No evidence for pre-copulatory sexual selection on sperm length in a passerine bird

There is growing evidence that post-copulatory sexual selection, mediated by sperm competition, influences the evolution of sperm phenotypes. Evidence for pre-copulatory sexual selection effects on sperm traits, on the other hand, is rather scarce. A recent paper on the pied flycatcher, Ficedula hypoleuca, reported phenotypic associations between sperm length and two sexually selected male traits, i.e. plumage colour and arrival date, thus invoking pre-copulatory sexual selection for longer sperm. We were unable to replicate these associations with a larger data set from the same and two additional study populations; sperm length was not significantly related to either male plumage colour or arrival date. Furthermore, there was no significant difference in sperm length between populations despite marked differences in male plumage colour. We also found some evidence against the previously held assumption of longer sperm being qualitatively superior; longer sperm swam at the same speed as shorter sperm, but were less able to maintain speed over time. We argue that both empirical evidence and theoretical considerations suggest that the evolution of sperm morphology is not primarily associated with pre-copulatory sexual selection on male secondary sexual traits in this or other passerine bird species. The relatively large between-male variation in sperm length in this species is probably due to relaxed post-copulatory sexual selection.     

No evidence for a role of pollinator discrimination in causing selection on flower size through female reproduction

The preference for certain floral phenotypes by flower visiting animals may fuel the evolution of floral traits because variation in flower visitation rates may lead to fitness variation within a population. Here, I examine the importance of flower size for pollinator visitation rate, seed set, and seed mass in two alpine populations of the insect-pollinated herb Ranunculus acris L. during two seasons. There was no pollen limitation of seed set or mass. Pollinators discriminated strongly against flowers experimentally reduced in size. Despite this, there were no signs of any significant impact of flower size on female reproductive success. The results show that although pollinators discriminate strongly among floral phenotypes, this may not always result in female fitness differences within a population because seed set or mass is not limited by pollen availability alone. Probably abiotic environmental constraints prevent plants with high pollinator visitation from capitalizing on the high pollen deposition.     

Paternity in mallards: effects of sperm quality and female sperm selection for inbreeding avoidance

Postcopulatory processes might play an important role in sexualselection. In theory, fertilization success could be controlledby females via selection of particular sperm within their reproductivetract, or it could be determined by sperm competition per se.In practice, these two mechanisms are difficult to disentangle.To assess the relative importance of both mechanisms we usedartificial insemination in combination with measurements ofsperm quality (swimming speed and motility) in mallards. Inthis species, females often lack behavioral control over copulationsand hence may use postcopulatory mechanisms to optimize theirreproductive output. One important factor affecting female fitnessmay be selection of genetically compatible males. To investigatethe influence of sperm quality and parental relatedness on paternitywe inseminated 12 groups of related females with a sperm mixturecontaining equal numbers of sperm from a brother and from anunrelated male. Paternity was independent of the relatednessof the siring male to the female but was significantly affectedby long-term sperm swimming speed and motility. No interactionbetween relatedness and sperm quality on paternity was observed.These results suggest that female mallards are not able to selectsperm on a purely genetic basis and emphasize the importanceof sperm quality in gaining paternity.     

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

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

No evidence of sperm conjugate formation in an Australian mouse bearing sperm with three hooks

Sperm conjugation occurs when two or more sperm physically unite for motility or transport through the female reproductive tract. In many muroid rodent species, sperm conjugates have been shown to form by a single, conspicuous apical hook located on the sperm head. These sperm “trains” have been reported to be highly variable in size and, despite all the heads pointing in roughly the same direction, exhibit a relatively disordered arrangement. In some species, sperm “trains” have been shown to enhance sperm swimming speed, and thus have been suggested to be advantageous in sperm competition. Here, we assessed the behavior of sperm in the sandy inland mouse (Pseudomys hermannsburgensis), a muroid rodent that bears sperm with three apical hooks. First, we accrued genetic evidence of multiple paternity within “wild” litters to unequivocally show that sperm competition does occur in this species. Following this we utilized both in vitro and in vivo methodologies to determine whether sandy inland mouse sperm conjugate to form motile trains. Our observations of in vitro preparations of active sperm revealed that sandy inland mouse sperm exhibit rapid, progressive motility as individual cells only. Similarly, histological sections of the reproductive tracts of mated females revealed no in vivo evidence of sperm conjugate formation. We conclude that the unique, three‐hooked morphology of the sandy inland mouse sperm does not facilitate the formation of motile conjugates, and discuss our findings in relation to the different hypotheses for the evolution of the muroid rodent hook/s.     

Nonfertilizing sperm in Lepidoptera show little evidence for recurrent positive selection

Sperm are among the most variable cells in nature. Some of this variation results from nonadaptive errors in spermatogenesis, but many species consistently produce multiple sperm morphs, the adaptive significance of which remains unknown. Here, we investigate the evolution of dimorphic sperm in Lepidoptera, the butterflies and moths. Males of this order produce both fertilizing sperm and a secondary, nonfertilizing type that lacks DNA. Previous organismal studies suggested a role for nonfertilizing sperm in sperm competition, but this hypothesis has never been evaluated from a molecular framework. We combined published data sets with new sequencing in two species, the monandrous Carolina sphinx moth and the highly polyandrous monarch butterfly. Based on population genetic analyses, we see evidence for increased adaptive evolution in fertilizing sperm, but only in the polyandrous species. This signal comes primarily from a decrease in nonsynonymous polymorphism in sperm proteins compared to the rest of the genome, suggesting stronger purifying selection, consistent with selection via sperm competition. Nonfertilizing sperm proteins, in contrast, do not show an effect of mating system and do not appear to evolve differently from the background genome in either species, arguing against the involvement of nonfertilizing sperm in direct sperm competition. Based on our results and previous work, we suggest that nonfertilizing sperm may be used to delay female remating in these insects and decrease the risk of sperm competition rather than directly affect its outcome.     

No evidence of trade‐offs in the evolution of sperm numbers and sperm size in mammals

Post‐copulatory sexual selection, in the form sperm competition, has influenced the evolution of several male reproductive traits. However, theory predicts that sperm competition would lead to trade‐offs between numbers and size of spermatozoa because increased costs per cell would result in a reduction of sperm number if both traits share the same energetic budget. Theoretical models have proposed that, in large animals, increased sperm size would have minimal fitness advantage compared with increased sperm numbers. Thus, sperm numbers would evolve more rapidly than sperm size under sperm competition pressure. We tested in mammals whether sperm competition maximizes sperm numbers and size, and whether there is a trade‐off between these traits. Our results showed that sperm competition maximizes sperm numbers in eutherian and metatherian mammals. There was no evidence of a trade‐off between sperm numbers and sperm size in any of the two mammalian clades as we did not observe any significant relationship between sperm numbers and sperm size once the effect of sperm competition was taken into account. Maximization of both numbers and size in mammals may occur because each trait is crucial at different stages in sperm's life; for example size‐determined sperm velocity is a key determinant of fertilization success. In addition, numbers and size may also be influenced by diverse energetic budgets required at different stages of sperm formation.     

Experimental evidence that female ornamentation increases the acquisition of sperm and signals fecundity

Mate choice can lead to the evolution of sexual ornamentation. This idea rests on the assumption that individuals with more elaborate ornaments than competitors have higher reproductive success due to gaining greater control over mating decisions and resources provided by partners. Nevertheless, how the resources and quality of sexual partners that individuals gain access to are influenced by the ornamentation of rival individuals remains unclear. By experimentally concealing and subsequently revealing female ornaments to males, we confirm in the fowl, Gallus gallus, that female ornamentation influences male mating decisions. We further show, by manipulating the relative ornament size of females, that when females had larger ornaments than competitors they were more often preferred by males and obtained more sperm, especially from higher quality males, as measured by social status. Males may benefit by investing more sperm in females with larger ornaments as they were in better condition and produced heavier eggs. Female ornament size also decreased during incubation, providing a cue for males to avoid sexually unreceptive females. This study reveals how inter-sexual selection can lead to the evolution of female ornaments and highlights how the reproductive benefits gained from mate choice and bearing ornaments can be dependent upon social context.     

No evidence of inbreeding depression in sperm performance traits in wild song sparrows

Inbreeding is widely hypothesized to shape mating systems and population persistence, but such effects will depend on which traits show inbreeding depression. Population and evolutionary consequences could be substantial if inbreeding decreases sperm performance and hence decreases male fertilization success and female fertility. However, the magnitude of inbreeding depression in sperm performance traits has rarely been estimated in wild populations experiencing natural variation in inbreeding. Further, the hypothesis that inbreeding could increase within‐ejaculate variation in sperm traits and thereby further affect male fertilization success has not been explicitly tested. We used a wild pedigreed song sparrow (Melospiza melodia) population, where frequent extrapair copulations likely create strong postcopulatory competition for fertilization success, to quantify effects of male coefficient of inbreeding (f) on key sperm performance traits. We found no evidence of inbreeding depression in sperm motility, longevity, or velocity, and the within‐ejaculate variance in sperm velocity did not increase with male f. Contrary to inferences from highly inbred captive and experimental populations, our results imply that moderate inbreeding will not necessarily constrain sperm performance in wild populations. Consequently, the widely observed individual‐level and population‐level inbreeding depression in male and female fitness may not stem from reduced sperm performance in inbred males.     

Directional postcopulatory sexual selection is associated with female sperm storage in Trinidadian guppies

Female sperm storage (FSS) is taxonomically widespread and often associated with intense sperm competition, yet its consequences on postcopulatory sexual selection (PCSS) are poorly known. Theory predicts that FSS will reduce the strength of PCSS, because sperm characteristics favored before and after FSS may be traded‐off, and opportunities for nondirectional PCSS should increase. We explored these questions in the guppy (Poecilia reticulata), by allowing females to mate multiply and by comparing the paternity pattern in two successive broods. Contrary to predictions, the variance in male fertilization success increased after FSS, driven by a change in male paternity share across broods. This change was positively associated with sperm velocity (measured before FSS) but not with the duration of FSS, indirectly suggesting that faster sperm were better in entering female storage organs, rather than in persisting within them. Other male traits, such as male size and orange color, heterozygosity, and relatedness to the female, did not influence paternity after FSS. These results indicate that processes associated with FSS tend to reinforce the strength of PCSS in guppies, rather than weaken it. Further work is necessary to test whether this pattern changes in case of more prolonged FSS.     

Cryopreservation of epididymal sperm in tree shrews (Tupaia belangeri)

Cryopreservation of sperm from tree shrews, which are considered primitive primates, would enhance genetic management and breeding programs. Epididymal sperm were surgically harvested from male tree shrews, cryopreserved in two Tes-Tris-based cryodiluents, and used in four experiments. In Experiment 1, there were no significant differences in motility and acrosome integrity among five concentrations of egg yolk in TTE after cooling to 4 °C. However, sperm frozen in TTE containing 20% egg yolk at −172 °C/min had better (P < 0.05) post-thaw motility and acrosome integrity. In Experiment 2, sperm held for 10 min prior to storage in liquid nitrogen had greater motility than those held for 5 or 15 min (P < 0.05), but acrosome integrity was not different (P > 0.05) among treatments. In Experiment 3, sperm frozen in TTE diluent had higher (P < 0.05) motility and acrosome integrity than those in TEST diluent. In Experiment 4, there were no differences (P > 0.05) in the fertilization rate of oocytes and the proportion of tree shrews yielding fertilized oocytes, following AI with fresh versus frozen sperm. In conclusion, tree shrew epididymal sperm were successfully cryopreserved, as assessed by post-thaw motility, acrosome integrity, and fertilizing ability.     

No evidence of an MHC-based female mating preference in great reed warblers

Female mate-choice based on genetic compatibility is an area of growing interest. The major histocompatibility complex (MHC) genes are likely candidates for such mate-choice since these highly polymorphic genes may both increase offspring viability and also provide direct cues for mate-choice. In great reed warblers, females actively choose a breeding partner out of a handful of males that they visit and evaluate; thus, female preference for compatible or heterozygous MHC genes could have evolved. Here, I investigate whether great reed warbler females preferentially mate with males with dissimilar MHC class I alleles or with males that are heterozygous at MHC class I. Despite favourable conditions, a thorough screening method and a large sample size, there was no evidence of an MHC-based female mating preference based on either genetic compatibility or heterozygosity in this population. Power analyses of the data sets revealed that relatively small differences (15% and 8%, respectively) between true and random pairs should have been detected.     

树鼩进化分类地位的分子证据

树鼩隶属攀鼩目,广泛分布于东南亚、南亚和中国南部等地区。由于其独特的特点,如体型小、脑-体重比例高、生殖周期短、寿命短和饲养成本低等,在生物医学研究中被认为是可望替代灵长类动物的新型实验动物。然而,关于树鼩与灵长类动物的亲缘关系一直存在争议。明确树鼩的分类地位是创建实验动物的重要研究基础。该文介绍了近年来关于树鼩分类地位探讨的分子证据。在现有的研究中,大部分核DNA序列研究,包括近期树鼩全基因组序列分析,都支持树鼩是灵长动物的近缘旁系群,然而绝大部分基于线粒体DNA序列的研究却显示树鼩与啮齿动物的亲缘关系更为接近。这样的分歧主要是由于线粒体序列和核基因数据的差异以及不同的算法导致。综合现有不同DNA数据的研究结果,作者认为树鼩作为灵长类的近亲这一结论应该成为共识。     

No evidence that sexual selection is an 'engine of speciation' in birds

Abstract Sexual selection has been implicated as having a role in promoting speciation, as it should increase the rate of evolution of reproductive isolation, and there is some comparative evidence that sexual selection may be related to imbalances in clade size seen in resolved phylogenies. By employing a new comparative method we are able to investigate the role of sexual selection in explaining the patterns of species richness across birds. We used data for testes size as an index of post‐mating sexual selection, and sexual size dimorphism and sexual dichromatism as indices of pre‐mating sexual selection. These measures were obtained for 1031 species representing 467 genera. None of the variables investigated explained the patterns of species richness. As sexual selection may also increase extinction rates, the net effect on species richness in any given clade will depend on the balancing effects of sexual selection upon speciation and extinction rates. We suggest that variance across clades in this balance may have resulted in the lack of a relationship between species richness and sexual selection seen in birds.     

Sperm competition games: sperm selection by females

We analyse a co-evolutionary sexual conflict game, in which males compete for fertilizations (sperm competition) and females operate sperm selection against unfavourable ejaculates (cryptic female choice). For simplicity, each female mates with two males per reproductive event, and the competing ejaculates are of two types, favourable (having high viability or success) or unfavourable (where progeny are less successful). Over evolutionary time, females can increase their level of sperm selection (measured as the proportion of unfavourable sperm eliminated) by paying a fecundity cost. Males can regulate sperm allocations depending on whether they will be favoured or disfavoured, but increasing sperm allocation reduces their mating rate. The resolution of this game depends on whether males are equal, or unequal. Males could be equal: each is favoured with probability, p, reflecting the proportion of females in the population that favour his ejaculate (the 'random-roles' model); different males are favoured by different sets of females. Alternatively, males could be unequal: given males are perceived consistently by all females as two distinct types, favoured and disfavoured, where p is now the frequency of the favoured male type in the population (the 'constant-types' model). In both cases, the evolutionarily stable strategy (ESS) is for females initially to increase sperm selection from zero as the viability of offspring from unfavourable ejaculates falls below that of favourable ejaculates. But in the random-roles model, sperm selection decreases again towards zero as the unfavourable ejaculates become disastrous (i.e. as their progeny viability decreases towards zero). This occurs because males avoid expenditure in unfavourable matings, to conserve sperm for matings in the favoured role where their offspring have high viability, thus allowing females to relax sperm selection. If sperm selection is costly to females, ESS sperm selection is high across a region of intermediate viabilities. If it is uncostly, there is no ESS in this region unless sperm limitation (i.e. some eggs fail to be fertilized because sperm numbers are too low) is included into the model. In the constant-types model, no relaxation of sperm selection occurs at very low viabilities of disfavoured male progeny. If sperm selection is sufficiently costly, ESS sperm selection increases as progeny viability decreases down towards zero; but if it is uncostly, there is no ESS at the lowest viabilities, and unlike the random-roles model, this cannot be stabilized by including sperm limitation. Sperm allocations in the ESS regions differ between the two models. With random roles, males always allocate more sperm in the favoured role. With constant types, the male type that is favoured allocates less sperm than the disfavoured type. These results suggests that empiricists studying cryptic female choice and sperm allocation patterns need to determine whether sperm selection is applied differently, or consistently, on given males by different females in the same population.     

Stabilizing selection on sperm number revealed by artificial selection and experimental evolution

Sperm competition is taxonomically widespread in animals and is usually associated with large sperm production, being the number of sperm in the competing pool the prime predictor of fertilization success. Despite the strong postcopulatory selection acting directionally on sperm production, its genetic variance is often very high. This can be explained by trade‐offs between sperm production and traits associated with mate acquisition or survival, that may contribute to generate an overall stabilizing selection. To investigate this hypothesis, we first artificially selected male guppies (Poecilia reticulata) for high and low sperm production for three generations, while simultaneously removing sexual selection. Then, we interrupted artificial selection and restored sexual selection. Sperm production responded to divergent selection in one generation, and when we restored sexual selection, both high and low lines converged back to the mean sperm production of the original population within two generations, indicating that sperm number is subject to strong stabilizing total sexual selection (i.e., selection acting simultaneously on all traits associated with reproductive success). We discuss the possible mechanisms responsible for the maintenance of high genetic variability in sperm production despite strong selection acting on it.     

No evidence of mitochondrial genetic variation for sperm competition within a population of Drosophila melanogaster

Recent studies have advocated a role for mitochondrial DNA (mtDNA) in sperm competition. This is controversial because earlier theory and empirical work suggested that mitochondrial genetic variation for fitness is low. Yet, such studies dealt only with females and did not consider that variation that is neutral when expressed in females, might be non-neutral in males as, in most species, mtDNA is never selected in males. We measured male ability to compete for fertilizations, at young and late ages, across 25 cytoplasms expressed in three different nuclear genetic backgrounds, within a population of Drosophila melanogaster. We found no cytoplasmic (thus no mtDNA) genetic variation for either male offence or offensive sperm competitiveness. This contrasts with previous findings demonstrating cytoplasmic genetic variation for female fitness and female ageing across these same lines. Taken together, this suggests that mitochondrial genes do not contribute to variation in sperm competition at the within-population level.     

The capture of voles and shrews by male and female ermine Mustela erminea in captivity

The capture of meadow vole Microtus pennsylvanicus and short-tailed shrew Blarina brevicauda by male and female ermine Mustela erminea was observed in an indoor enclosure which simulated field conditions. We made 114 trials using 15 different male and 17 different female ermine. Each trial involved one ermine deprived of food for either 0, 10 or 20 h, and only one individual of either prey species. In most trials, the hunting efficiency (indicated by search and handling durations, and number of captures) of male ermine was greater than that of females. Females hardly ever captured shrews, whereas males hunted them successfully independently of the level of food deprivation. However, when deprived of food for 20 h, females spent less time than males in searching for voles. These results suggest that smaller females may be specialized and more effective at hunting meadow voles than larger males under severe food deprivation. This advantage may allow them to channel a greater proportion of energy into reproduction.     

No evidence for presence of maternal mitochondrial DNA in the sperm of Mytilus galloprovincialis males

Species of the mussel family Mytilidae have a special mitochondrial DNA (mtDNA) transmission system, known as doubly uniparental inheritance (DUI), which consists of a maternally inherited (F) and a paternally inherited (M) mitochondrial genome. Females are normally homoplasmic for the F genome and males are heteroplasmic mosaics, with their somatic tissues dominated by the maternal and their gonads dominated by the paternal genome. Several studies have indicated that the maternal genome may often be present in the male germ line. Here we report the results from the examination of mtDNA in pure sperm from more than 30 males of Mytilus galloprovincialis. In all cases, except one, we detected only the M genome. In the sperm of one male, we detected a paternal genome with an F-like primary sequence that was different from the sequence of the maternal genome in the animal's somatic tissues. We conclude that the male germ line is protected against invasion by the maternal genome. This is important because fidelity of gamete-specific transmission of the two mitochondrial genomes is a basic requirement for the stability of DUI.