are unjustified without supporting data.     

High variation in sperm precedence and last male advantage in the scorpionfly Panorpa germanica L. (Mecoptera, Panorpidae): possible causes and consequences

In the last decades, many insect species have been studied in terms of sperm competition. Patterns of sperm use are often inferred from the mean species value of P(2), defined as the mean proportion of offspring sired by the second male in double-mating trials. In Panorpa germanica (Mecoptera, Panorpidae), P(2) largely depends on relative copulation durations of both males, but with the second male on average having some advantage over the first male. Estimating the presence of fertile sperm inside the female's reproductive tract in relation to time after copulation we conclude this partial last male sperm precedence not to be caused by natural death, loss, or depletion of first male sperm. Estimating sperm transfer rates of both mates of a female we, furthermore, found that the high intraspecific variance in P(2) that can be observed cannot solely be explained by variances in sperm transfer rates among P. germanica males. Other factors possibly causing the observed patterns of paternity success are discussed.     

Sperm transfer and paternity in the scorpionfly <Emphasis Type="Italic">Panorpa cognata</Emphasis>: large variance in traits favoured by post-copulatory episodes of sexual selection

Post-copulatory episodes of sexual selection can be a powerful selective force influencing the reproductive success of males. In order to understand variation in male fertilisation success, we first need to consider the pattern of sperm utilisation by females following matings with more than one male. Second, we need to study those traits responsible for male success in sperm competition. Here we study both male sperm transfer characteristics as well as offspring paternity of females mated to two males in the scorpionfly Panorpa cognata. By repeatedly mating males to virgin females and interrupting copulation at defined time points, we found for all males that sperm transfer set off after approximately 40 min. During the remaining copulation, sperm transfer of individual males was continuous and with constant rate. Yet the rate of sperm transfer differed between individual males from about one sperm per minute to more than eight sperm per minute for the most successful males. In addition, we measured the fertilisation success in sperm competition of males with known sperm transfer capability. The relative number of sperm transferred by males during copulation, estimated from copulation duration and the males’ individual sperm transfer rate, explained a large proportion of variation in offspring paternity. The mode of sperm competition in this species, thus, conforms largely to a fair raffle following complete mixing of sperm prior to fertilisation. Hence, male differences in both the ability to copulate for long and of rapid sperm transfer will translate directly into differences in reproductive success.     

Resolving mechanisms of short-term competitive fertilization success in the red flour beetle

Postcopulatory sexual selection occurs when sperm from multiple males occupy a female’s reproductive tract at the same time and is expected to generate strong selection pressures on traits related to competitive fertilization success. However, knowledge of competitive fertilization success mechanisms and characters targeted by resulting selection is limited, partially due to the difficulty of discriminating among sperm from different males within the female reproductive tract. Here, we resolved mechanisms of competitive fertilization success in the promiscuous flour beetle Tribolium castaneum. Through creation of transgenic lines with fluorescent-tagged sperm heads, we followed the fate of focal male sperm in female reproductive tracts while tracking paternity across numerous rematings. Our results indicate that a given male’s sperm persist and fertilize eggs through at least seven rematings. Additionally, the proportion of a male’s sperm in the bursa (the site of spermatophore deposition), which is influenced by both timing of female’s ejecting excess sperm and male size, significantly predicted paternity share in the 24 h following a mating. Contrary to expectation, proportional representation of sperm within the female’s specialized sperm-storage organ did not significantly predict paternity, though spermathecal sperm may play a role in fertilization when females do not have access to mates for longer time periods. We address the adaptive significance of the identified reproductive mechanisms in the context of T. castaneum’s unique mating system and ecology.     

A species‐specific multigene family mediates differential sperm displacement in Drosophila melanogaster

Sperm competition is a postcopulatory sexual selection mechanism in species in which females mate with multiple males. Despite its evolutionary relevance in shaping male traits, the genetic mechanisms underlying sperm competition are poorly understood. A recently originated multigene family specific to Drosophila melanogaster, Sdic, is important for the outcome of sperm competition in doubly mated females, although the mechanistic nature of this phenotype remained unresolved. Here, we compared doubly mated females, second mated to either Sdic knockout or nonknockout males, and directly visualize sperm dynamics in the female reproductive tract. We found that a less effective removal of first‐to‐mate male's sperm within the female's sperm storage organs is consistent with a reduced sperm competitive ability of the Sdic knockout males. Our results highlight the role young genes can play in driving the evolution of sperm competition.     

Determinants of sperm precedence in a noctuid moth Heliothis virescens: a role for male age

1. Females of the noctuid moth Heliothis virescens F. mate more than once. Thus, sperm from two or more males normally compete for fertilisations within the female reproductive tract. The eggs are typically fertilised by sperm from only one male, either the female's last mate or an earlier mate. Twice‐mated females store only one ejaculate's worth of fertilising sperm (eupyrene) but nearly two ejaculates' worth of a nonfertilising sperm morph (apyrene), which is thought to play a role in sperm competition. 2. The mechanism of sperm use in H. virescens was investigated by examining factors that vary with paternity, which was assigned based on allozyme variation. The factors included male and female body masses and ages, male genital characters, the size of the sperm package, and the number of sperm stored by the female. 3. One male typically gained sperm precedence; this was nearly twice as likely to be the second male as it was to be the first. Two factors were found to vary significantly with paternity: female mass and male age. The second male to mate was more likely to gain sperm precedence if the female was larger and if the male was older than the female's first mate. 4. The significance of male age and female mass to several hypothetical models of the mechanism of sperm use is discussed.     

Longer exaggerated male genitalia confer defensive sperm-competitive benefits in an earwig

As evidence mounts that male genitalia can affect relative fertilisation success, the role that sexual selection has played in the rapid and divergent evolution of genitalia is becoming increasingly recognized. Unfortunately, the limited functional understanding of these complex structures and their interactions with the female reproductive tract often limit interpretation regarding their evolution. Here, we address this issue using the earwig Euborellia brunneri, where both the male intromittent organ and the female spermatheca are highly exaggerated in length yet structurally simple. In a double mating design, we use the sterile male technique to study how sperm precedence patterns are affected by male genital length, male age, and the size of the male sperm storage organ, the seminal vesicle. Relative fertilisation success exhibited considerable variation around modest last-male paternity. Only an interaction between first and second male genital length affected paternity, where males gained reduced paternity when preceded by rivals with longer genitalia. Longer genitalia confer defensive benefits in sperm competition by apparently depositing ejaculate deeper in the tubular spermatheca, safe from removal by rivals. Paternity similarly depended on an interaction between the ages of both males, likely mediated by sperm traits as testes size decreased with age. Seminal vesicle size showed positive allometry but did not affect paternity; instead, greater seminal vesicle size in last males expedited oviposition. The exaggerated yet relatively simple genitalia of E. brunneri facilitate an unusually clear example of post-copulatory selection on phenotypic variation in multiple reproductive traits.     

Multiple sperm storage organs facilitate female control of paternity

It has been proposed that multiple sperm storage organs (spermathecae) could allow polyandrous females to control paternity. There is little conclusive evidence for this since insemination of individual spermathecae is generally not experimentally manipulable. Here, we examined sperm use patterns in the Australian redback spider (Latrodectus hasselti), which has paired, independent spermathecae. We assessed paternity when two rivals were forced to inseminate a single storage organ or opposite storage organs. When males inseminated a single spermatheca, mean paternity of the female's first mate was 79.8% (median 89.4%), and 38% of first mates achieved 100% paternity. In contrast, when males inseminated opposite organs, the mean paternity of the first mate was 49.3% (median 49.9%), only 10% of males achieved complete precedence, and paternity was normally distributed, suggesting sperm mixing. Males responded to this difference by avoiding previously inseminated female reproductive tracts. Complete sperm precedence can only be achieved if females permit males to copulate with both reproductive tracts. Females often cannibalize smaller males during their first copulation, thus limiting their paternity to 50%. These data show that multiple sperm storage organs can increase female control of paternity.     

Old‐male paternity advantage is a function of accumulating sperm and last‐male precedence in a butterfly

Old‐male mating advantage has been convincingly demonstrated in Bicyclus anynana butterflies. This intriguing pattern may be explained by two alternative hypotheses: (i) an increased aggressiveness and persistence of older males during courtship, being caused by the older males' low residual reproductive value; and (ii) an active preference of females towards older males what reflects a good genes hypothesis. Against this background, we here investigate postcopulatory sexual selection by double‐mating Bicyclus anynana females to older and younger males, thus allowing for sperm competition and cryptic mate choice, and by genotyping the resulting offspring. Virgin females were mated with a younger virgin (2–3 days old) and afterwards an older virgin male (12–13 days old) or vice versa. Older males had a higher paternity success than younger ones, but only when being the second (=last) mating partner, while paternity success was equal among older and younger males when older males were the first mating partner. Older males produced larger spermatophores with much higher numbers of fertile sperm than younger males. Thus, we found no evidence for cryptic female mate choice. Rather, the findings reported here seem to result from a combination of last‐male precedence and the number of sperm transferred upon mating, both increasing paternity success.     

Sperm competition promotes the exploitation of rival ejaculates

In many polyandrous species, the second male to mate with a female has fertilization priority. Strategic mechanisms generating this pattern tend to involve facultative increases in ejaculate size by informed males. Here we suggest a novel mechanism to generate second male precedence in internal fertilizers, based on the established fact that the female site of insemination is hostile to sperm, and that non-sperm components of the ejaculate frequently act to ameliorate these hostile conditions. We suggest that in species where female remating is frequent and rapid, second male sperm precedence may be due to the prior buffering of the female tract by previous males' ejaculates. In this scenario, second male costs are lower due to reduced sperm mortality and/or the redundancy of seminal fluid. Contrary to many classical predictions, second males may gain a paternity advantage despite investing less in an ejaculate.     

Biases in sperm use in the mallard: no evidence for selection by females based on sperm genotype

If we are to understand fully the factors influencing fertilization success, it is essential to untangle male and female effects on sperm use. In many species, differences in fertilizing ability have been found between males or male genotypes, but the impact of female effects is less clear and may vary between taxa. Here, we examine sperm use in the mallard (Anas platyrhynchos), a species of bird in which forced copulation forms a major component of the mating system, to investigate whether there is any evidence for post-insemination female choice or rejection of particular sperm genotypes. Current models of sperm use in birds suggest observed patterns of paternity are a result of passive sperm loss from the reproductive tract and the relative timing of inseminations. Although this type of model successfully predicted average values of last male precedence observed in this species, there was considerable variation between females in their pattern of sperm use, with a tendency for females to use sperm of a single genotype. However, females did not consistently prefer one genotype over another in repeated inseminations with identical sperm mixtures, suggesting that post-insemination female preference based on sperm genotype did not account for this variation.     

Mating duration and sperm precedence in the spider <Emphasis Type="Italic">Linyphia triangularis</Emphasis>

In many animal species, mating behaviour is highly ritualised, which may allow us to relate some of its consequences, e.g. male paternity and female receptivity, to the progression of phases in the mating sequence; at the same time, ritualisation raises the question of to what extent the partners, especially the males, are able to influence the outcome of mating for their own benefit. We studied the linyphiid spider Linyphia triangularis in which mating follows a strict sequence during which the male inducts two droplets of sperm and transfers them to the female. We performed sperm competition experiments (sterile-male technique) including four treatments, in which the copulation of the first male was interrupted at prescribed phases of the mating sequence, while the second male was allowed a complete mating. Second males spent a shorter time than first males on the behaviours prior to sperm transfer, but the amount of sperm (2 droplets) and the time spent in sperm transfer were independent of the females’ mating status. The proportion of females accepting the second male depended on the mating duration of the first male, i.e. whether the first male had transferred one or two sperm droplets. After a complete first mating, most females accepted no further males. A last-male sperm precedence was apparent if only half of the first sperm droplet had been transferred by the first male, but this switched to a first male precedence if one full sperm droplet had been transferred. Thus, even in the face of sperm competition, it is sufficient for the first male to transfer one sperm droplet. The second sperm droplet and the extended copulatory courtship associated with its transfer may serve to induce a lack of receptivity in the female, but the males seem unable to enhance their reproductive success through variable copulatory tactics.     

Factors influencing paternity success in Antechinus agilis: last‐male sperm precedence,timing of mating and genetic compatibility

We describe the patterns of paternity success from laboratory mating experiments conducted in Antechinus agilis, a small size dimorphic carnivorous marsupial (males are larger than females). A previous study found last‐male sperm precedence in this species, but they were unable to sample complete litters, and did not take male size and relatedness into account. We tested whether last‐male sperm precedence regardless of male size still holds for complete litters. We explored the relationship between male mating order, male size, timing of mating and relatedness on paternity success. Females were mated with two males of different size with either the large or the small male first, with 1 day rest between the matings. Matings continued for 6 h. In these controlled conditions male size did not have a strong effect on paternity success, but mating order did. Males mating second sired 69.5% of the offspring. Within first mated males, males that mated closer to ovulation sired more offspring. To a lesser degree, variation appeared also to be caused by differences in genetic compatibility of the female and the male, where high levels of allele‐sharing resulted in lower paternity success.