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Sexual selection

Publications filtered by: Sexual selection

The relative nature of fertilization success: implications for the study of post-copulatory sexual selection
Garcia-Gonzalez, F. 2008 The relative nature of fertilization success: implications for the study of post-copulatory sexual selection BMC Evolutionary Biology, 8:140
Background: The determination of genetic variation in sperm competitive ability is fundamental to distinguish between post-copulatory sexual selection models based on good-genes vs compatible genes. The sexy-sperm and the good-sperm hypotheses for the evolution of polyandry require additive (intrinsic) effects of genes influencing sperm competitiveness, whereas the genetic incompatibility hypothesis invokes non-additive genetic effects. A male's sperm competitive ability is typically estimated from his fertilization success, a measure that is dependent on the ability of rival sperm competitors to fertilize the ova. It is well known that fertilization success may be conditional to genotypic interactions among males as well as between males and females. However, the consequences of effects arising from the random sampling of sperm competitors upon the estimation of genetic variance in sperm competitiveness have been overlooked. Here I perform simulations of mating trials performed in the context of sibling analysis to investigate whether the ability to detect additive genetic variance underlying the sperm competitiveness phenotype is hindered by the relative nature of fertilization success measurements. Results: Fertilization success values render biased sperm competitive ability values. Furthermore, asymmetries among males in the errors committed when estimating sperm competitive abilities are likely to exist as long as males exhibit variation in sperm competitiveness. Critically, random effects arising from the relative nature of fertilization success lead to an underestimation of underlying additive genetic variance in sperm competitive ability. Conclusion: The results show that, regardless of the existence of genotypic interactions affecting the output of sperm competition, fertilization success is not a perfect predictor of sperm competitive ability because of the stochasticity of the background used to obtain fertilization success measures. Random effects need to be considered in the debate over the maintenance of genetic variation in sperm competitiveness, and when testing good-genes and compatible-genes processes as explanations of polyandrous behaviour using repeatability/heritability data in sperm competitive ability. These findings support the notion that the genetic incompatibility hypothesis needs to be treated as an alternative hypothesis, rather than a null hypothesis, in studies that fail to detect intrinsic sire effects on the sperm competitiveness phenotype.
Garcia-Gonzalez, F. 2008 Male genetic quality and the inequality between paternity success and fertilization success: consequences for studies of sperm competition and the evolution of polyandry Evolution, 62:1653-1665
Studies of postcopulatory sexual selection typically estimate a male’s fertilization success from his paternity success (P2) calculated at hatching or birth. However, P2 may be affected by differential embryo viability, thereby confounding estimations of true fertilization success (F2). This study examines the effects of variation in the ability of males to influence embryo viability upon the inequality between P2 and F2. It also investigates the consequences of this inequality for testing the hypothesis that polyandrous females accrue viability benefits for their offspring through facilitation of sperm competition (good-sperm model). Simulations of competitive mating trials show that although relative measures of male reproductive success tend to underestimate the strength of underlying good-sperm processes, good-sperm processes can be seriously overestimated using P2 values if males influence the viability of the embryos they sire. This study cautions the interpretation of P2 values as a proxy for fertilization success or sperm competitiveness in studies of postcopulatory sexual selection, and highlights that the good-sperm hypothesis needs empirical support from studies able to identify and separate unequivocally the males’ ability to win fertilizations from their ability to influence the development of embryos.
Gomendio, M., Garcia-Gonzalez, F., Reguera, P. & Rivero A. 2008 Male egg-carrying in Phyllomorpha laciniata is favoured by natural not sexual selection Animal Behaviour, 75:763-770

Two hypotheses could explain the evolution of paternal care: caring males are more attractive to females and mate more often (sexual selection); males care when the benefits in terms of offspring survival exceed the costs (natural selection). To test these hypotheses we used Phyllomorpha laciniata: females can choose whether to lay eggs on plants or on conspecifics, and the extent to which males carry eggs varies between populations. Our results do not support the sexual selection hypothesis: females did not choose to mate with egg-carrying males in either natural populations or experimental contexts. We compared two populations that differ in the extent of male egg carrying and we show that in the population where male egg carrying was more prevalent, parasitism pressure was higher. Field experiments revealed that, in the population with high parasitism rate, egg mortality as a result of parasitoid attack was up to 10 times higher on plants than on conspecifics. Egg carrying is thus an effective strategy that protects eggs against parasitoids. We conclude that the main benefit derived by males from egg carrying is an increase in offspring survival, and that males are sensitive to interpopulation differences in egg mortality risks. Male care in this system has evolved despite intermediate levels of paternity certainty because the impact on offspring survival is high, and the costs in terms of loss of mating opportunities low. Thus, our findings support the natural selection hypothesis, although additional work on more populations is needed to verify this.

Paternal indirect genetic effects on offspring viability and the benefits of polyandry
Garcia-Gonzalez, F. & Simmons, L. W. 2007 Paternal indirect genetic effects on offspring viability and the benefits of polyandry Current Biology, 17: 32-36
Although females are expected to maximize their reproductive success with only one or a few matings, the females of many species mate with multiple partners. Experimental studies have found evidence for an increase in egg or embryo viability when females mate polyandrously. These studies have been interpreted in the context of genetic-benefit models that propose that multiple mating increases offspring viability because it allows females to select male genotypes that influence viability directly or because it allows females to avoid genetic incompatibility. However, no studies have examined directly the precise mechanisms by which parents influence embryo viability. Using a morphological marker that enabled us to determine paternity and survival of embryos sired by individual male crickets in both sperm competitive and -noncompetitive situations, we show that males inducing high embryo viability enhance the viability of embryos sired by inferior males. These results indicate that paternal effects and interacting phenotypes determine embryo viability. They show that a male’s reproductive success is modified by the interaction between indirect genetic effects of sperm competitors. Importantly, our findings show that the benefits accruing to offspring of multiply mated females need not be transmitted genetically.
Shorter sperm confer higher competitive fertilization success
Garcia-Gonzalez, F. & Simmons, L. W. 2007 Shorter sperm confer higher competitive fertilization success Evolution, 61: 816-824

Spermatozoa exhibit taxonomically widespread patterns of divergent morphological evolution. However, the adaptive significance of variation in sperm morphology remains unclear. In this study we examine the role of natural variation in sperm length on fertilization success in the dung beetle Onthophagus taurus. We conducted sperm competition trials between males that differed in the length of their sperm and determined the paternity of resulting offspring using amplified fragment length polymorphism (AFLP) markers. We also quantified variation in the size and shape of the female’s sperm storage organ to determine whether female morphology influenced the competitiveness of different sperm morphologies. We found that fertilization success was biased toward males with relatively shorter sperm, but that selection on sperm length was dependent on female tract morphology; selection was directional for reduced sperm length across most of the spermathecal size range, but stabilizing in females with the smallest spermathecae. Our data provide empirical support for the theory that sperm competition should favor the evolution of numerous tiny sperm. Moreover, because sperm length is both heritable and genetically correlated with condition, our results are consistent with a process by which females can accrue genetic benefits for their offspring from the incitement of sperm competition and/or cryptic female choice, as proposed by the “sexy sperm” and “good sperm” models for the evolution of polyandry.

Female crickets trade offspring viability for fecundity
Simmons, L. W. & Garcia-Gonzalez, F. 2007 Female crickets trade offspring viability for fecundity Journal of Evolutionary Biology, 20: 1617-1623
A growing number of studies are suggesting that females can improve the viability of their embryos by mating with multiple males. However, the reason why females should have low rates of embryo viability is puzzling. Here we conduct a quantitative genetic study of maternal effects on embryo viability in the field cricket Teleogryllus oceanicus. After controlling for female body size, we find significant additive genetic variance for ovary weight, a measure of fecundity, and egg hatching success, a measure of embryo viability. Moreover, we show a genetic trade-off between these traits that is predicted from life-history theory. High rates of embryo mortality in this highly fecund species might therefore be explained by selection favouring an optimum balance between fecundity and embryo viability that maximizes maternal fitness. Paternal effects on female fecundity and embryo viability are often seen as benefits driving the evolution of polyandrous behaviour. However, we raise the alternative possibility that paternal effects might shift females from their naturally selected optimum, and present some support for the notion that sexual conflict over a female’s optimal fecundity and embryo viability might generate antagonistic coevolution between the sexes.
Sources of genetic and phenotypic variance in sperm performance and larval traits in a sea urchin
Evans, J. P., Garcia-Gonzalez, F. & Marshall, D. J. 2007 Sources of genetic and phenotypic variance in sperm performance and larval traits in a sea urchin Evolution, 61: 2832-2838
In nonresource based mating systems females are thought to derive indirect genetic benefits by mating with high-quality males. Such benefits can be due either to the intrinsic genetic quality of sires or to beneficial interactions between maternal and paternal haplotypes. Animals with external fertilization and no parental care offer unrivaled opportunities to address these hypotheses. With these systems, cross-classified breeding designs and in vitro fertilization can be used to disentangle sources of genetic and environmental variance in offspring fitness. Here, we employ these approaches in the Australian sea urchin Heliocidaris erythrogramma and explore how sire–dam identities influence fertilization rates, embryo viability (survival to hatching), and metamorphosis, as well as the interrelationships between these potential fitness traits. We show that fertilization is influenced by a combination of strong maternal effects and intrinsic male effects. Our subsequent analysis of embryo viability, however, revealed a highly significant interaction between parental genotypes, indicating that partial incompatibilities can severely limit offspring survival at this life-history stage. Importantly, we detected no significant relationship between fertilization rates and embryo viability. This finding suggests that fertilization rates should not be inferred from hatching rates, which is commonly practiced in species in which it is not possible to estimate fertilization at conception. Finally, we detected significant additive genetic variance due to sires in rates of juvenile metamorphosis, and a positive correlation between fertilization rates and metamorphosis. This latter finding indicates that the performance of a male’s ejaculate in noncompetitive IVF trials predicts heritable offspring traits, although the fitness implications of variance in rates of spontaneous juvenile metamorphosis have yet to be determined.
The adaptive significance of male egg carrying in the golden egg bug: defining research avenues. A reply to Härdling et al
Garcia-Gonzalez, F., Roldan, E. R. S., Ponz, F. & Gomendio, M. 2007 The adaptive significance of male egg carrying in the golden egg bug: defining research avenues. A reply to Härdling et al Ecological Entomology, 32: 578-581
The evolution of polyandry: intrinsic sire effects contribute to embryo viability
Garcia-Gonzalez, F. & Simmons, L. W. 2005 The evolution of polyandry: intrinsic sire effects contribute to embryo viability Journal of Evolutionary Biology, 18:1097-1103
Females typically mate with more than one male despite the costs incurred, thus questioning Bateman’s principle. A series of genetic benefits have been proposed to account for the evolution of polyandry, including the acquisition of viability genes for offspring. The ‘intrinsic male quality’ hypothesis suggests that polyandry increases the probability that females produce offspring sired by males that bestow high viability on their offspring. Heritable variation in viability is the basic requirement for the occurrence of this genetic benefit. By using a half-sib breeding design with a species of cricket in which polyandry is known to increase hatching success, we present clear experimental evidence that intrinsic male quality contributes to embryo viability. Despite recent support for the evolution of polyandry based on compatibility of genotypes between males and females, we show that hatching success is not determined by an interaction between paternal and maternal genotypes but rather that sons inherit paternal genes that influence the viability of eggs laid by their mates. Moreover, our data implicate a potential role for indirect genetic effects of male accessory gland products on embryo viability. Additive genetic contributions to embryo viability may be an important factor underlying the frequently observed benefits of polyandrous behaviour.
Sperm viability matters in insect sperm competition
Garcia-Gonzalez, F. & Simmons, L. W. 2005 Sperm viability matters in insect sperm competition Current Biology, 15: 271-275

Female promiscuity often results in the ejaculates of different males competing to fertilize a female's ova. Experimental studies in insects have shown how sperm competition can be a potent selective force acting on an array of male reproductive traits, including features of the ejaculate such as sperm numbers or sperm size. However, experimental analysis of the role of sperm quality in determining paternity in insects has been neglected, despite the fact that sperm quality has been shown to influence the outcome of sperm competition in vertebrates. A recent comparative analysis found that males of polyandrous insect species show a higher proportion of live sperm in their stores, suggesting that sperm competition has shaped the quality of insect sperm. Here we test the hypothesis that sperm viability influences paternity at the within-species level. We use the cricket Teleogryllus oceanicus to conduct sperm competition trials involving pre-screened males that differ in the viability of their sperm. We find that paternity success is determined by the proportion of live sperm in a male's ejaculate. Furthermore, we were able to predict the patterns of paternity observed on the basis of the males' relative representation of viable sperm in the female's sperm storage organ. Our findings provide the first experimental evidence for the theory that sperm competition selects for higher sperm quality in insects, and indicate that between-male variation in sperm quality needs to be considered in theoretical and experimental studies of insect sperm competition.