Sperm Precedence Research Articles

Reproductive interference hampers species coexistence despite conspecific sperm precedence.

Negative interspecific mating interactions, known as reproductive interference, can hamper species coexistence in a local patch and promote niche partitioning or geographical segregation of closely related species. Conspecific sperm precedence (CSP), which occurs when females that have mated with both conspecific and heterospecific males preferentially use conspecific sperm for fertilization, might contribute to species coexistence by mitigating the costs of interspecific mating and hybridization. We discussed whether two species exhibiting CSP can coexist in a local environment in the presence of reproductive interference. First, using a behaviorally explicit mathematical model, we demonstrated that two species characterized by negative mating interactions are unlikely to coexist because the costs of reproductive interference, such as loss of mating opportunity with conspecific partners, are inevitably incurred when individuals of both species are present. Second, we experimentally examined differences in mating activity and preference in two Harmonia ladybird species known to exhibit CSP. These behavioral differences may lead to local extinction of H. yedoensis because of reproductive interference by H. axyridis. This prediction is consistent with field observations that H. axyridis uses various food sources and habitats whereas H. yedoensis is confined to a less preferred prey item and a pine tree habitat. Finally, by a comparative approach, we observed that niche partitioning or parapatric distribution, but not sympatric coexistence in the same habitat, is maintained between species with CSP belonging to a wide range of taxa, including vertebrates and invertebrates living in aquatic or terrestrial environments. Taken together, it is possible that reproductive interference may destabilize local coexistence even in closely related species that exhibit CSP.

Within-population sperm competition intensity does not predict asymmetry in conpopulation sperm precedence.

Postcopulatory sexual selection can generate evolutionary arms races between the sexes resulting in the rapid coevolution of reproductive phenotypes. As traits affecting fertilization success diverge between populations, postmating prezygotic (PMPZ) barriers to gene flow may evolve. Conspecific sperm precedence is a form of PMPZ isolation thought to evolve early during speciation yet has mostly been studied between species. Here, we show conpopulation sperm precedence (CpSP) between Drosophila montana populations. Using Pool-seq genomic data we estimate divergence times and ask whether PMPZ isolation evolved in the face of gene flow. We find models incorporating gene flow fit the data best indicating populations experienced considerable gene flow during divergence. We find CpSP is asymmetric and mirrors asymmetry in non-competitive PMPZ isolation, suggesting these phenomena have a shared mechanism. However, we show asymmetry is unrelated to the strength of postcopulatory sexual selection acting within populations. We tested whether overlapping foreign and coevolved ejaculates within the female reproductive tract altered fertilization success but found no effect. Our results show that neither time since divergence nor sperm competitiveness predicts the strength of PMPZ isolation. We suggest that instead cryptic female choice or mutation-order divergence may drive divergence of postcopulatory phenotypes resulting in PMPZ isolation.This article is part of the theme issue ‘Fifty years of sperm competition’.

Effects of caffeine on mating behavior and sperm precedence in Tribolium castaneum

AbstractBiogenic amines such as dopamine are physiologically neuroactive substances that affect behavioral and physiological traits in invertebrates, and it has long been known that these substances affect mating behavior in insects. Caffeine is a dopamine activator and thus enhances dopamine receptor activity. However, the effects of caffeine intake on insect mating behavior have been largely unexplored. Therefore, we examined the effect of caffeine on mating behavior in the red flour beetle Tribolium castaneum. Caffeine, which activates dopamine, affected the mating behavior of T. castaneum males. Males who orally ingested caffeine courted faster than males who did not, resulting in faster mounting of females and less time to a male's external aedeagus protrusion. However, the present results showed no difference in sperm precedence measured as a P2 value between males fed caffeine and males not fed caffeine. We discuss the effects of caffeine on insect mating and the possibility that caffeine consumption may cause males to mate with more females in the laboratory.

Sperm competition in golden cuttlefish Sepia esculenta: The impact of mating order and male size

Sperm competition, as the last form of male competition prior to fertilization, is an important component in the reproductive and evolutionary biology of polyandrous species. Golden cuttlefish (Sepia esculenta) is a commercially important annual cephalopod, in which females often mate with multiple males during their mating season and maintain viable sperm in their sperm storage organs for a long time. This multiple-mating system leads to male competition and ultimately sperm competition. To explore the mechanisms of sperm competition in cuttlefish, a laboratory study was conducted using three experimental approaches. First, two similar-sized males were introduced to sequentially mate with a single female (experiment 1), to investigate mating order effects on paternity. Thereafter, a four-male experiment was run (experiment 2), which again tested for mating sequence on paternal contribution. Finally, to test whether male size is a factor affecting paternity, we compared the paternity of two different-sized males that were sequentially mated with a single female (experiment 3). The paternity of the offspring was analyzed using six microsatellite DNA markers on fertilized eggs. The results revealed last-male sperm precedence in all experiments, and no effect of male size was detected. For the two-male experiment, the last male to mate had a progeny share (P2) of 42.86%–61.90%. For the four-male experiment, the last mated male had the highest progeny share (P4) with a proportion of 35.56%–43.75%. Even with differential male sizes in the two-male experiment, the last mated male retained the highest progeny share (P2, 58.33%–91.67%). This study shows a consistent pattern of last-sperm advantage across different experiments and contributes to growing research on cephalopod mating systems. Proper mating schemes should be considered during seedling production for S. esculenta to ascertain efficient resource use and genetic diversity in produced offspring.

Spermathecal variation in temperate Opiliones.

Most arachnid fertilization occurs internally, allowing for a variety of post-copulatory mechanisms to take place. Females are expected to exert some level of control over sperm fate when 1) the point of gametic fusion is particularly distant from the point of oogenesis, 2) the time of syngamy is significantly later than the time of mating, 3) sperm are non-motile, and/or 4) the morphology of females allows for selective containment of sperm. Many of these conditions are met in Opiliones (a.k.a. "harvesters," "harvestmen," or "daddy-longlegs"), where we have evidence of sexual antagonism, multiple mating, and delayed oviposition for a number of species. We used confocal laser scanning microscopy to capture and analyze images of harvester spermathecae, structures within the genitalia of female arthropods that store and maintain sperm after copulation. Spermathecal morphology may have critical function in controlling seminal movement. We anticipated that species with previously identified traits associated with sexual antagonism would also have thicker and/or relatively more complex spermathecae. We examined spermathecal morphology in thirteen species of Leiobunum and one species of Hadrobunus, which were collected from North America and Japan. Our results show that eight species had structures consisting of a single chamber with no or partial invagination, and the remainder had multiple cuticular invaginations producing 2-3 lumina within the spermathecae. Using phylogenetic multivariate comparative methods, we estimated a trend towards cross-correlation between conflict and spermathecal traits. Some, but not all, of the species with thicker, more complex spermathecae had morphological traits associated with sexual conflict (larger body size, thicker genital muscle). In conclusion, we discuss methods to elucidate spermathecal mechanism and sperm precedence in these species. Confocal microscopy allowed us to visualize internal structures difficult to interpret with two-dimensional brightfield microscopy, a technique that could be applied to the characterization of internal reproductive structures in other arthropods.

High variation in last male sperm precedence and genital morphology in the emerald damselfly, Lestes sponsa

Abstract In organisms in which individuals mate multiply, knowledge of the proportion of offspring sired by the last male to mate (P2) under field conditions is important for a thorough understanding of how sexual selection works in nature. In many insect groups, pronounced intraspecific variation in P2 is commonplace. Interestingly, however, in stark contrast to these observations, compilation of P2 data in dragonflies and damselflies (Odonata) indicates that a high P2, seldom below 0.95, is a feature of this taxon. Here we used double digest restriction-site associated DNA sequencing to generate a panel of single nucleotide polymorphisms (SNPs) with which we could determine paternity and estimate values of P2 in the offspring of 19 field-collected pairs of the emerald damselfly Lestes sponsa. We also estimated the relationship between P2 and male genital shape of 16 males using geometric morphometric analysis. P2 was variable (range = 0.0–1.0; mean = 0.5), and there was a marginally non-significant (P = 0.069) relationship between genital shape and P2, suggesting that males with a high P2 had an aedeagus with a broader tip. We suggest that the high P2-values reported in past studies in Odonata are partly due to the methods used to infer paternity. Use of SNPs to determine patterns of paternity and P2 in odonates is needed for a better appraisal of fitness in odonates, and would open many future avenues for use of odonates as models of sexual selection.

Costs and benefits of multiple mating in a species with first-male sperm precedence.

Different patterns of sperm precedence are expected to entail different costs and benefits of mating for each sex that translate into distinct predictions regarding mating system evolution. Still, most studies addressing these costs and benefits have focused on species with mixed paternity or last male precedence, neglecting first-male sperm precedence. We attempted to understand whether this latter pattern of sperm precedence translates into different costs and benefits for each sex in the haplodiploid spider mite Tetranychus urticae, a species in which female multiple mating is prevalent but most offspring aresired by first males. First, we assessed the stability of the sperm precedence pattern. To do so, we measured offspring paternity after exposing females to a different number of matings and mating intervals. Next, to determine the potential costs or benefits of multiple mating for females under different contexts, we measured the fecundity and survival of females that re-mated at different time points. To measure the potential costs of multiple mating for males, we analysed male survival in the presence of different numbers of virgin or mated females. We also tested whether males can reduce offspring production of their competitors, by reducing the production of fertilized offspring of mated females. We found no change in the pattern of sperm precedence, independently of the mating interval between matings and the number of matings. Females paid a cost of mating, as multiply-mated females laid fewer eggs than once-mated females. However, while males had reduced survival when exposed to an intermediate number of virgin females, they paid no additional costs of mating with mated females. Moreover, females that mated multiple times produced fewer fertilized offspring than females that mated once. Thus, males that copulated with mated females reduced the fitness of other males, potentially leading to a relative fitness benefit for themselves. Our results show that complex costs and benefits may arise in males in species with first-male sperm precedence. How these costs and benefits affect the maintenance of selection for polyandry remains an open question.

Mating failure shapes the patterns of sperm precedence in an insect

Estimates of last male sperm precedence (P2) are often used to infer mechanisms of sperm competition, a form of post-copulatory sexual selection. However, high levels of mating failure (i.e. copulations resulting in no offspring) in a population can lead to misinterpretations of sperm competition mechanisms. Through simulations, García-González (2004) illustrated how mating failure could cause bimodal distributions of paternity with peaks at P2 = 0 and 1, under a random sperm mixing mechanism. Here, we demonstrate this effect empirically with the seed bug Lygaeus simulans, a species known to exhibit high levels of mating failure (40–60%), using a morphological marker to estimate paternity. Contrary to previous findings in a sister species, we did not find strong evidence for last male sperm precedence. There was a tendency towards last male precedence (P2 = 0.58) but within the expected range for random sperm mixing. Instead, P2 was highly variable, with a bimodal distribution, as predicted by García-González (2004). After taking mating failure into account, the strongest driver of paternity outcome was copulation duration. Furthermore, we found evidence that mating failure could partly be a female-associated trait. Some doubly-mated females were more likely to produce no offspring or produce offspring from two different sires than expected by chance. Therefore, some females are more prone to experience mating failure than others, a result that mirrors an earlier result in male L. simulans. Our results confirm that mating failure needs to be considered when interrogating mechanisms of post-copulatory sexual selection.Significance statementMating failure arises when animals fail to produce offspring across their lifetime. This may be due to a failure to find a mate or a failure to produce offspring after one or more apparently successful matings. Sperm competition is when ejaculates of rival males compete to fertilize a female’s eggs. Estimates of second male paternity (P2) are often used to infer mechanisms of sperm competition (i.e. which male “wins” and how). However, García-González (2004) suggested that high levels of mating failure can skew paternity (i.e. give spuriously high/low levels of P2) and lead to misinterpretations of these mechanisms. We carried out sperm competition experiments on Lygaeus simulans seed bugs using a morphological marker to estimate paternity. We show empirically that mating failure does skew patterns of paternity, causing a bimodal distribution of P2. Therefore, by disrupting patterns of sperm competition, mating failure influences both the action of post-copulatory sexual selection and also our understanding of the mechanisms of sperm competition.

Body weight of the two sexes determines the occurrence of polyandry in a moth

Polyandry has been a central topic in the study of animal behaviour. Yet, it is still not clear how the body weight of each sex affects the probability of female remating, and what each sex gains from polyandry. Here we aimed to shed some light on these questions using the Mediterranean flour moth, Ephestia kuehniella (Lepidoptera: Pyralidae). We found that virgin pairs mated regardless of their weight to ensure fertilization. However, only 58% of once-mated females remated. The probability of female remating increased when the female and/or the second male was heavier. Our findings indicate that fecundity, polygamy and longevity are functions of body weight in each sex. The nature of the last male sperm precedence in this species allows males and nonvirgin females to regulate their mating decisions. We propose that once-mated females would benefit from accepting heavier second males to replace most sperm from the lighter first mates and rejecting lighter second males to avoid displacement of sperm from the first heavier mates. Heavier males are more coercive, probably also preferred by females, and thus more likely to achieve mating with heavier nonvirgin females to replace most sperm from the previous males and dominate paternity. As body weight is heritable, we suggest that the fitness of both sexes rises when they pass on genes for this trait through female encouragement or avoidance of sperm displacement and male coercion of mated females into remating. We conclude that the two sexes play interactive roles in determining polyandrous patterns depending on their body weight.

Sperm competition tactics shape paternity: adaptive role of extremely long copulations in a wolf spider

Polyandry may occur in many species. In spiders when multiple males mate sequentially, conservative anatomical female traits largely, although not entirely, determine whether the first or last male gains precedence in siring the offspring. More flexible behavioural traits show remarkable variation among spiders, including the occurrence of extremely long copulation times. In the wolf spider Pardosa agrestis copulations can last several hours, even though complete fertilization of the female's eggs requires a small fraction of this time. We tested the hypothesis that long copulations in P. agrestis may be a way to overcome a strict first-male precedence, expected from the genital anatomy of female entelegyne spiders. In a large number of mating tests we observed complete and interrupted copulations in both positions of a double mating set-up. Using the sterile male technique we found that paternity was not exclusively determined by the mating order of males; interruption of mating had a dramatic effect on paternity, but did not affect offspring number in a single mating scenario (i.e. the copulation duration was sufficient for the fertilization of all eggs). Females that had an uninterrupted mating were less willing to mate again and became choosy, while females whose first mating was interrupted behaved similarly to virgin females. Also, males seemed to adjust copulation duration to female reproductive status, mating for less time with nonvirgin females, irrespective of whether the female's first mating was interrupted or not. We conclude that extremely long copulations are likely to be an adaptive behaviour manipulating female reproductive behaviour and enabling males to overcome phylogenetically determined limitations in paternity.

Female remating rate and pattern of sperm use suggest intense sperm competition in Drosophila antonietae (Diptera: Drosophilidae)

Sperm competition is conceived as a powerful selective force which acts on genes related to reproduction, promoting the evolution of reproductive adaptations and inter-population divergence that may ultimately lead to speciation. Such selective pressure is believed to be particularly strong in species from the Drosophila repleta group since females of most of these species are highly promiscuous. However, sperm competition is determined by many other factors far less explored than female remating rate; consequently, its actual intensity as a selective pressure continues to remain unknown across most taxa. In this paper, we aim to estimate the intensity of sperm competition in Drosophila antonietae, a South American species from the D. repleta group, by means of behavioural and genetic analyses. In sum, our results, while failing to reveal sperm precedence in twice-mated females, show that sperm competition is likely to occur and may strongly reduce males’ reproductive output. In addition, using highly polymorphic microsatellite loci, we confirmed a high incidence of multiple paternity in wild-inseminated females. We discuss the results within an evolutionary context comparing the mating system between D. antonietae and closely related species.

When is the change in sperm precedence in the millipede <em>Centrobolus inscriptus</em> (Attems, 1928) (Diplopoda, Pachybolidae)?

There were three scenarios of sperm precedence; first-male, last-male and no order effect. I replicated the H3+ radiolabel technique and showed sperm precedence patterns in the sexually dimorphic millipede Centrobolus inscriptus (Attems, 1928). Disintegration rates (D.P.M.) from single labelled mating were significantly lower than those of unlabelled-labelled mating (U=-2.6, n=13, p=0.005) but not to labelled-unlabelled double mating (U=0.45, n=14, p=0.65). In the initial experiment without the interval V2=43.66% and with the interval V2=74.59%. D.P.M. from the immediate dissections of twice mated females were higher (Z = 3.81, n = 12, 12, P = 0.0001) than females with a remating interval of up to 24 hours. When the experiment was replicated significant differences (H = 30.67, d.f. = 5, P = 0.00001) between total D.P.M values were again found. D.P.M. values were higher when second mating followed immediately after the first (V2= 21%). D.P.M. from labelled-unlabelled mating were significantly lower than the D.P.M. of unlabelled-labelled mating after 10 hours (600 minutes) resulting in a reduction of up to 57% in favour of the second male when second-male ejaculate volume precedence arises to 77%.

The effects of heterospecific mating frequency on the strength of cryptic reproductive barriers.

Heterospecific mating frequency is critical to hybrid zone dynamics and can directly impact the strength of reproductive barriers and patterns of introgression. The effectiveness of post-mating prezygotic (PMPZ) reproductive barriers, which include reduced fecundity via heterospecific matings and conspecific sperm precedence, may depend on the number, identity and order of mates. Studies of PMPZ barriers suggest that they may be important in many systems, but whether these barriers are effective at realistic heterospecific mating frequencies has not been tested. Here, we evaluate the strength of cryptic reproductive isolation in two leaf beetles (Chrysochus auratus and C.cobaltinus) in the context of a range of heterospecific mating frequencies observed in natural populations. We found both species benefited from multiple matings, but the benefits were greater in C.cobaltinus and extended to heterospecific matings. We found that PMPZ barriers greatly limited hybrid production by C.auratus females with moderate heterospecific mating frequencies, but that their effectiveness diminished at higher heterospecific mating frequencies. In contrast, there was no evidence for PMPZ barriers in C.cobaltinus females at any heterospecific mating frequency. We show that integrating realistic estimates of cryptic isolation with information on relative abundance and heterospecific mating frequency in the field substantially improves our understanding of the strong directional bias in F1 production previously documented in the Chrysochus hybrid zone. Our results demonstrate that heterospecific mating frequency is critical to understanding the impact of cryptic post-copulatory barriers on hybrid zone structure and dynamics, and that future studies of such barriers should incorporate field-relevant heterospecific mating frequencies.

The effect of nuptial gift number on fertilization success in a Hawaiian swordtail cricket

In many insects, males transfer nuptial gifts to females prior to, during, or after copulation. Nuptial gifts comprise any form of non-gametic material transfer, such as glandular secretions, prey items, and spermatophores. These gifts can increase male fitness by facilitating copulation or sperm transfer or by promoting the female’s use of the focal male’s sperm. However, when a female mates with multiple males, the relationships between the relative size, number, or quality of nuptial gifts that each male donates and the proportion of offspring that each male sires are unknown in many cases. Here, we assessed these relationships in the Hawaiian swordtail cricket Laupala cerasina. We experimentally manipulated the number of gifts a female received from two male mating partners in sequence, with the first male giving either more, fewer, or an equal number of gifts relative to the second male. We found that there was a significant positive relationship between nuptial gift number and paternity share. In addition, we found considerable first-male sperm precedence, regardless of gift contribution. Finally, offspring from second males tended to be produced later and to be part of larger families. These results show that both nuptial gift number and mating order play important roles in sperm use patterns and fertilization success in this species. SIGNIFICANCE STATEMENT: In many insects, males transfer nuptial gifts (a token, often nutritional, gift) to females prior to, during, or after copulation. In the Hawaiian swordtail cricket Laupala cerasina, males transfer a series of nuptial gifts in the form of spermless spermatophores, to females during courtship. Moreover, males experience sperm competition as females of this species are known to mate multiply. By manipulating the number of gifts transferred, we tested the hypothesis that males who transfer more nuptial gifts achieve higher fitness by fathering more offspring. We found a significant positive relationship between gift number and the proportion of offspring a male sires. In addition, controlling for gift number, the first male to mate with a female achieves more paternity. These results show that both nuptial gift number and mating order affect sperm use patterns and fertilization success in this species.

Strong first sperm precedence in the freshwater hermaphroditic snail Planorbella trivolvis

ABSTRACTPost-copulatory sexual selection, through either sperm competition or cryptic female choice, is an important aspect of sexual selection in hermaphrodites. We investigated sperm precedence, non-random fertilization of eggs by multiple partners, to determine the potential for sperm competition in the hermaphroditic freshwater snail Planorbella trivolvis. We provided snails with two sequential partners, and assessed paternity using albinism as a genetic marker. We mated albino snails first to a pigmented partner then an albino (PA; n = 26) or first to an albino then a pigmented individual (AP; n = 27) and calculated the proportion of eggs fertilized by the second partner (P2) during the 1st, 3rd, and 5th week post-mating. We collected all egg masses laid each week (~300) and examined the embryos to determine paternity by the presence or absence of eye pigmentation. P2 values were quite low (0.18 ± 0.04) indicating strong first sperm precedence and thus little opportunity for sperm competition. We also found that mating order influenced precedence; when the first mate was an albino P2 values were higher than when the first mate was pigmented, suggesting a difference in sperm transfer or viability or a preference for pigmented partners.

Fish sperm competition in hatcheries and between wild and hatchery origin fish in nature

Males compete pre- and post-mating to fertilize the maximum number of eggs. In polyandry, sperm competition occurs when sperm from two or more males compete to fertilize eggs from a female. Here we review how sperm competition from hatchery origin fish can cause loss of genetic variability in fish populations kept in captivity and in wild populations. In fish hatchery practices, sperm competition occurs in mass spawners that release gametes in tanks, and in artificial fertilizations when pooled semen is used. In mass spawnings sperm competition is difficult to tease apart from pre-mating competition and other post-mating selective mechanisms, whereas, studies focused on the use of pooled semen in different fish species have shown a clear relationship between sperm motility parameters and precedence in fertilization. In both situations, sperm competition will result in a loss of genetic variability that accumulates over generations, but hatchery protocols can be adjusted to mitigate it. Another source of concern regarding sperm competition for hatchery produced fish is the spatial and temporal overlap in spawning with wild individuals, either via aquaculture escapees or purposeful stocking programs. This may result in sperm competition between hatchery origin and wild males and impact natural populations. Our review suggests that in order to give every adult selected as broodstock an equal opportunity to produce offspring in captivity, mass spawning and the use of pooled semen should be limited.

Natural history and ecology of the New Zealand sheet-web spiders Cambridgea plagiata and C. foliata (Araneae: Desidae)

ABSTRACTWe investigate key ecological differences and track population dynamics in two sympatric species of sheet-web spider, Cambridgea plagiata and Cambridgea foliata throughout a single breeding season. We undertook a series of weekly population surveys in plots that included both C. plagiata and C. foliata, recording features of their web ecology and of each species’ population dynamics over a single breeding season. We found significant differences in the distribution, web size and substrate, and predominant diet of C. plagiata and C. foliata with the former feeding predominantly on amphipods, building smaller webs and associating with rock walls, resulting in areas of densely aggregated webs. By contrast, moths were the predominant prey type found in C. foliata webs which were larger and more common in open vegetation, resulting in a more uniform distribution of webs within our plots. Additionally, both species exhibit pre-copulatory guarding consistent with the expectations of a first-male sperm priority pattern, suggesting male competition over virgin females. This is despite a consistent female-biased sex ratio. This research not only provides the first ecological data for C. plagiata and C. foliata but provides important context for sexually selected behaviours performed by males in these species.

Underlying sperm precedence pattern in the millipede Centrobolus inscriptus (Attems, 1928) (Diplopoda, Pachybolidae)

Three scenarios exist for sperm precedence; first-male, last-male and no order effect. I replicated the H3+ radiolabel technique to show sperm precedence patterns in the arthropod Centrobolus inscriptus (Attems, 1928). Male and female morphometric parameters illustrated a clear sexual dimorphism. Disintegration rates (d.p.m.) from immediate dissections of once-mated females were higher than females dissected after 24 hours (L (0) » L (24)). A significant difference between mean d.p.m. values occurred in double mating (L-UL vs. UL- L). D.p.m. values were higher when second mating followed immediately after the first (UL-L (O) >UL-L (24) and L-UL (0) » L-UL (24)). D.p.m. from L mating were significantly lower than those of UL-L mating but not then L-UL double mating. There was a reduction of up to 57% in favour of the second male over 24 hours in C. inscriptus.

Conspecific sperm precedence is reinforced, but postcopulatory sexual selection weakened, in sympatric populations of Drosophila.

Sexual selection can accelerate speciation by driving the evolution of reproductive isolation, but forces driving speciation could also reciprocally feedback on sexual selection. This might be particularly important in the context of 'reinforcement', where selection acts directly to increase prezygotic barriers to reduce the cost of heterospecific matings. Using assays of sperm competition within and between two sister species, we show a signature of reinforcement where these species interact: populations of Drosophila pseudoobscura that co-occur with sister species D. persimilis have an elevated ability to outcompete heterospecific sperm, consistent with selection for increased postcopulatory isolation. We also find these D. pseudoobscura populations have decreased sperm competitive ability against conspecifics, reducing the opportunity for sexual selection within these populations. Our findings demonstrate that direct selection to increase reproductive isolation against other species can compromise the efficacy of sexual selection within species, a collateral effect of reproductive traits responding to heterospecific interactions.

Bad tenants: female sheet-web spiders, Cambridgea foliata (Araneae: Desidae), lose feeding opportunities when cohabiting with males

In web-building spiders, females are often too widely distributed across the landscape for males to monopolize more than one mate. Consequently, males seek females one at a time and may cohabit with females in their webs. Pre-copulatory cohabitation is most common in araneomorphs, which suggests that the first male to mate with a female will have a greater share of paternity than any subsequent mates (first sperm precedence). However, pairs of adult New Zealand sheet-web spiders (Cambridgea foliata (L. Koch, 1872); Desidae) cohabit for longer than required to achieve copulation. This is counter-intuitive as it suggests that males defend females they have already copulated with, in lieu of seeking additional mating opportunities. To investigate the costs and benefits of extended cohabitation on male and female C. foliata, we conducted surveys of webs of solitary and paired males and females. We found that solitary spiders of both sexes consistently position themselves in the center of their webs but that when in pairs, females are displaced from the webs by males and will frequently leave the web altogether. Males in pairs would respond to vibrations simulating prey, while females would not respond. This strongly suggests that extended cohabitation should be costly for females. By contrast, for males, cohabitation is a valuable foraging strategy which, combined with the advantages of mate-guarding, may compensate for any lost mating opportunities due to foregoing searching for further mates.