Relative influence of male and female genital morphology on paternity in the dung beetle Onthophagus taurus

Studies of fertilization success have demonstrated that maleeffects are often a strong and important source of variationin P₂ (the proportion of offspring that are fertilized by thesecond male to mate). More recently there has been emphasison female processes that occur during and after copulation thatmight bias the outcome of male-male interactions. Here we usedthe sterile male technique to evaluate whether female genitalmorphology influences the repeatability of P₂ when the samepair of male dung beetles, Onthophagus taurus, copulated witha series of full-sib females or unrelated females that wereall unrelated to the male pair. Repeatability estimates of measuresof female genital morphology showed that full-sib females variedless in their genital morphology than did unrelated females.Therefore, if female genital traits are an important sourceof variation in male fertilization success, P₂ should be morerepeatable across full-sib than unrelated females. Contraryto this prediction, we show that the repeatability of P₂ didnot differ between female groups. Moreover, specific dimensionsof the female genitalia (sclerotized vagina and bursa) did notcontribute significantly to variance in P₂. In contrast, maleeffects had consistent and repeatable influences on paternityacross females. These were partly explained by variation inthe morphology of male genital sclerites.     

Long sperm fertilize more eggs in a bird

Sperm competition, in which the ejaculates of multiple males compete to fertilize a female''s ova, results in strong selection on sperm traits. Although sperm size and swimming velocity are known to independently affect fertilization success in certain species, exploring the relationship between sperm length, swimming velocity and fertilization success still remains a challenge. Here, we use the zebra finch (Taeniopygia guttata), where sperm size influences sperm swimming velocity, to determine the effect of sperm total length on fertilization success. Sperm competition experiments, in which pairs of males whose sperm differed only in length and swimming speed, revealed that males producing long sperm were more successful in terms of (i) the number of sperm reaching the ova and (ii) fertilizing those ova. Our results reveal that although sperm length is the main factor determining the outcome of sperm competition, complex interactions between male and female reproductive traits may also be important. The mechanisms underlying these interactions are poorly understood, but we suggest that differences in sperm storage and utilization by females may contribute to the outcome of sperm competition.     

Sperm competitiveness in frogs: slow and steady wins the race

When sperm compete to fertilize available ova, selection is expected to favour ejaculate traits that contribute to a male''s fertilization success. While there is much evidence to show that selection favours increased numbers of sperm, only a handful of empirical studies have examined how variation in sperm form and function contributes to competitive fertilization success. Here, we examine selection acting on sperm form and function in the externally fertilizing myobatrachid frog, Crinia georgiana. Using in vitro fertilization techniques and controlling for variation in the number of sperm contributed by males in competitive situations, we show that males with a greater proportion of motile sperm, and motile sperm with slower swimming velocities, have an advantage when competing for fertilizations. Sperm morphology and the degree of genetic similarity between putative sires and the female had no influence on competitive fertilization success. These unusual patterns of selection might explain why frog sperm typically exhibit relatively slow swimming speeds and sustained longevity.     

Sperm competition and the evolution of the sperm hook in house mice

Sperm morphology varies considerably both between and within species. The sperm of many muroid rodents bear an apical hook at the proximal end of the head. The curvature of the sperm hook varies greatly across species, however the adaptive significance of the sperm hook is currently not known. In wood mice the apical hooks intertwine to form sperm ‘trains’, which exhibit faster swimming velocities than single cells. Thus, it has been suggested that if sperm ‘trains’ were advantageous in a competitive situation, then the apical sperm hook might be an evolutionary product of selection via sperm competition. A comparative study of rodent species provided support for the hypothesis, and showed that species with higher levels of sperm competition had more reflected sperm hooks. Here, we tested this hypothesis at the intraspecific level. We quantified sperm hook morphology from seven house mouse populations, and found that interpopulation variation in hook curvature was not explained by variation in sperm competition risk. Furthermore, observations of ejaculated sperm revealed that sperm groups are not a common characteristic of mouse ejaculates. We suggest that selection for sperm attachment to the oviduct epithelium, and thus better retainment of sperm fertilizing potential, may provide a more general explanation of the evolutionary relationship between sperm competition risk and the curvature of the sperm hook among rodents, and provide a phylogenetic comparison among rodent species that supports our hypothesis.     

Sperm competition and maternal effects differentially influence testis and sperm size in Callosobruchus maculatus

The evolutionary factors affecting testis size are well documented, with sperm competition being of major importance. However, the factors affecting sperm length are not well understood; there are no clear theoretical predictions and the empirical evidence is inconsistent. Recently, maternal effects have been implicated in sperm length variation, a finding that may offer insights into its evolution. We investigated potential proximate and microevolutionary factors influencing testis and sperm size in the bruchid beetle Callosobruchus maculatus using a combined approach of an artificial evolution experiment over 90 generations and an environmental effects study. We found that while polyandry seems to select for larger testes, it had no detectable effect on sperm length. Furthermore, population density, a proximate indicator of sperm competition risk, was not significantly associated with sperm length or testis size variation. However, there were strong maternal effects influencing sperm length.     

Variation in paternity in the field cricket Teleogryllus oceanicus: no detectable influence of sperm numbers or sperm length

Recent attention has focused on the role that sperm competitionmay play in the evolution of sperm morphology. Theoretical analysespredict increased sperm size, decreased sperm size, and no changein sperm size in response to sperm competition, depending onthe assumptions made concerning the life history and functionof sperm. However, although there is good evidence that spermmorphology varies widely within and between species, the adaptivesignificance of this variation has not been examined. Here wedocument significant intraspecific variation in sperm lengthin the field cricket, Teleogryllus oceanicus. Sperm length didnot influence the rate of migration of sperm from the spermatophoreto the female's spermatheca. We performed sperm competitiontrials in which we varied the numbers of sperm transferred byeach of two males that differed in the length of sperm theyproduced. Neither sperm length nor the number of sperm transferredinfluenced paternity. The same results were obtained using twodifferent methods for assigning paternity. The distributionof paternity across a female's mates was highly variable, withfrequently one, or more in the case of females mated to fourmales, principal sire. There were no mating order effects onpaternity. These data show that sperm do not mix randomly inthe female's spermatheca. We discuss several alternative explanationsfor the patterns of paternity observed.     

Evolution of female multiple mating: A quantitative model of the “sexually selected sperm” hypothesis

Explaining the evolution and maintenance of polyandry remains a key challenge in evolutionary ecology. One appealing explanation is the sexually selected sperm (SSS) hypothesis, which proposes that polyandry evolves due to indirect selection stemming from positive genetic covariance with male fertilization efficiency, and hence with a male's success in postcopulatory competition for paternity. However, the SSS hypothesis relies on verbal analogy with “sexy-son” models explaining coevolution of female preferences for male displays, and explicit models that validate the basic SSS principle are surprisingly lacking. We developed analogous genetically explicit individual-based models describing the SSS and “sexy-son” processes. We show that the analogy between the two is only partly valid, such that the genetic correlation arising between polyandry and fertilization efficiency is generally smaller than that arising between preference and display, resulting in less reliable coevolution. Importantly, indirect selection was too weak to cause polyandry to evolve in the presence of negative direct selection. Negatively biased mutations on fertilization efficiency did not generally rescue runaway evolution of polyandry unless realized fertilization was highly skewed toward a single male, and coevolution was even weaker given random mating order effects on fertilization. Our models suggest that the SSS process is, on its own, unlikely to generally explain the evolution of polyandry.     

Coevolution of non-fertile sperm and female receptivity in a butterfly

Sexual conflict can promote rapid evolution of male and female reproductive traits. Males of many polyandrous butterflies transfer nutrients at mating that enhances female fecundity, but generates sexual conflict over female remating due to sperm competition. Butterflies produce both normal fertilizing sperm and large numbers of non-fertile sperm. In the green-veined white butterfly, Pieris napi, non-fertile sperm fill the females'' sperm storage organ, switching off receptivity and thereby reducing female remating. There is genetic variation in the number of non-fertile sperm stored, which directly relates to the female''s refractory period. There is also genetic variation in males'' sperm production. Here, we show that females'' refractory period and males'' sperm production are genetically correlated using quantitative genetic and selection experiments. Thus selection on male manipulation may increase the frequency of susceptible females to such manipulations as a correlated response and vice versa.     

The frequency of multiple paternity predicts variation in testes size among island populations of house mice

Polyandry generates selection on males through sperm competition, which has broad implications for the evolution of ejaculates and male reproductive anatomy. Comparative analyses across species and competitive mating trials within species have suggested that sperm competition can influence the evolution of testes size, sperm production and sperm form and function. Surprisingly, the intraspecific approach of comparing among population variation for investigating the selective potential of sperm competition has rarely been explored. We sampled seven island populations of house mice and determined the frequency of multiple paternity within each population. Applying the frequency of multiple paternity as an index of the risk of sperm competition, we looked for selective responses in male reproductive traits. We found that the risk of sperm competition predicted testes size across the seven island populations of house mice. However, variation in sperm traits was not explained by sperm competition risk. We discuss these findings in relation to sperm competition theory, and other intrinsic and extrinsic factors that might influence ejaculate quality.