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.     

Paternity analysis of wild‐caught females shows that sperm package size and placement influence fertilization success in the bushcricket Pholidoptera griseoaptera

In species where females store sperm, males may try to influence paternity by the strategic placement of sperm within the female's sperm storage organ. Sperm may be mixed or layered in storage organs, and this can influence sperm use beyond a ‘fair raffle’. In some insects, sperm from different matings is packaged into discrete packets (spermatodoses), which retain their integrity in the female's sperm storage organ (spermatheca), but little is known about how these may influence patterns of sperm use under natural mating conditions in wild populations. We examined the effect of the size and position of spermatodoses within the spermatheca and number of competing ejaculates on sperm use in female dark bushcrickets (Pholidoptera griseoaptera) that had mated under unmanipulated field conditions. Females were collected near the end of the mating season, and seven hypervariable microsatellite loci were used to assign paternity of eggs laid in the laboratory. Females contained a median of three spermatodoses (range 1–6), and only six of the 36 females contained more than one spermatodose of the same genotype. Both the size and relative placement of the spermatodoses within the spermatheca had a significant effect on paternity, with a bias against smaller spermatodoses and those further from the single entrance/exit of the spermatheca. A higher number of competing males reduced the chances of siring offspring for each male. Hence, both spermatodose size and relative placement in the spermatheca influence paternity success.     

Estimating genetic benefits of polyandry from experimental studies: a meta‐analysis

The consequences of polyandry for female fitness are controversial. Sexual conflict studies and a meta‐analysis of mating rates in insects suggest that there is a longevity cost when females mate repeatedly. Even so, compensatory material benefits can elevate egg production and fertility, partly because polyandry ensures an adequate sperm supply. Polyandry can therefore confer direct benefits. The main controversy surrounds genetic benefits. The argument is analogous to that surrounding the evolution of conventional female mate choice, except that with polyandry it is post‐copulatory mechanisms that might bias paternity towards males with higher breeding values for fitness. Recent meta‐analyses of extra‐pair copulations in birds have cast doubt on whether detectable genetic benefits exist. By contrast, another meta‐analysis showed that polyandry elevates egg hatching success (possibly due to a fertilization bias towards sperm with paternal genes that elevate embryo survival) in insects. A detailed summary of whether polyandry elevates other components of offspring performance is lacking. Here we present a comprehensive meta‐analysis of 232 effect sizes from 46 experimental studies. These experiments were specifically designed to try to quantify the potential genetic benefits of polyandry by controlling fully for the number of matings by females assigned to monandry and polyandry treatments. The bias‐corrected 95% confidence intervals for egg hatching success (d = ?0.01 to 0.61), clutch production (d = 0.07 to 0.45) and fertility (d = 0.04 to 0.40) all suggest that polyandry has a beneficial effect (although P values from parametric tests were marginally non‐significant at P = 0.075, 0.052 and 0.058, respectively). Polyandry was not significantly beneficial for any single offspring performance trait (e.g. growth rate, survival, adult size), but the test power was low due to small sample sizes (suggesting that many more studies are still needed). We then calculated a composite effect size that provides an index of general offspring performance. Depending on the model assumptions, the mean effect of polyandry was either significantly positive or marginally non‐significant. A possible role for publication bias is discussed. The magnitude of the reported potential genetic benefits (d = 0.07 to 0.19) are larger than those from two recent meta‐analyses comparing offspring sired by social and extra‐pair mates in birds (d = 0.02 to 0.04). This difference raises the intriguing possibility that cryptic, post‐copulatory female choice might be more likely to generate ‘good gene’ or ‘compatible gene’ benefits than female choice of mates based on the expression of secondary sexual traits.     

In the intimacy of the darkness: Genetic polyandry in deep-sea luminescent lanternsharks Etmopterus spinax and Etmopterus molleri (Squaliformes,Etmopteridae)

Multiple paternity seems common within elasmobranchs. Focusing on two deep-sea shark species, the velvet belly lanternshark (Etmopterus spinax) and the slendertail lanternshark (Etmopterus molleri) we inferred the paternity in 31 E. spinax litters from Norway (three to 18 embryos per litter) and six E. molleri litters from Japan (three to six embryos), using 21 and 10 specific microsatellites, respectively. At least two E. spinax litters were sired from multiple fathers each, with highly variable paternal skew (1:1 to 9:1). Conversely, no clear signal of genetic polyandry was found in E. molleri.     

Evidence of developmental niche construction in dung beetles: effects on growth,scaling and reproductive success

Niche construction occurs when organisms modify their environments and alter selective conditions through their physiology and behaviours. Such modifications can bias phenotypic variation and enhance organism–environment fit. Yet few studies exist that experimentally assess the degree to which environmental modifications shape developmental and fitness outcomes, how their influences may differ among species and identify the underlying proximate mechanisms. Here, we experimentally eliminate environmental modifications from the developmental environment of Onthophagus dung beetles. We show that these modifications (1) differentially influence growth among species, (2) consistently shape scaling relationships in fitness‐related traits, (3) are necessary for the maintenance of sexual dimorphism, (4) influence reproductive success among females of at least one species and (5) implicate larval cultivation of an external rumen as a possible mechanism for environmental modification. Our results present evidence that Onthophagus larvae engage in niche construction, and that this is a fundamental component of beetle development and fitness.     

Cuticular hydrocarbons as a basis for chemosensory self‐referencing in crickets: a potentially universal mechanism facilitating polyandry in insects

Females of many species obtain benefits by mating polyandrously, and often prefer novel males over previous mates. However, how do females recognise previous mates, particularly in the face of cognitive constraints? Female crickets appear to have evolved a simple but effective solution: females imbue males with their own cuticular hydrocarbons (CHCs) at mating and utilise chemosensory self‐referencing to recognise recent mates. Female CHC profiles exhibited significant additive genetic variation, demonstrating that genetically unique chemical cues are available to support chemosensory self‐referencing. CHC profiles of males became more similar to those of females after mating, indicating physical transfer of CHCs between individuals during copulation. Experimental perfuming of males with female CHCs resulted in a female aversion to males bearing chemical cues similar to their own. Chemosensory self‐referencing, therefore, could be a widespread mechanism by which females increase the diversity of their mating partners.     

Sex‐specific selection under environmental stress in seed beetles

Sexual selection can increase rates of adaptation by imposing strong selection in males, thereby allowing efficient purging of the mutation load on population fitness at a low demographic cost. Indeed, sexual selection tends to be male‐biased throughout the animal kingdom, but little empirical work has explored the ecological sensitivity of this sex difference. In this study, we generated theoretical predictions of sex‐specific strengths of selection, environmental sensitivities and genotype‐by‐environment interactions and tested them in seed beetles by manipulating either larval host plant or rearing temperature. Using fourteen isofemale lines, we measured sex‐specific reductions in fitness components, genotype‐by‐environment interactions and the strength of selection (variance in fitness) in the juvenile and adult stage. As predicted, variance in fitness increased with stress, was consistently greater in males than females for adult reproductive success (implying strong sexual selection), but was similar in the sexes in terms of juvenile survival across all levels of stress. Although genetic variance in fitness increased in magnitude under severe stress, heritability decreased and particularly so in males. Moreover, genotype‐by‐environment interactions for fitness were common but specific to the type of stress, sex and life stage, suggesting that new environments may change the relative alignment and strength of selection in males and females. Our study thus exemplifies how environmental stress can influence the relative forces of natural and sexual selection, as well as concomitant changes in genetic variance in fitness, which are predicted to have consequences for rates of adaptation in sexual populations.     

Polyandrous females provide sons with more competitive sperm: Support for the sexy‐sperm hypothesis in the rattlebox moth (Utetheisa ornatrix)

Given the costs of multiple mating, why has female polyandry evolved? Utetheisa ornatrix moths are well suited for studying multiple mating in females because females are highly polyandrous over their life span, with each male mate transferring a substantial spermatophore with both genetic and nongenetic material. The accumulation of resources might explain the prevalence of polyandry in this species, but another, not mutually exclusive, possibility is that females mate multiply to increase the probability that their sons will inherit more‐competitive sperm. This latter “sexy‐sperm” hypothesis posits that female multiple mating and male sperm competitiveness coevolve via a Fisherian runaway process. We tested the sexy‐sperm hypothesis by using competitive double matings to compare the sperm competition success of sons of polyandrous versus monandrous females. In accordance with sexy‐sperm theory, we found that in 511 offspring across 17 families, the male whose polyandrous mother mated once with each of three different males sired significantly more of all total offspring (81%) than did the male whose monandrous mother was mated thrice to a single male. Interestingly, sons of polyandrous mothers had a significantly biased sex ratio of their brood toward sons, also in support of the hypothesis.     

High levels of polyandry,but limited evidence for multiple paternity,in wild populations of the western rock lobster (Panulirus cygnus)

Polyandry, where multiple mating by females results in the temporal and spatial overlap of ejaculates from two or more males, is taxonomically widespread and occurs in varying frequencies within and among species. In decapods (crabs, lobsters, crayfish, and prawns), rates of polyandry are likely to be variable, but the extent to which patterns of multiple paternity reflect multiple mating, and thus are shaped by postmating processes that bias fertilization toward one or a subset of mated males, is unclear. Here, we use microsatellite markers to examine the frequency of multiple mating (the presence of spermatophores from two or more males) and patterns of paternity in wild populations of western rock lobster (Panulirus cygnus). Our data confirm that >45% of females had attached spermatophores arising from at least two males (i.e., confirming polyandry), but we found very limited evidence for multiple paternity; among 24 clutches sampled in this study, only two arose from fertilizations by two or more males. Single inferred paternal genotypes accounted for all remaining progeny genotypes in each clutch, including several instances when the mother had been shown to mate with two or more males. These findings highlight the need for further work to understand whether polyandry is adaptive and to uncover the mechanisms underlying postmating paternity biases in this system.     

Insights into the mating habits of the tiger salamander (Ambystoma tigrinum tigrinum) as revealed by genetic parentage analyses

Among urodeles, ambystomatid salamanders are particularly amenable to genetic parentage analyses because they are explosive aggregate breeders that typically have large progeny arrays. Such analyses can lead to direct inferences about otherwise cryptic aspects of salamander natural history, including the rate of multiple mating, individual reproductive success, and the spatial distribution of clutches. In 2002, we collected eastern tiger salamander (Ambystoma tigrinum tigrinum) egg masses (> 1000 embryos) from a approximately 80 m linear transect in Indiana, USA. Embryos were genotyped at four variable microsatellite loci and the resulting progeny array data were used to reconstruct multilocus genotypes of the parental dams and sires for each egg mass. UPGMA analysis of genetic distances among embryos resolved four instances of egg mass admixture, where two or more females had oviposited at exactly the same site resulting in the mixing of independent cohorts. In total, 41 discrete egg masses were available for parentage analyses. Twenty-three egg masses (56%) consisted exclusively of full-siblings (i.e. were singly sired) and 18 (44%) were multiply sired (mean 2.6 males/clutch). Parentage could be genetically assigned to one of 17 distinct parent pairs involving at least 15 females and 14 different males. Reproductive skew was evident among males who sired multiply sired clutches. Additional evidence of the effects of sexual selection on male reproductive success was apparent via significant positive correlations between male mating and reproductive success. Females frequently partitioned their clutches into multiple discrete egg masses that were separated from one another by as many as 43 m. Collectively, these data provide the first direct evidence for polygynandry in a wild population of tiger salamanders.     

Increase in multiple paternity across the reproductive lifespan in a sperm‐storing,hermaphroditic freshwater snail

Polyandry is a common phenomenon and challenges the traditional view of stronger sexual selection in males than in females. In simultaneous hermaphrodites, the physical proximity of both sex functions was long thought to preclude the operation of sexual selection. Laboratory studies suggest that multiple mating and polyandry in hermaphrodites may actually be common, but data from natural populations are sparse. We therefore estimated the rate of multiple paternity and its seasonal variability in the annual, sperm‐storing, simultaneously hermaphroditic freshwater snail Radix balthica for the entire duration of the reproductive lifespan. We also tested whether multiple paternity was associated with clutch size or embryonic development. To obtain these data, we measured and genotyped 60 field‐collected egg clutches using nine highly polymorphic microsatellite markers. Overall, 50% of the clutches had multiple fathers, and both the frequency (20–93% of clutches) and magnitude of multiple paternity (mean 1.3–3.8 fathers per clutch) substantially increased over time, probably because of extensive sperm storage. Most multiply sired clutches (83%) had a dominant father, but neither clutch size nor the proportion of developed embryos per clutch was associated with levels of multiple paternity. Both the evident promiscuity and the frequent skew of paternity shares suggest that sexual selection may be an important evolutionary force in the study population.     

Sperm competition and the evolution of precopulatory weapons: Testis size and amplexus position,but not arm strength,affect fertilization success in a chorusing frog

Trade‐offs between pre‐ and postcopulatory traits influence their evolution, and male expenditure on such traits is predicted to depend on the number of competitors, the benefits from investing in weapons, and the risk and intensity of sperm competition. Males of the chorusing frog Crinia georgiana use their arms as weapons in contest competition. Previously, we showed that increased numbers of rivals elevated the risk and intensity of sperm competition due to multimale amplexus, and caused a reversal in the direction of precopulatory selection on arm girth. Here, we focused on the factors affecting postcopulatory fertilization success during group spawning, using paternity data from natural choruses. Competitive fertilization success depended on the time spent amplexed and amplexus position. Relative testes size but not arm girth, contributed to fertilization success, but the effect of testes size depended on amplexus position. Our findings offer within species empirical support for recent sperm competition models that incorporate precopulatory male–male competition, and show why an understanding of the evolution of animal weapons requires a consideration of both pre‐ and postcopulatory episodes of sexual selection.     

No evidence that within‐group male relatedness reduces harm to females in Drosophila

Conflict between males and females over whether, when, and how often to mate often leads to the evolution of sexually antagonistic interactions that reduce female reproductive success. Because the offspring of relatives contribute to inclusive fitness, high relatedness between rival males might be expected to reduce competition and result in the evolution of reduced harm to females. A recent study investigated this possibility in Drosophila melanogaster and concluded that groups of brothers cause less harm to females than groups of unrelated males, attributing the effect to kin selection. That study did not control for the rearing environment of males, rendering the results impossible to interpret in the context of kin selection. Here, we conducted a similar experiment while manipulating whether males developed with kin prior to being placed with females. We found no difference between related and unrelated males in the harm caused to females when males were reared separately. In contrast, when related males developed and emerged together before the experiment, female reproductive output was higher. Our results show that relatedness among males is insufficient to reduce harm to females, while a shared rearing environment – resulting in males similar to or familiar with one another – is necessary to generate this pattern.     

Patterns of molecular evolution in dioecious and non‐dioecious Silene

Dioecy (i.e. having separate sexes) is a rather rare breeding system in flowering plants. Such rareness may result from a high probability of extinction in dioecious species because of less efficient dispersal and the costs of sexual selection, which are expected to harm dioecious species' survival on the long term. These handicaps should decrease the effective population size () of dioecious species, which in turn should reduce the efficacy of selection. Moreover, sexual selection in dioecious species is expected to specifically affect some genes, which will evolve under positive selection. The relative contribution of these effects is currently unknown and we tried to disentangle them by comparing sequence evolution between dioecious and non‐dioecious species in the Silene genus (Caryophyllaceae), where dioecy has evolved at least twice. For the dioecious species in the section Melandrium, where dioecy is the oldest, we found a global reduction of purifying selection, while on some, male‐biased genes, positive selection was found. For section Otites, where dioecy evolved more recently, we found no significant differences between dioecious and non‐dioecious species. Our results are consistent with the view that dioecy is an evolutionary dead end in flowering plants, although other scenarios for explaining reduced cannot be ruled out. Our results also show that contrasting forces act on the genomes of dioecious plants, and suggest that some time is required before the genome of such plants bears the footprints of dioecy.     

No evidence for MHC‐based mate choice in wild giant pandas

Major histocompatibility complex genes (MHC), a gene cluster that controls the immune response to parasites, are regarded as an important determinant of mate choice. However, MHC‐based mate choice studies are especially rare for endangered animals. The giant panda (Ailuropoda melanoleuca), a flagship species, has suffered habitat loss and fragmentation. We investigated the genetic variation of three MHC class II loci, including DRB1, DQA1, and DQA2, for 19 mating‐pairs and 11 parent‐pairs of wild giant pandas based on long‐term field behavior observations and genetic samples. We tested four hypotheses of mate choice based on this MHC variation. We found no supporting evidence for the MHC‐based heterosis, genetic diversity, genetic compatibility and “good gene” hypotheses. These results suggest that giant pandas may not use MHC‐based signals to select mating partners, probably because limited mating opportunities or female‐biased natal dispersal restricts selection for MHC‐based mate choice, acknowledging the caveat of the small sample size often encountered in endangered animal studies. Our study provides insight into the mate choice mechanisms of wild giant pandas and highlights the need to increase the connectivity and facilitate dispersal among fragmented populations and habitats.     

The genetic legacy of the 19th‐century decline of the British polecat: evidence for extensive introgression from feral ferrets

In the 19th century, the British polecat suffered a demographic contraction, as a consequence of direct persecution, reaching its lowest population in the years that preceded the First World War. The polecat is now recovering and expanding throughout Britain, but introgressive hybridization with feral ferrets has been reported, which could be masking the true range of the polecat and introducing domestic genes into the species. We used a fragment of the mitochondrial DNA control region and 11 microsatellite loci to characterize the frequency and extent of hybridization and introgression between the two species and assess whether the 19th‐century decline corresponded to a genetic bottleneck in the polecat. The proportion of admixture detected in the wild was high (31%) and hybrids were more frequently found outside Wales, suggesting that hybridization is more likely to occur along the eastern edge of the polecat's range expansion. The patterns observed in the mitochondrial and nuclear DNA data show that introgression was mediated by crosses between male polecats and female ferrets, whose offspring backcrossed with polecats. No first‐generation (F₁) hybrids were identified, and the broad range of observed admixture proportions agrees with a scenario of past extensive hybridization between the two species. Using several different methods to investigate demographic history, we did not find consistent evidence for a genetic bottleneck in the British polecat, a result that could be interpreted as a consequence of hybridization with ferrets. Our results highlight the importance of the Welsh polecat population for the conservation and restoration of the genetic identity of the British polecat.     

Hit‐and‐run trophallaxis of small hive beetles

Some parasites of social insects are able to exploit the exchange of food between nestmates via trophallaxis, because they are chemically disguised as nestmates. However, a few parasites succeed in trophallactic solicitation although they are attacked by workers. The underlying mechanisms are not well understood. The small hive beetle (=SHB), Aethina tumida, is such a parasite of honey bee, Apis mellifera, colonies and is able to induce trophallaxis. Here, we investigate whether SHB trophallactic solicitation is innate and affected by sex and experience. We quantified characteristics of the trophallactic solicitation in SHBs from laboratory‐reared individuals that were either bee‐naïve or had 5 days experience. The data clearly show that SHB trophallactic solicitation is innate and further suggest that it can be influenced by both experience and sex. Inexperienced SHB males begged more often than any of the other groups had longer breaks than their experienced counterparts and a longer soliciting duration than both experienced SHB males and females, suggesting that they start rather slowly and gain more from experience. Successful experienced females and males were not significantly different from each other in relation to successful trophallactic interactions, but had a significantly shorter soliciting duration compared to all other groups, except successful inexperienced females. Trophallactic solicitation success, feeding duration and begging duration were not significantly affected by either SHB sex or experience, supporting the notion that these behaviors are important for survival in host colonies. Overall, success seems to be governed by quality rather than quantity of interactions, thereby probably limiting both SHB energy investment and chance of injury (<1%). Trophallactic solicitation by SHBs is a singular example for an alternative strategy to exploit insect societies without requiring chemical disguise. Hit‐and‐run trophallaxis is an attractive test system to get an insight into trophallaxis in the social insects.