Multiple paternity in the freshwater snail,Potamopyrgus antipodarum

Mating multiply may incur costs, such as exposure to predators and to sexually transmitted diseases. Nevertheless, it may be favored, in spite of these costs, as a way to increase the genetic diversity of offspring through fertilization by multiple males. Here, we tested for multiple paternity in a freshwater snail (Potamopyrgus antipodarum), which is host to several species of sterilizing trematode worms. Using microsatellites markers, we found multiple paternity in two different snail populations, with as many as seven males fertilizing a single female. In addition, high evenness of sire fertilization was found within individual broods. Multiple paternity can occur for a variety of reasons; however, given that these populations experience high risk of infection by a sterilizing trematode, one potential explanation may be that multiple paternity and high evenness of sire fertilizations increase the chances of the production of parasite‐resistant offspring.     

Females benefit from multiple mating but not multiple mates in the burying beetle Nicrophorus vespilloides

Male reproductive success generally increases with number of mates but this need not be true for females. If females are the limiting sex, as few as one mate can be optimal. Despite the theoretical differences driving multiple mating in the sexes, multiple mating is the norm rather than the exception. Empirical investigations are therefore required to determine why females mate with multiple males. Both nonadaptive (correlated responses to selection on males, given the mean mating rates have to be the same) and adaptive (direct or indirect fitness benefits) can drive the evolution of multiple mating in females. Females of the burying beetle Nicorphorus vespilloides often mate repeatedly with the same male, but this appears to be a correlated response to selection on males rather than reflecting direct benefits to females for multiple mating. However, an unexamined alternative to this nonadaptive explanation is that females benefit by mating with multiple different males and therefore are selected for general promiscuity. Here we examine if mating polyandrously provides fitness benefits by examing the effects of number of mates (1, 2 or 3), mating system (monogamous, polyandrous) and their interaction. The only significant influence was mating more than once. This did not depend on type of mating. We suggest that unlike most other species examined, in N. vespilloides mating with the same male repeatedly or with several different males reflects an indiscriminate willingness to mate as a result of correlated selection on males for high rates of mating.     

Genetic covariance between components of male reproductive success: within‐pair vs. extra‐pair paternity in song sparrows

The evolutionary trajectories of reproductive systems, including both male and female multiple mating and hence polygyny and polyandry, are expected to depend on the additive genetic variances and covariances in and among components of male reproductive success achieved through different reproductive tactics. However, genetic covariances among key components of male reproductive success have not been estimated in wild populations. We used comprehensive paternity data from socially monogamous but genetically polygynandrous song sparrows (Melospiza melodia) to estimate additive genetic variance and covariance in the total number of offspring a male sired per year outside his social pairings (i.e. his total extra‐pair reproductive success achieved through multiple mating) and his liability to sire offspring produced by his socially paired female (i.e. his success in defending within‐pair paternity). Both components of male fitness showed nonzero additive genetic variance, and the estimated genetic covariance was positive, implying that males with high additive genetic value for extra‐pair reproduction also have high additive genetic propensity to sire their socially paired female's offspring. There was consequently no evidence of a genetic or phenotypic trade‐off between male within‐pair paternity success and extra‐pair reproductive success. Such positive genetic covariance might be expected to facilitate ongoing evolution of polygyny and could also shape the ongoing evolution of polyandry through indirect selection.     

Asynchronous arrival pattern, operational sex ratio and occurrence of multiple paternities in a territorial breeding anuran, Rana dalmatina

Understanding why females mate multiply is a major issue in evolutionary ecology. We investigated the consequences of an asynchronous arrival pattern on male competition and multiple paternity in the apparently monoandrous agile frog ( Rana dalmatina ). The largest frogs arrived first and both males and females lost weight significantly during the spawning period. Asynchronous arrival at breeding sites resulted in a male-biased operational sex ratio (OSR). The OSR was more strongly male-biased at the beginning and at the end of the breeding period when the number of satellite males increased. All females mated only once, but multiple paternity within clutches occurred at the beginning and the end of the breeding period. The influence of asynchronous arrival and biased sex ratio suggests that reduced variance or bet-hedging promoting female fitness had only a reduced role in the evolution of polyandry, and polyandry is likely to be associated with male benefits. Polyandry in frogs can be explained either by forced mating as a result of sexual conflict or by clutch piracy. By modifying intrasexual competition, asynchronous arrival and changes in OSR may have a decisive influence upon the evolution of mating systems and favour both polyandry and stable coexistence of alternative mating behaviour.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 191–200.     

Effects of two seminal fluid transcripts on post‐mating behaviour in the simultaneously hermaphroditic flatworm Macrostomum lignano

The seminal fluid proteins (SFPs) transferred to mating partners along with sperm often play crucial roles in mediating post‐mating sexual selection. One way in which sperm donors can maximize their own reproductive success is by modifying the partner's (sperm recipient's) post‐copulatory behaviour to prevent or delay re‐mating, thereby decreasing the likelihood or intensity of sperm competition. Here, we adopted a quantitative genetic approach combining gene expression and behavioural data to identify candidates that could mediate such a response in the simultaneously hermaphroditic flatworm Macrostomum lignano. We identified two putative SFPs—Mlig‐pro46 and Mlig‐pro63—linked to both mating frequency and ‘suck’ frequency, a distinctive behaviour, in which, upon ejaculate receipt, the worm places its pharynx over its female genital opening and apparently attempts to remove the received ejaculate. We, therefore, performed a manipulative experiment using RNA interference‐induced knockdown to ask how the loss of Mlig‐pro46 and Mlig‐pro63 expression, singly and in combination, affects mating frequency, partner suck propensity and sperm competitive ability. None of the knockdown treatments impacted strongly on the mating frequency or sperm competitive ability, but knockdown of Mlig‐pro63 resulted in a significantly decreased suck propensity of mating partners. This suggests that Mlig‐pro63 may normally act as a cue in the ejaculate to trigger recipient suck behaviour and—given that other proteins in the ejaculate have the opposite effect—could be one component of an ongoing arms race between donors and recipients over the control of ejaculate fate. However, the adaptive significance of Mlig‐pro46 and Mlig‐pro63 from a donor perspective remains enigmatic.     

High multiple paternity and low last-male sperm precedence in a hermaphroditic planarian flatworm: consequences for reciprocity patterns

It is difficult to predict a priori how mating success translates into fertilization success in simultaneous hermaphrodites with internal fertilization. Whereas insemination decisions will be determined by male interests, fertilization will depend on female interests, possibly leading to discrepancies between insemination and fertilization patterns. The planarian flatworm Schmidtea polychroa, a simultaneous hermaphrodite in which mating partners trade sperm was studied. Sperm can be stored for months yet individuals mate frequently. Using microsatellites, maternity and paternity data were obtained from 748 offspring produced in six groups of 10 individuals during four weeks. Adults produced young from four mates on average. Reciprocal fertilization between two mates was found in only 41 out of 110 registered mate combinations, which is clearly less than what is predicted from insemination patterns. Multiple paternity was high: > 80% of all cocoons had two to five fathers for only three to five offspring per cocoon. Because animals were collected from a natural population, 28% of all hatchlings were sired by unknown sperm donors in the field, despite a 10-day period of acclimatization and within-group mating. This percentage decreased only moderately throughout the experiment, showing that sperm can be stored and used for at least a month, despite frequent mating and sperm digestion. The immediate paternity a sperm donor could expect to obtain was only about 25%. Male reproductive success increased linearly with the number of female partners, providing support for Bateman's principle in hermaphrodites. Our results suggest that hermaphrodites do not trade fertilizations when trading sperm during insemination, lending support to the view that such conditional sperm exchange is driven by exchange of resources.     

Heterosis and inbreeding depression in bottlenecked populations: a test in the hermaphroditic freshwater snail Lymnaea stagnalis

Small population size is expected to induce heterosis, due to the random fixation and accumulation of mildly deleterious mutations, whereas within‐population inbreeding depression should decrease due to increased homozygosity. Population bottlenecks, although less effective, may have similar consequences. We tested this hypothesis in the self‐fertile freshwater snail Lymnaea stagnalis, by subjecting experimental populations to a single bottleneck of varied magnitude. Although patterns were not strong, heterosis was significant in the most severely bottlenecked populations, under stressful conditions. This was mainly due to hatching rate, suggesting that early acting and highly deleterious alleles were involved. Although L. stagnalis is a preferential outcrosser, inbreeding depression was very low and showed no clear relationship with bottleneck size. In the less reduced populations, inbreeding depression for hatching success increased under high inbreeding. This may be consistent with the occurence of synergistic epistasis between fitness loci, which may contribute to favour outcrossing in L. stagnalis.     

Multiple paternity is a shared reproductive strategy in the live‐bearing surfperches (Embiotocidae) that may be associated with female fitness

According to Bateman's principle, female fecundity is limited relative to males, setting the expectation that males should be promiscuous, while females should be choosy and select fewer mates. However, several surfperches (Embiotocidae) exhibit multiple paternity within broods indicating that females mate with multiple males throughout the mating season. Previous studies found no correlation between mating success and reproductive success (i.e., a Bateman gradient). However, by including samples from a broader range of reproductive size classes, we found evidence of a Bateman gradient in two surfperch species from distinct embiotocid clades. Using microsatellite analyses, we found that 100% of the spotfin surfperch families sampled exhibit multiple paternity (Hyperprosopon anale, the basal taxon from the only clade that has not previously been investigated) indicating that this tactic is a shared reproductive strategy among surfperches. Further, we detected evidence for a Bateman gradient in H. anale; however, this result was not significant after correction for biases. Similarly, we found evidence for multiple paternity in 83% of the shiner surfperch families (Cymatogaster aggregata) sampled. When we combine these data with a previous study on the same species, representing a larger range of reproductive size classes and associated brood sizes, we detect a Bateman gradient in shiner surfperch for the first time that remains significant after several conservative tests for bias correction. These results indicate that sexual selection is likely complex in this system, with the potential for conflicting optima between sexes, and imply a positive shift in fertility (i.e., increasing number) and reproductive tactic with respect to the mating system and number of sires throughout the reproductive life history of females. We argue that the complex reproductive natural history of surfperches is characterized by several traits that may be associated with cryptic female choice, including protracted oogenesis, uterine sac complexity, and sperm storage.     

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.     

Sperm competition and the significance of female multiple mating in the predatory mite Parasitus fimetorum

Females of the predatory mite Parasitus fimetorum (Gamasida; Parasitina) inhabiting animal manure indiscriminately copulate with many mates. The sperm competition between the males was estimated by electrophoresis of allozymes and the effects of multiple mating on female reproduction were investigated. When females were forced to mate only once, their fecundity decreased drastically compared to the case of multiple mating (but longevity was unaffected). When one female mated with two males, the outcome of sperm competition depended greatly upon the mating interval. When the second mating occurred immediately after the first, the female fecundity decreased as in the case of single mating and the second male fertilized only a few eggs. However, when there was an interval of 1 day between the two matings, the females achieved normal fecundity and the second male fertilized approximately half the eggs. This suggests that the spermatophore deposited by the first male may act as a short-term copulatory plug in the female's genital opening. When one female mated with several males with 1 day intervals, three or more males shared fertilization of the eggs. This study suggests that the multiple mating of females is a necessary stimulus to continue oogenesis and some physiological factors for this stimulation may exist in spermatophores.     

Genetic diversity does not explain variation in extra‐pair paternity in multiple populations of a songbird

Many songbirds are socially monogamous but genetically polyandrous, mating with individuals outside their pair bonds. Extra‐pair paternity (EPP) varies within and across species, but reasons for this variation remain unclear. One possible source of variation is population genetic diversity, which has been shown in interspecific meta‐analyses to correlate with EPP but which has limited support from intraspecific tests. Using eight populations of the genetically polyandrous red‐winged blackbird (Agelaius phoeniceus), including an island population, we investigated whether population‐level differences in genetic diversity led to differences in EPP. We first measured genetic diversity over 10 microsatellite loci and found, as predicted, low genetic diversity in the island population. Additional structure analyses with multilocus genotypes and mtDNA showed the island population to be distinct from the continental populations. However, the island population's EPP rate fell in the middle of the continental populations' distribution, whereas the continental populations themselves showed significant variation in EPP. This result suggests that genetic diversity by itself is not a predictor of EPP rate. We discuss reasons for the departure from previous results, including hypotheses for EPP that do not solely implicate female‐driven behaviour.     

Multiple paternity and sperm competition in the sibling species Drosophila buzzatii and Drosophila koepferae

Sperm competition (SC) is a major component of sexual selection that enhances intra‐ and intersexual conflicts and may trigger rapid adaptive evolution of sexual characters. The actual role of SC on rapid evolution, however, is poorly understood. Besides, the relative contribution of distinctive features of the mating system to among species variation in the strength of SC remains unclear. Here, we assessed the strength of SC and mating system factors that may account for it in the closely related species Drosophila buzzatii and Drosophila koepferae. Our analyses reveal higher incidence of multiple paternity and SC risk in D. buzzatii wild‐inseminated females. The estimated number of fathers per brood was 3.57 in D. buzzatii and 1.95 in D. koepferae. In turn, the expected proportion of females inseminated by more than one male was 0.89 in D. buzzatii and 0.58 in D. koepferae. Laboratory experiments show that this pattern may be accounted for by the faster rate of stored sperm usage observed in D. koepferae and by the greater female remating rate exhibited by D. buzzatii. We also found that the male reproductive cost of SC is also higher in D. buzzatii. After a female mated with a second male, first‐mating male fertility was reduced by 71.4% in D. buzzatii and only 33.3% in D. koepferae. Therefore, we may conclude that postmating sexual selection via SC is a stronger evolutionary force in D. buzzatii than in its sibling.     

Benefits of polyandry: Molecular evidence from field‐caught dung beetles

When females mate with multiple males, they set the stage for postcopulatory sexual selection via sperm competition and/or cryptic female choice. Surprisingly little is known about the rates of multiple mating by females in the wild, despite the importance of this information in understanding the potential for postcopulatory sexual selection to drive the evolution of reproductive behaviour, morphology and physiology. Dung beetles in the genus Onthophagus have become a laboratory model for studying pre‐ and postcopulatory sexual selection, yet we still lack information about the reproductive behaviour of female dung beetles in natural populations. Here, we develop microsatellite markers for Onthophagus taurus and use them to genotype the offspring of wild‐caught females and to estimate natural rates of multiple mating and patterns of sperm utilization. We found that O. taurus females are highly polyandrous: 88% of females produced clutches sired by at least two males, and 5% produced clutches with as many as five sires. Several females (23%) produced clutches with significant paternity skew, indicating the potential for strong postcopulatory sexual selection in natural populations. There were also strong positive correlations between the number of offspring produced and both number of fathers and paternity skew, which suggests that females benefit from mating polyandrously by inciting postcopulatory mechanisms that bias paternity towards males that can sire more viable offspring. This study evaluates the fitness consequences of polyandry for an insect in the wild and provides strong evidence that female dung beetles benefit from multiple mating under natural conditions.     

Random size‐assortative mating despite size‐dependent fecundity in a Neotropical amphibian with explosive reproduction

Sexual selection theory predicts that, when body size is correlated with fecundity, there should be fitness advantages for mate choice of the largest females. Moreover, because larger males are expected to monopolise the largest females, this should result in an assortative mating based on body size. Although such patterns could be expected in both explosive and prolonged breeders, non‐assortative mating should be more widespread in species under time constraints. However, patterns of sexual selection are largely unexplored in explosive breeding species, and contrasting patterns have been found previously. We expect that the active choice of partners may be particularly risky when the time period during which sexual partners are available is severely limited. Therefore, to avoid missing an entire reproductive act, males and females should pair irrespective of traits, such as body size. We tested this hypothesis by investigating the mating patterns of the Pacific horned toad, Ceratophrys stolzmanni, a short‐lived fossorial species inhabiting Neotropical dry forests. This species is particularly adequate to test our prediction because it reproduces explosively over the course of a single night per year. Although the number of eggs laid was proportional to the size of females, and individuals of both sexes showed variation in body size, there was no assortative mating based either on size, body condition or age of mates. Egg size was not influenced by either female size or clutch size. The larger body size of females compared to males is likely due to fecundity selection, that is, the selective pressure that enhances reproductive output. Although we cannot dismiss the possibility that individuals could select their partners based on other criteria than those related to size or age, the results fit well our prediction, showing that the explosive breeding makes improbable an active choice of partners in both sexes and therefore favours a random mating pattern.     

The accumulation of reproductive isolation in early stages of divergence supports a role for sexual selection

Models of speciation by sexual selection propose that male–female coevolution leads to the rapid evolution of behavioural reproductive isolation. Here, we compare the strength of behavioural isolation to ecological isolation, gametic incompatibility and hybrid inviability in a group of dichromatic stream fishes. In addition, we examine whether any of these individual barriers, or a combined measure of total isolation, is predicted by body shape differences, male colour differences, environmental differences or genetic distance. Behavioural isolation reaches the highest values of any barrier and is significantly greater than ecological isolation. No individual reproductive barrier is associated with any of the predictor variables. However, marginally significant relationships between male colour and body shape differences with ecological and behavioural isolation are discussed. Differences in male colour and body shape predict total reproductive isolation between species; hierarchical partitioning of these two variables' effects suggests a stronger role for male colour differences. Together, these results suggest an important role for divergent sexual selection in darter speciation but raise new questions about the mechanisms of sexual selection at play and the role of male nuptial ornaments.     

Patterns of extra‐pair paternity in mountain chickadees

Extra‐pair paternity (EPP) is common in chickadees and often attributed to the good genes hypothesis. Females generally seek dominant males, who are typically larger, older and sing at higher rates than subordinate males, as extra‐pair sires. In other songbird species, habitat quality and urbanization have been found to influence EPP. Mountain chickadees commonly inhabit suburban habitat, and previous research on our population has shown urbanization may provide benefits to these adaptable songbirds. Here, we ask how individual condition and urbanization influence rates of EPP in mountain chickadees. Over three breeding seasons, we monitored mountain chickadee nests in urban and rural habitat, and determined parentage by genotyping adults and nestlings at six microsatellite loci. Extra‐pair paternity is common in mountain chickadees, with extra‐pair offspring (EPO) in 43.2% of nests and accounting for 17.9% of offspring. We found tenuous support for the good genes hypothesis with females tending to engage in EPCs with older males. However, we did not find an influence of male or female condition on the proportion of EPO in a nest. In addition, we did not find a significant effect of habitat on EPP rates, suggesting the impacts of urbanization on mountain chickadee reproduction may not extend to altering extra‐pair behaviour.     

Heritability,evolvability, phenotypic plasticity and temporal variation in sperm‐competition success of Drosophila melanogaster

Sperm‐competition success (SCS) is seen as centrally important for evolutionary change: superior fathers sire superior sons and thereby inherit the traits that make them superior. Additional hypotheses, that phenotypic plasticity in SCS and sperm ageing explain variation in paternity, are less considered. Even though various alleles have individually been shown to be correlated with variation in SCS, few studies have addressed the heritability, or evolvability, of overall SCS. Those studies that have addressed found low or no heritability and have not examined evolvability. They have further not excluded phenotypic plasticity, and temporal effects on SCS, despite their known dramatic effects on sperm function. In Drosophila melanogaster, we found that both standard components of sperm competition, sperm defence and sperm offence, showed nonsignificant heritability across several offspring cohorts. Instead, our analysis revealed, for the first time, the existence of phenotypic plasticity in SCS across an extreme environment (5% CO₂), and an influence of sperm ageing. Evolvability of SCS was substantial for sperm defence but weak for sperm offence. Our results suggest that the paradigm of explaining evolution by sperm competition is more complex and will benefit from further experimental work on the heritability or evolvability of SCS, measuring phenotypic plasticity, and separating the effects of sperm competition and sperm ageing.     

Genetic variation in nuclear and mitochondrial markers supports a large sex difference in lifetime reproductive skew in a lekking species

Sex differences in skews of vertebrate lifetime reproductive success are difficult to measure directly. Evolutionary histories of differential skew should be detectable in the genome. For example, male‐biased skew should reduce variation in the biparentally inherited genome relative to the maternally inherited genome. We tested this approach in lek‐breeding ruff (Class Aves, Philomachus pugnax) by comparing genetic variation of nuclear microsatellites (θ_n; biparental) versus mitochondrial D‐loop sequences (θ_m; maternal), and conversion to comparable nuclear (N_e) and female (N_ef) effective population size using published ranges of mutation rates for each marker (μ). We provide a Bayesian method to calculate N_e (θ_n = 4N_eμ_n) and N_ef (θ_m = 2N_efμ_m) using 95% credible intervals (CI) of θ_n and θ_m as informative priors, and accounting for uncertainty in μ. In 96 male ruffs from one population, N_e was 97% (79–100%) lower than expected under random mating in an ideal population, where N_e:N_ef = 2. This substantially lower autosomal variation represents the first genomic support of strong male reproductive skew in a lekking species.