Artificial selection and heritability of sperm length in Gryllus bimaculatus

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulations of high promiscuity (10 mates per female) showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2-3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Furthermore, when promiscuity is low, disruptive selection on male traits will occur under much more limited circumstances (i.e. only with higher among-female variation) than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.

Reproductive context of extremely short sperm in the parasitic wasp Cotesia congregata (Hymenoptera: Braconidae)

Among adaptive traits under sexual selection, the length of spermatozoa shows high interspecific variation. In insects, extremes exist for both short and long sperm. The spermatozoa of the endoparasitic wasp Cotesia congregata (Say) are the shortest flagellated sperm described in animals, 6.6 µm in length. By comparison, the sperm of Drosophila bifurca are almost 6000 times longer. Thus, C. congregata has the potential to shed light on the selection pressures that drive variation in sperm length in relation to their production and use. The reproductive organs, sperm counts, controlled oviposition and sex ratios were investigated. The testes showed stratified differentiation stages of spermatogenesis, and sperm counts revealed continuous spermatogenesis in the late pupal stage. The small female spermatheca stored ~1000 sperm, resulting in an extremely high sperm concentration. The number of progeny per brood decreased over time until depletion of eggs. Females produced up to 370 daughters, corresponding to the effective use of 34% of the average sperm stock. Haploid males made up a greater proportion of broods in later parasitisms. Sperm miniaturization may be an adaptation to transfer increased quantities for the entire reproductive life of females in the absence of sperm competition but in the reduced space offered by the spermatheca.

Complex interactions between sperm viability and female fertility

Sperm viability is a major male fitness component, with higher sperm viability associated with enhanced sperm competitiveness. While many studies have focussed on sperm viability from the male fitness standpoint, its impact on female fitness is less clear. Here we used a panel of 32 isogenic Drosophila simulans lines to test for genetic variation in sperm viability (percentage of viable cells). We then tested whether sperm viability affected female fitness by mating females to males from low or high sperm viability genotypes. We found significant variation in sperm viability among genotypes, and consistent with this, sperm viability was highly repeatable within genotypes. Additionally, females mated to high sperm viability males laid more eggs in the first seven hours after mating, and produced more offspring in total. However, the early increase in oviposition did not result in more offspring in the 8 hours following mating, suggesting that mating with high sperm-viability genotypes leads to egg wastage for females shortly after copulation. Although mating with high sperm-viability males resulted in higher female fitness in the long term, high quality ejaculates would result in a short-term female fitness penalty, or at least lower realised fitness, potentially generating sexual conflict over optimal sperm viability.

Natural Variation and Genetic Determinants of Caenorhabditis elegans Sperm Size

The diversity in sperm shape and size represents a powerful paradigm to understand how selection drives the evolutionary diversification of cell morphology. Experimental work on the sperm biology of the male-hermaphrodite nematode Caenorhabditis elegans has elucidated diverse factors important for sperm fertilization success, including the competitive superiority of larger sperm. Yet despite extensive research, the molecular mechanisms regulating C. elegans sperm size and the genetic basis underlying natural variation in sperm size remain unknown. To address these questions, we quantified male sperm size variation of a worldwide panel of 97 genetically distinct C. elegans strains, allowing us to uncover significant genetic variation in male sperm size. Aiming to characterize the molecular genetic basis of C. elegans male sperm size variation using a genome-wide association study, we did not detect any significant quantitative trait loci. We therefore focused on the genetic analysis of pronounced sperm size differences observed between recently diverged laboratory strains (N2 vs. LSJ1/2). Using mutants and quantitative complementation tests, we demonstrate that variation in the gene nurf-1 underlies the evolution of small sperm in the LSJ lineage. Given the previous discovery that this same nurf-1 variation was central for hermaphrodite laboratory adaptation, the evolution of reduced male sperm size in LSJ strains likely reflects a pleiotropic consequence. Together, our results provide a comprehensive quantification of natural variation in C. elegans sperm size and first insights into the genetic determinants of Caenorhabditis sperm size, pointing at an involvement of the NURF chromatin remodeling complex.

The problem of measuring trait-preference correlations without disrupting them

A key element at the heart of the Fisher–Lande–Kirkpatrick model of the sexual selection process is the genetic correlation between (male) trait and (female) preference. The strength of this association is critical in determining a population’s evolutionary trajectory, which is why estimating its magnitude is so important. In the Lande model, the trait-preference correlation is solely established and maintained by mate choice, and although it is unclear how exclusively mate choice does this in nature, the experimental designs typically employed to measure trait-preference genetic correlations could be systematically weakening estimates by not allowing free mate choice (similarly with husbandry practices). The precise impact of the problem is unknown, and possibly unknowable, but simple solutions can be applied to ensure the accuracy of trait-preference correlation estimates is maximized.

Moderate heritability and low evolvability of sperm morphology in a species with high risk of sperm competition, the collared flycatcher Ficedula albicollis

Spermatozoa represent the morphologically most diverse type of animal cells and show remarkable variation in size across and also within species. To understand the evolution of this diversity, it is important to reveal to what degree this variation is genetic or environmental in origin and whether this depends on species' life histories. Here we applied quantitative genetic methods to a pedigreed multigenerational data set of the collared flycatcher Ficedula albicollis, a passerine bird with high levels of extra-pair paternity, to partition genetic and environmental sources of phenotypic variation in sperm dimensions for the first time in a natural population. Narrow-sense heritability (h² ) of total sperm length amounted to 0.44 ± 0.14 SE, whereas the corresponding figure for evolvability (estimated as coefficient of additive genetic variation, CV_a ) was 0.02 ± 0.003 SE. We also found an increase in total sperm length within individual males between the arrival and nestling period. This seasonal variation may reflect constraints in the production of fully elongated spermatozoa shortly after arrival at the breeding grounds. There was no evidence of an effect of male age on sperm dimensions. In many previous studies on laboratory populations of several insect, mammal and avian species, heritabilities of sperm morphology were higher, whereas evolvabilities were similar. Explanations for the differences in heritability may include variation in the environment (laboratory vs. wild), intensity of sexual selection via sperm competition (high vs. low) and genetic architecture that involves unusual linkage disequilibrium coupled with overdominance in one of the studied species.

No support for the sexy-sperm hypothesis in the seed beetle: Sons of monandrous females fare better in post-copulatory competition

The sexy-sperm hypothesis posits that polyandrous females derive an indirect fitness benefit from multi-male mating because they increase the probability their eggs are fertilized by males whose sperm have high fertilizing efficiency, which is assumed to be heritable and conferred on their sons. However, whether this process occurs is contentious because father-to-son heritability may be constrained by the genetic architecture underlying traits important in sperm competition within certain species. Previous empirical work has revealed such genetic constraints in the seed beetle, Callosobruchus maculatus, a model system in sperm competition studies in which female multi-male mating is ubiquitous. Using the seed beetle, I tested a critical prediction of the sexy-sperm hypothesis that polyandrous females produce sons that are on average more successful under sperm competition than sons from monandrous females. Contrary to the prediction of the sexy-sperm hypothesis, I found that sons from monandrous females had significantly higher relative paternity in competitive double matings. Moreover, post hoc analyses revealed that these sons produced significantly larger ejaculates when second to mate, despite being smaller. This study is the first to provide empirical evidence for post-copulatory processes favoring monandrous sons and discusses potential explanations for the unexpected bias in paternity.

The repeatability of mating failure in a polyandrous bug

Mating failure, characterized by the lack of production of offspring following copulation, is relatively common across taxa yet is little understood. It is unclear whether mating failures are stochastic occurrences between incompatible mating partners or represent a persistent, meaningful phenotype on the part of one or other sex. Here we test this in the seed bug Lygaeus simulans, by sequentially mating families of males with randomly allocated unrelated females and calculating the repeatability of mating outcome for each individual male and family. Mating outcome is significantly repeatable within individual males but not across full‐sib brothers. We conclude that mating failure represents a consistent male‐associated phenotype with low heritability in this species, affected by as yet undetermined environmental influences on males.

Experimental removal of sexual selection leads to decreased investment in an immune component in female Tribolium castaneum

Because of divergent selection acting on males and females arising from different life-history strategies, polyandry can be expected to promote sexual dimorphism of investment into immune function. In previous work we have established the existence of such divergence within populations where males and females are exposed to varying degrees of polyandry. We here test whether the removal of sexual selection via enforced monogamy generates males and females that have similar levels of investment in immune function. To test this prediction experimentally, we measured differences between the sexes in a key immune measurement (phenoloxidase (PO) activity) and resistance to the microsporidian Paranosema whitei in Tribolium castaneum lines that evolved under monogamous (sexual selection absent) vs polyandrous (sexual selection present) mating systems. At generation 49, all selected lines were simultaneously assessed for PO activity and resistance to their natural parasite P. whitei after two generations of relaxed selection. We found that the polyandrous regime was associated with a clear dimorphism in immune function: females had significantly higher PO activities than males in these lines. In contrast, there was no such difference between the sexes in the lines evolving under the monogamous regime. Survival in the infection experiment did not differ between mating systems or sexes. Removing sexual selection via enforced monogamy thus seems to erase intersexual differences in immunity investment. We suggest that higher PO activities in females that have evolved under sexual selection might be driven by the increased risk of infections and/or injuries associated with exposure to multiple males.

Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology

Rapid diversification of sexual traits is frequently attributed to sexual selection, though explicit tests of this hypothesis remain limited. Spermatozoa exhibit remarkable variability in size and shape, and studies report a correlation between sperm morphology (sperm length and shape) and sperm competition risk or female reproductive tract morphology. However, whether postcopulatory processes (e.g., sperm competition and cryptic female choice) influence the speed of evolutionary diversification in sperm form is unknown. Using passerine birds, we quantified evolutionary rates of sperm length divergence among lineages (i.e., species pairs) and determined whether these rates varied with the level of sperm competition (estimated as relative testes mass). We found that relative testes mass was significantly and positively associated with more rapid phenotypic divergence in sperm midpiece and flagellum lengths, as well as total sperm length. In contrast, there was no association between relative testes mass and rates of evolutionary divergence in sperm head size, and models suggested that head length is evolutionarily constrained. Our results are the first to show an association between the strength of sperm competition and the speed of sperm evolution, and suggest that postcopulatory sexual selection promotes rapid evolutionary diversification of sperm morphology.

Insect host-parasite coevolution in the light of experimental evolution

The many ways parasites can impact their host species have been the focus of intense study using a range of approaches. A particularly promising but under-used method in this context is experimental evolution, because it allows targeted manipulation of known populations exposed to contrasting conditions. The strong potential of applying this method to the study of insect hosts and their associated parasites is demonstrated by the few available long-term experiments where insects have been exposed to parasites. In this review, we summarize these studies, which have delivered valuable insights into the evolution of resistance in response to parasite pressure, the underlying mechanisms, as well as correlated genetic responses. We further assess findings from relevant artificial selection studies in the interrelated contexts of immunity, life history, and reproduction. In addition, we discuss a number of well-studied Tribolium castaneum-Nosema whitei coevolution experiments in more detail and provide suggestions for research. Specifically, we suggest that future experiments should also be performed using nonmodel hosts and should incorporate contrasting experimental conditions, such as population sizes or environments. Finally, we expect that adding a third partner, for example, a second parasite or symbiont, to a host-parasite system could strongly impact (co)evolutionary dynamics.

The heritability of mating behaviour in a fly and its plasticity in response to the threat of sperm competition

Phenotypic plasticity is a key mechanism by which animals can cope with rapidly changeable environments, but the evolutionary lability of such plasticity remains unclear. The socio-sexual environment can fluctuate very rapidly, affecting both the frequency of mating opportunities and the level of competition males may face. Males of many species show plastic behavioural responses to changes in social environment, in particular the presence of rival males. For example, Drosophila pseudoobscura males respond to rivals by extending mating duration and increasing ejaculate size. Whilst such responses are predicted to be adaptive, the extent to which the magnitude of response is heritable, and hence selectable, is unknown. We investigated this using isofemale lines of the fruit fly D. pseudoobscura, estimating heritability of mating duration in males exposed or not to a rival, and any genetic basis to the change in this trait between these environments (i.e. degree of plasticity). The two populations differed in population sex ratio, and the presence of a sex ratio distorting selfish chromosome. We find that mating duration is heritable, but no evidence of population differences. We find no significant heritability of plasticity in mating duration in one population, but borderline significant heritability of plasticity in the second. This difference between populations might be related to the presence of the sex ratio distorting selfish gene in the latter population, but this will require investigation in additional populations to draw any conclusions. We suggest that there is scope for selection to produce an evolutionary response in the plasticity of mating duration in response to rivals in D. pseudoobscura, at least in some populations.

Ecological selection as the cause and sexual differentiation as the consequence of species divergence?

Key conceptual issues about speciation go unanswered without consideration of non-mutually exclusive factors. With tests based on speciation theory, we exploit the island distribution and habitat differences exhibited by the Caribbean cricket Amphiacusta sanctaecrucis, and with an analysis of divergent ecological selection, sexually selected differentiation and geographical isolation, address how these different factors interact. After testing for divergent selection by comparing neutral genetic and morphological divergence in one ecological (mandible shape) and one sexual (male genitalia shape) trait, we examine whether ecological or sexual selection is the primary mechanism driving population divergence. We find that all three factors--isolation, ecological and sexual selection--contribute to divergence, and that their interaction determines the stage of completeness achieved during the speciation process, as measured by patterns of genetic differentiation. Moreover, despite the striking diversity in genitalic shapes across the genus Amphiacusta, which suggests that sexual selection drives speciation, the significant differences in genitalia shape between forest habitats revealed here implies that ecological divergence may be the primary axis of divergence. Our work highlights critical unstudied aspects in speciation-differentiating the cause from the consequence of divergence-and suggests avenues for further disentangling the roles of natural and sexual selection in driving divergence in Amphiacusta.

Response to selection and realized heritability of sperm length in the yellow dung fly (Scathophaga stercoraria)

Sperm length shows considerable phenotypic variation both inter- and intra-specifically, but a general explanation for this variation is lacking. In addition, our understanding of the genetic variation underlying sperm length variation is also limited because there have been few studies on the genetics of sperm size. One factor that could explain the variation in sperm length is that length influences sperm competitiveness, and there is some evidence for this. However, in yellow dung flies (Scathophaga stercoraria), microevolutionary responses to experimental variation at levels of sperm competition indicate that sperm length does not influence sperm competitiveness, although this lack of response may simply indicate sperm length lacks evolutionary potential (that is, it is constrained in some way), in spite of evidence that sperm length is heritable. Here we report on a laboratory study, in which we artificially selected upwards and downwards on sperm length in S. stercoraria. We found that sperm length significantly diverged after four generations of selection, but the response to selection was asymmetrical: upward selection generated a rapid response, but downward did not. We estimated the realized heritability of sperm length to be approximately 50%, which is consistent with previous sire-son estimates. We also assessed the fertility of males from upward and downward lines and found they did not differ. Results are discussed in the context of sperm competition.

Mitochondrial haplotype does not affect sperm velocity in the zebra finch Taeniopygia guttata

Mitochondrial DNA (mtDNA) variation has been suggested as a possible cause of variation in male fertility because sperm activity is tightly coupled to mitochondrial oxidative phosphorylation and ATP production, both of which are sensitive to mtDNA mutations. Since male-specific phenotypes such as sperm have no fitness consequences for mitochondria due to maternal mitochondrial (and mtDNA) inheritance, mtDNA mutations that are deleterious in males but which have negligible or no fitness effect in females can persist in populations. How often such mutations arise and persist is virtually unknown. To test whether there were associations between mtDNA variation and sperm performance, we haplotyped 250 zebra finches Taeniopygia guttata from a large pedigreed-population and measured sperm velocity using computer-assisted sperm analysis. Using quantitative genetic 'animal' models, we found no effect of mtDNA haplotype on sperm velocity. Therefore, there is no evidence that in this system mitochondrial mutations have asymmetric fitness effects on males and females, leading to genetic variation in male fertility that is blind to natural selection.

Maternal effects, but no good or compatible genes for sperm competitiveness in Australian crickets

Explanations for the evolution of polyandry often center on the idea that females garner genetic benefits for their offspring by mating multiply. Furthermore, postcopulatory processes are thought to be fundamental to enabling polyandrous females to screen for genetic quality. Much attention has focused on the potential for polyandrous females to accrue such benefits via a sexy- or good-sperm mechanism, whereby additive variation exists among males in sperm competitiveness. Likewise, attention has focused on an alternative model, in which offspring quality (in this context, the sperm competitiveness of sons) hinges on an interaction between parental haplotypes (genetic compatibility). Sperm competitiveness that is contingent on parental compatibility will exhibit nonadditive genetic variation. We tested these models in the Australian cricket, Teleogryllus oceanicus, using a design that allowed us to partition additive, nonadditive genetic, and parental variance for sperm competitiveness. We found an absence of additive and nonadditive genetic variance in this species, challenging the direct relevance of either model to the evolution of sperm competitiveness in particular, and polyandry in general. Instead, we found maternal effects that were possibly sex-linked or cytoplasmically linked. We also found effects of focal male age on sperm competitiveness, with small increments in age conferring more competitive sperm.

Intra-specific variation of sperm length in the malaria vector Anopheles gambiae: males with shorter sperm have higher reproductive success

Background

Intra-specific variation in sperm length influences male reproductive success in several species of insects. In males of the malaria vector Anopheles gambiae, sperm length is highly variable but the significance of this variation is unknown. Understanding what determines the reproductive success of male mosquitoes is critical for controlling malaria, and in particular for replacing natural populations with transgenic, malaria-resistant mosquitoes.

Methods

A laboratory population of A. gambiae males was tested for intra-specific variation in sperm length. A full-sib quantitative genetic design was used to test for a genetic component of sperm length in A. gambiae males and estimate its heritability. This study also tested for a relationship between sperm length and male reproductive success in A. gambiae. Male reproductive success was measured as the proportions of inseminated and ovipositing females.

Results

There was intra-specific variation of sperm length in A. gambiae. There was no significant genetic variation in sperm length and its heritability was low (h² = 0.18) compared to other insects. Sperm length was correlated with male body size (measured as wing length). Males with short sperm had significantly higher reproductive success than males with long sperm and this was independent of body size.

Conclusion

This is the first study to demonstrate intra-specific variation in sperm length in A. gambiae and that males with short sperm have higher reproductive success. That sperm length influences female oviposition is important for any strategy considering the release of transgenic males.

Adaptation to long sperm in Drosophila: correlated development of the sperm roller and sperm packaging

Sperm are generally small and produced in huge numbers, but some species combine exaggerated sperm length with extremely limited numbers of sperm, an evolutionary trend that deviates from the theory of anisogamy. Sperm gigantism has arisen recurrently in various species, but insects exhibit the longest sperm, with some species of the Drosophilidae family producing sperm up to 6 cm in length. The anatomical, cytological, and physiological requirements for males to cope with these giant sperm were hitherto poorly understood. In this paper, we investigate the internal morphology of the male reproductive tract, and highlight specific features that may be linked to this increase in sperm size. We focus on species in the repleta group, within which sperm length varies by a factor of 35. An associated development of the sperm roller, a special twisting device inserted between the testis and the seminal vesicle, is demonstrated. Its length and the number of coils involved increase with sperm size, and it allows individual sperm to swell and roll into a spermatic pellet before reaching the seminal vesicle. This process occurs independently of and in addition to the sperm bundle coiling that takes place at the base of the testis. It is suggested that the emergence and development of the sperm roller may be a male adaptation to sperm gigantism.

Sexual selection in the cricket Gryllus bimaculatus: no good genes?

Recent studies have suggested that females of the field cricket Gryllus bimaculatus exercise post-copulatory choice over the paternity of their offspring. There is evidence that these choices are made in relation to the genetic compatibility of mates rather than their absolute quality, but the magnitude of heritable differences in males has not been thoroughly examined. Using a half-sib breeding design we measured additive genetic variance and dam effects in a suite of reproductive and non-reproductive traits. Both components explained relatively little of the phenotypic variance across traits. The dam component in our design contains variance caused by both maternal effects and dominance. If maternal effects are negligible as suggested by previous studies, our data suggest that dominance variance is an important source of variation in these traits. The lack of additive genetic variation, but possible existence of large amounts of non-additive genetic variation is consistent with the idea that female mate choice and multiple mating may be driven by differences in genetic compatibility between potential mates rather than by differences in genetic quality.

Selfish genetic elements and sexual selection: their impact on male fertility

Females of many species mate with more than one male (polyandry), yet the adaptive significance of polyandry is poorly understood. One hypothesis to explain the widespread occurrence of multiple mating is that it may allow females to utilize post-copulatory mechanisms to reduce the risk of fertilizing their eggs with sperm from incompatible males. Selfish genetic elements (SGEs) are ubiquitous in eukaryotes, frequent sources of reproductive incompatibilities, and associated with fitness costs. However, their impact on sexual selection is largely unexplored. In this review we examine the link between SGEs, male fertility and sperm competitive ability. We show there is widespread evidence that SGEs are associated with reduced fertility in both animals and plants, and present some recent data showing that males carrying SGEs have reduced paternity in sperm competition. We also discuss possible reasons why male gametes are particularly vulnerable to the selfish actions of SGEs. The widespread reduction in male fertility caused by SGEs implies polyandry may be a successful female strategy to bias paternity against SGE-carrying males.

Reproductive traits: evidence for sexually selected sperm

Sperm exhibit extraordinary morphological divergence yet precise evolutionary causes often remain elusive. A quantitative genetic study sheds light on the major role postcopulatory sexual selection could play in determining sperm size.

Genetic variation of male reproductive success in a laboratory population of Anopheles gambiae

Background

For Anopheline mosquitoes, the vectors of human malaria, genetic variation in male reproductive success can have important consequences for any control strategy based on the release of transgenic or sterile males.

Methods

A quantitative genetics approach was used to test whether there was a genetic component to variation in male reproductive success in a laboratory population of Anopheles gambiae. Swarms of full sibling brothers were mated with a fixed number of females and their reproductive success was measured as (1) proportion of ovipositing females, (2) proportion of ovipositing females that produced larvae, (3) proportion of females that produced larvae, (4) number of eggs laid per female, (5) number of larvae per ovipositing female and (6) number of larvae per female.

Results

The proportion of ovipositing females (trait 1) and the proportion of ovipositing females that produced larvae (trait 2) differed among full sib families, suggesting a genetic basis of mating success. In contrast, the other measures of male reproductive success showed little variation due to the full sib families, as their variation are probably mostly due to differences among females. While age at emergence and wing length of the males were also heritable, they were not associated with reproductive success. Larger females produced more eggs, but males did not prefer such partners.

Conclusion

The first study to quantify genetic variation for male reproductive success in A. gambiae found that while the initial stages of male reproduction (i.e. the proportion of ovipositing females and the proportion of ovipositing females that produced larvae) had a genetic basis, the overall reproductive success (i.e. the mean number of larvae per female) did not.

Sexual selection in the cricket Gryllus bimaculatus: no good genes?

Recent studies have suggested that females of the field cricket Gryllus bimaculatus exercise post-copulatory choice over the paternity of their offspring. There is evidence that these choices are made in relation to the genetic compatibility of mates rather than their absolute quality, but the magnitude of heritable differences in males has not been thoroughly examined. Using a half-sib breeding design we measured additive genetic variance and dam effects in a suite of reproductive and non-reproductive traits. Both components explained relatively little of the phenotypic variance across traits. The dam component in our design contains variance caused by both maternal effects and dominance. If maternal effects are negligible as suggested by previous studies, our data suggest that dominance variance is an important source of variation in these traits. The lack of additive genetic variation, but possible existence of large amounts of non-additive genetic variation is consistent with the idea that female mate choice and multiple mating may be driven by differences in genetic compatibility between potential mates rather than by differences in genetic quality.

Selfish genetic elements and sexual selection: their impact on male fertility

Females of many species mate with more than one male (polyandry), yet the adaptive significance of polyandry is poorly understood. One hypothesis to explain the widespread occurrence of multiple mating is that it may allow females to utilize post-copulatory mechanisms to reduce the risk of fertilizing their eggs with sperm from incompatible males. Selfish genetic elements (SGEs) are ubiquitous in eukaryotes, frequent sources of reproductive incompatibilities, and associated with fitness costs. However, their impact on sexual selection is largely unexplored. In this review we examine the link between SGEs, male fertility and sperm competitive ability. We show there is widespread evidence that SGEs are associated with reduced fertility in both animals and plants, and present some recent data showing that males carrying SGEs have reduced paternity in sperm competition. We also discuss possible reasons why male gametes are particularly vulnerable to the selfish actions of SGEs. The widespread reduction in male fertility caused by SGEs implies polyandry may be a successful female strategy to bias paternity against SGE-carrying males.

The genetic basis of traits regulating sperm competition and polyandry: can selection favour the evolution of good- and sexy-sperm?

The good-sperm and sexy-sperm (GS-SS) hypotheses predict that female multiple mating (polyandry) can fuel sexual selection for heritable male traits that promote success in sperm competition. A major prediction generated by these models, therefore, is that polyandry will benefit females indirectly via their sons' enhanced fertilization success. Furthermore, like classic 'good genes' and 'sexy son' models for the evolution of female preferences, GS-SS processes predict a genetic correlation between genes for female mating frequency (analogous to the female preference) and those for traits influencing fertilization success (the sexually selected traits). We examine the premise for these predictions by exploring the genetic basis of traits thought to influence fertilization success and female mating frequency. We also highlight recent debates that stress the possible genetic constraints to evolution of traits influencing fertilization success via GS-SS processes, including sex-linked inheritance, nonadditive effects, interacting parental genotypes, and trade-offs between integrated ejaculate components. Despite these possible constraints, the available data suggest that male traits involved in sperm competition typically exhibit substantial additive genetic variance and rapid evolutionary responses to selection. Nevertheless, the limited data on the genetic variation in female mating frequency implicate strong genetic maternal effects, including X-linkage, which is inconsistent with GS-SS processes. Although the relative paucity of studies on the genetic basis of polyandry does not allow us to draw firm conclusions about the evolutionary origins of this trait, the emerging pattern of sex linkage in genes for polyandry is more consistent with an evolutionary history of antagonistic selection over mating frequency. We advocate further development of GS-SS theory to take account of the complex evolutionary dynamics imposed by sexual conflict over mating frequency.

Effects of cytoplasmic genes on sperm viability and sperm morphology in a seed beetle: implications for sperm competition theory?

Sperm competition theory predicts that sperm traits influencing male fertilizing ability will evolve adaptively. However, it has been suggested that some sperm traits may be at least partly encoded by mitochondrial genes. If true, this may constrain the adaptive evolution of such traits because mitochondrial DNA (mtDNA) is maternally inherited and there is thus no selection on mtDNA in males. Phenotypic variation in such traits may nevertheless be high because mutations in mtDNA that have deleterious effects on male traits, but neutral or beneficial effects in females, may be maintained by random processes or selection in females. We used backcrossing to create introgression lines of seed beetles (Callosobruchus maculatus), carrying orthogonal combinations of distinct lineages of cytoplasmic and nuclear genes, and then assayed sperm viability and sperm length in all lines. We found sizeable cytoplasmic effects on both sperm traits and our analyses also suggested that the cytoplasmic effects varied across nuclear genetic backgrounds. We discuss some potential implications of these findings for sperm competition theory.

Mother's curse: the effect of mtDNA on individual fitness and population viability

The mitochondrial genome is considered generally to be an innocent bystander in adaptive evolution; however, there is increasing evidence that mitochondrial DNA (mtDNA) is an important contributor to viability and fecundity. Some of this evidence is now well documented, with mtDNA mutations having been shown to play a causal role in degenerative diseases, ageing, and cancer. However, most research on mtDNA has ignored the possibility that other instances exist where mtDNA mutations could have profound fitness consequences. Recent work in humans and other species now indicates that mtDNA mutations play an important role in sperm function, male fertility, and male fitness. Ironically, deleterious mtDNA mutations that affect only males, such as those that impair sperm function, will not be subject to natural selection because mitochondria are generally maternally inherited and could reach high frequencies in populations if the mutations are not disadvantageous in females. Here, we review how such mtDNA mutations might affect the viability of natural populations. We consider factors that increase or decrease the strength of the effect of mtDNA mutations on population viability and discuss what mechanisms exist to mitigate deleterious mtDNA effects.

Postcopulatory sexual selection is associated with reduced variation in sperm morphology

BACKGROUND

The evolutionary role of postcopulatory sexual selection in shaping male reproductive traits, including sperm morphology, is well documented in several taxa. However, previous studies have focused almost exclusively on the influence of sperm competition on variation among species. In this study we tested the hypothesis that intraspecific variation in sperm morphology is driven by the level of postcopulatory sexual selection in passerine birds.

METHODOLOGY/FINDINGS

Using two proxy measures of sperm competition level, (i) relative testes size and (ii) extrapair paternity level, we found strong evidence that intermale variation in sperm morphology is negatively associated with the degree of postcopulatory sexual selection, independently of phylogeny.

CONCLUSIONS/SIGNIFICANCE

Our results show that the role of postcopulatory sexual selection in the evolution of sperm morphology extends to an intraspecific level, reducing the variation towards what might be a species-specific 'optimum' sperm phenotype. This finding suggests that while postcopulatory selection is generally directional (e.g., favouring longer sperm) across avian species, it also acts as a stabilising evolutionary force within species under intense selection, resulting in reduced variation in sperm morphology traits. We discuss some potential evolutionary mechanisms for this pattern.

X chromosome influences sperm length in the stalk-eyed fly Cyrtodiopsis dalmanni

Whether sexually selected traits are sex linked can have profound effects on their evolution. In the diopsid stalk-eyed fly, Cyrtodiopsis dalmanni, sperm length and female reproductive tract morphology have coevolved across species, postcopulatory sexual selection is known to occur, and X-linked genes affect female ventral sperm receptacle size. Here, we estimate the location of quantitative trait loci (QTL) for spermatocyst tail length by using F2 progeny segregating for an X-linked factor that causes sex-ratio meiotic drive. We found two QTL, including a major X-linked QTL responsible for 25% of the variation in spermatocyst tail length 2.1 cM from the sex-ratio element and 0.8 cM from a major eye span QTL. Sex-ratio males produce shorter spermatocyst tails and shorter eye spans. Thus, X-linked factors affect both pre- and postcopulatory traits, and linkage between the alleles for short eye span and short spermatocyst tail length allow pre- and postcopulatory sexual selection to potentially act in concert against the transmission bias caused by the sex-ratio chromosome.

Heritability of sperm length in the bumblebee Bombus terrestris

Sperm length is highly variable, both between and within species, but the evolutionary significance of this variation is poorly understood. Sexual selection on sperm length requires a significant additive genetic variance, but few studies have actually measured this. Here we present the first estimates of narrow sense heritability of sperm length in a social insect, the bumblebee Bombus terrestris. In spite of a balanced and straightforward rearing design of colonies, and the possibility to replicate measurements of sperm within single males nested within colonies, the analysis proved to be complex. Several appropriate statistical models were derived, each depending on different assumptions. The heritability estimates obtained ranged from h (2) = 0.197 +/- 0.091 to h (2) = 0.429 +/- 0.154. All our estimates were substantially lower than previous estimates of sperm length heritability in non-social insects and vertebrates.