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Sexual selection

Publications filtered by: Sexual selection

Infertile matings and sperm competition: the effect of non sperm representation on intraspecific variation in sperm precedence patterns
Garcia-Gonzalez, F. 2004 Infertile matings and sperm competition: the effect of "non sperm representation" on intraspecific variation in sperm precedence patterns American Naturalist, 164: 457-472

In theoretical and experimental approaches to the study of sperm competition, it is often assumed that ejaculates always contain enough sperm of good quality and that they are successfully transferred and used for fertilization. However, this view neglects the potential effects of infertility and sperm limitation. Permanent or temporal male infertility due to male sterility, insemination failures, or failures to fertilize the ova implies that some males do not achieve sperm representation in the female reproductive tract after mating. A review of the literature suggests that rates of nonsperm representation may be high; values for the proportion of infertile matings across 30 insect species vary between 0% and 63%, with the median being 22%. I simulated P2 (the proportion of offspring fathered by the second male to copulate with a female in a double-mating trial) distributions under a mechanism of random sperm mixing when sample sizes and rates of male infertility varied. The results show that nonsperm representation can be responsible for high intraspecific variance in sperm precedence patterns and that it can generate misleading interpretations about the mechanism of sperm competition. Nonsperm representation might be a common obstacle in the studies of sperm competition and postcopulatory female choice.

Adjustment of copula duration and ejaculate size according to the risk of sperm competition in the golden egg bug (Phyllomorpha laciniata)
Garcia-Gonzalez, F. and Gomendio, M. 2004 Adjustment of copula duration and ejaculate size according to the risk of sperm competition in the golden egg bug (Phyllomorpha laciniata) Behavioral Ecology, 15:23-30
Several hypotheses have been proposed to explain the adaptive significance of prolonged copulations in insects, which include mate guarding and sperm loading functions. We have explored the adaptive significance of the prolonged copulations in the golden egg bug (copulations up to 50 h) and the effect of an increased risk of sperm competition on ejaculate investment. Our data support predictions derived from sperm competition theory, which posits that males are expected to increase ejaculate expenditure in response to an increased risk of sperm competition. Results show a combined response by males that has not been previously described: males in the presence of rivals increase copulation duration and the rate of sperm transfer. No relationship was found between male or female size and copulation duration or ejaculate size. Golden egg bug males transfer sperm slowly and gradually throughout copulation; thus an increase in the amount of sperm transferred and the corresponding increase in the male’s numerical representation in the female’s storage organs could be particularly important in a system in which so few sperm are transferred and in which so few sperm are stored by females. In addition, copulation duration may not only serve to increase the total amount of sperm transferred, but it may also increase the chances that the female will lay an egg soon after copulation has ended. This could explain why males tend to accept eggs after copulation, since they could be maximizing the chances that such eggs are fathered by them, and in this way they would substantially increase the survival rates of their offspring because eggs laid on plants suffer high mortality rates.
Sperm competition mechanisms, confidence of paternity, and the evolution of paternal care in the golden egg bug (Phyllomorpha laciniata)
Garcia-Gonzalez, F., Núñez, Y., Ponz, F., Roldán, E. R. S., and Gomendio, M. 2003 Sperm competition mechanisms, confidence of paternity, and the evolution of paternal care in the golden egg bug (Phyllomorpha laciniata) Evolution, 57:1078-1088

Theoretical models predict how paternal effort should vary depending on confidence of paternity and on the trade-offs between present and future reproduction. In this study we examine patterns of sperm precedence in Phyllomorpha laciniata and how confidence of paternity influences the willingness of males to carry eggs. Female golden egg bugs show a flexible pattern of oviposition behavior, which results in some eggs being carried by adults (mainly males) and some being laid on plants, where mortality rates are very high. Adults are more vulnerable to predators when carrying eggs; thus, it has been suggested that males should only accept eggs if there are chances that at least some of the eggs will be their true genetic offspring. We determined the confidence of paternity for naturally occurring individuals and its variation with the time. Paternity of eggs fertilized by the last males to mate with females previously mated in the field has been determined using amplified fragment length polymorphisms (AFLPs). The exclusion probability was 98%, showing that AFLP markers are suitable for paternity assignment. Sperm mixing seems the most likely mechanism of sperm competition, because the last male to copulate with field females sires an average of 43% of the eggs laid during the next five days. More importantly, the proportion of eggs sired does not change significantly during that period. We argue that intermediate levels of paternity can select for paternal care in this system because: (1) benefits of care in terms of offspring survival are very high; (2) males have nothing to gain from decreasing their parental effort in a given reproductive event because sperm mixing makes it difficult for males to reach high paternity levels and males are left with no cues to assess paternity; (3) males cannot chose to care for their offspring exclusively because they can neither discriminate their own eggs, nor can they predict when their own eggs will be produced; and (4) males suffer no loss of further matings with other females when they carry eggs. Thus, our findings do not support the traditional view that paternal investment is expected to arise only in species where confidence of paternity is high. The results suggest that females maximize the chances that several males will accept eggs at different times by promoting a mechanism of sperm mixing that ensures that all males that have copulated with a female have some chance of fathering offspring, that this probability remains constant with time, and that males have no cues as to when their own offspring will be produced.

A field test of the intraspecific brood parasitism hypothesis in the golden egg bug (Phyllormorpha laciniata)
Garcia-Gonzalez, F. and Gomendio, M. 2003 A field test of the intraspecific brood parasitism hypothesis in the golden egg bug (Phyllormorpha laciniata) Behavioral Ecology and Sociobiology, 53:332-339

In natural populations of golden egg bugs (Phyllomorpha laciniata), females lay eggs on plants where they develop unattended, or on conspecifics, where they remain firmly glued until the nymphs hatch and start an independent life. Mortality rates among eggs laid on plants are higher than among eggs carried by adults. Because females cannot lay eggs on themselves, in order to improve offspring survival, they have to lay eggs on other individuals. Two hypotheses have been proposed to explain egg carrying: (1) the mating pair intraspecific brood parasitism hypothesis suggests that females dump eggs on copulating pairs, and (2) the paternal care hypothesis suggests that the system is driven mainly by males accepting eggs to improve the survival rates of their own offspring. Our data from the field show that 77% of the eggs are carried by males, because more males than females carry eggs, and because males carry a greater number of eggs. In addition, we show that mating males carry more recently laid eggs than single males. These results support the view that egg carrying is performed predominantly by males and that eggs are laid on males by their current mating partner, probably between repeated copulations. Males are likely to accept eggs, despite intermediate levels of paternity, because they cannot discriminate in favour of their own eggs, because rejected eggs will face 97% mortality rates on plants, and because they do not suffer mating costs when they carry eggs. However, females carry 23% of the eggs, but no differences in egg carrying have been found between mating and single females, suggesting that this is not the result of egg dumping while females are copulating. Egg carrying by females could reflect low levels of intraspecific parasitism, which is likely to reflect the low rate of successful attempts by egg-laying females who try to oviposit on other conspecifics rather indiscriminately, in an effort to improve the survival of their offspring.

Variation in paternity in the field cricket Teleogryllus oceanicus: no influence of sperm numbers or sperm length
Simmons, L.W.; Wernham, J.; Garcia-Gonzalez, F. & Kamien, D. 2002 Variation in paternity in the field cricket Teleogryllus oceanicus: no influence of sperm numbers or sperm length Behavioral Ecology, 14: 539-545
Recent attention has focused on the role that sperm competition may play in the evolution of sperm morphology. Theoretical analyses predict increased sperm size, decreased sperm size, and no change in sperm size in response to sperm competition, depending on the assumptions made concerning the life history and function of sperm. However, although there is good evidence that sperm morphology varies widely within and between species, the adaptive significance of this variation has not been examined. Here we document significant intraspecific variation in sperm length in the field cricket, Teleogryllus oceanicus. Sperm length did not influence the rate of migration of sperm from the spermatophore to the female’s spermatheca. We performed sperm competition trials in which we varied the numbers of sperm transferred by each of two males that differed in the length of sperm they produced. Neither sperm length nor the number of sperm transferred influenced paternity. The same results were obtained using two different methods for assigning paternity. The distribution of paternity across a female’s mates was highly variable, with frequently one, or more in the case of females mated to four males, principal sire. There were no mating order effects on paternity. These data show that sperm do not mix randomly in the female’s spermatheca. We discuss several alternative explanations for the patterns of paternity observed.
Oviposition site selection and oviposition stimulation by conspecifics in the golden egg bug (Phyllormorpha laciniata): implications on female fitness
Garcia-Gonzalez, F. and Gomendio, M. 2003 Oviposition site selection and oviposition stimulation by conspecifics in the golden egg bug (Phyllormorpha laciniata): implications on female fitness Behavioral Ecology and Sociobiology, 53: 385-392
Phyllomorpha laciniata Vill. (Heteroptera, Coreidae) females lay eggs on the host plant and on the backs of conspecifics. Since egg survival is greater when eggs develop on the backs of conspecifics than when laid on plants, we predict that females should prefer to lay eggs on conspecifics. In addition, because conspecifics are a high-quality site that represents a limiting resource, females should experience oviposition stimulation upon an encounter with a conspecific. Our results reveal that, when both the host plant and conspecifics are available simultaneously, females lay eggs preferentially on conspecifics. The results also support the second prediction, since females housed with conspecifics lay more than twice the number of eggs than isolated females. Isolated females do not seem to retain eggs, suggesting that oviposition stimulation is the result of an acceleration of egg-maturation rates. Other studies have found oviposition stimulation by mating and have suggested that it is the result of male strategies to increase short-term male reproductive success at some cost to females. The evolutionary scenario of our model organism seems to be quite different since females benefit greatly from increasing egg laying when there are conspecifics, because the advantages in terms of offspring survival are likely to translate into substantial increases in female reproductive success.
Harm to females increases with male body size in Drosophila melanogaster
Pitnick, S. & Garcia-Gonzalez, F. 2002 Harm to females increases with male body size in Drosophila melanogaster Proceedings of the Royal Society of London B, 269: 1821-1828

Previous studies indicate that female Drosophila melanogaster are harmed by their mates through copulation. Here, we demonstrate that the harm that males inflict upon females increases with male size. Specifically, both the lifespan and egg-production rate of females decreased significantly as an increasing function of the body size of their mates. Consequently, females mating with larger males had lower lifetime fitness. The detrimental effect of male size on female longevity was not mediated by male effects on female fecundity, egg-production rate or female-remating behaviour. Similarly, the influence of male size on female lifetime fecundity was independent of the male-size effect on female longevity. There was no relationship between female size and female resistance to male harm. Thus, although increasing male body size is known to enhance male mating success, it has a detrimental effect on the direct fitness of their mates. Our results indicate that this harm is a pleiotropic effect of some other selected function and not an adaptation. To the extent that females prefer to mate with larger males, this choice is harmful, a pattern that is consistent with the theory of sexually antagonistic coevolution.

No evidence for heritability of male mating latency or copulation duration across social environments in Drosophila melanogaster
Taylor, M. L., Evans, J. P. and Garcia-Gonzalez, F. 2013 No evidence for heritability of male mating latency or copulation duration across social environments in Drosophila melanogaster PLoS ONE, 8:e77347
A key assumption underpinning major models of sexual selection is the expectation that male sexual attractiveness is heritable. Surprisingly, however, empirical tests of this assumption are relatively scarce. Here we use a paternal fullsib/half-sib breeding design to examine genetic and environmental variation in male mating latency (a proxy for sexual attractiveness) and copulation duration in a natural population of Drosophila melanogaster. As our experimental design also involved the manipulation of the social environment within each full-sibling family, we were able to further test for the presence of genotype-by-environment interactions (GEIs) in these traits, which have the potential to compromise mate choice for genetic benefits. Our experimental manipulation of the social environment revealed plastic expression of both traits; males exposed to a rival male during the sensitive period of adult sexual maturation exhibited shorter mating latencies and longer copulation durations than those who matured in isolation. However, we found no evidence for GEIs, and no significant additive genetic variation underlying these traits in either environment. These results undermine the notion that the evolution of female choice rests on covariance between female preference and male displays, an expectation that underpins indirect benefit models such as the good genes and sexy sons hypotheses. However, our results may also indicate depletion of genetic variance in these traits in the natural population studied, thus supporting the expectation that traits closely aligned with reproductive fitness can exhibit low levels of additive genetic variance.
Maternal sexual interactions affect offspring survival and ageing
Dowling, D. K., Williams, B. R., and Garcia-Gonzalez, F. 2014 Maternal sexual interactions affect offspring survival and ageing Journal of Evolutionary Biology, 27: 88-97

In many species, females exposed to increased sexual activity experience reductions in longevity. Here, in Drosophila melanogaster, we report an additional effect on females brought about by sexual interactions; an effect that spans generations. We subjected females to a sexual treatment consisting of different levels of sexual activity, and then investigated patterns of mortality in their offspring. We found reduced probabilities of survival, increases in the rate-of-senescence, and a pattern of reduced mean longevities, for offspring produced by mothers that experienced higher levels of sexual interaction. We contend that these effects constitute trans-generational costs of sexual conflict – the existence or implications of which have rarely been considered previously. Our results indicate that ongoing exposure by mothers to male pre-copulatory interactions is itself sufficient to drive trans-generational effects on offspring mortality. Thus, we show that increases in maternal sexual activity can produce trans-generational effects that permeate through to latter life-stages in the offspring. This helps to elucidate the complex interplay between sex and ageing, and provides new insights into the dynamics of adaptation under sexual selection.

Model Systems, Taxonomic Bias, And Sexual Selection: Beyond Drosophila
Zuk, M., Garcia-Gonzalez, F. Herbestein, M. E. and Simmons, L. W. 2014 Model Systems, Taxonomic Bias, And Sexual Selection: Beyond Drosophila Annual Review of Entomology, 59: 321-338

While model systems are useful in entomology, allowing generalizations to be made based on a few well-known species, they also have drawbacks. It can be difficult to know how far to generalize from information in a few species: are all flies like Drosophila? The use of model systems is particularly problematic in studying sexual selection, where variability among taxa is key to the evolution of different behaviors. A bias toward use of a few insect species, particularly Drosophila, is evident in the sexual selection and sexual conflict literature over the last several decades, although the diversity of study organisms has increased more recently. As the number of model systems used to study sexual conflict increased, support for the idea that sexual interactions resulted in harm to females decreased. Future work should choose model systems thoughtfully, combining well-known species with those that can add to the variation that allows us to make more meaningful generalizations.