Good genes and the maternal effects of polyandry on offspring reproductive success in the bulb mite

The effects of sexual selection on life-history traits: an experimental study on guppies

Sexual selection is often prevented during captive breeding in order to maximize effective population size and retain genetic diversity. However, enforcing monogamy and thereby preventing sexual selection may affect population fitness either negatively by preventing the purging of deleterious mutations or positively by reducing sexual conflicts. To better understand the effect of sexual selection on the fitness of small populations, we compared components of female fitness and the expression of male secondary sexual characters in 19 experimental populations of guppies (Poecilia reticulata) maintained under polygamous or monogamous mating regimes over nine generations. In order to generate treatments that solely differed by their level of sexual selection, the middle-class neighbourhood breeding design was enforced in the monogamous populations, while in the polygamous populations, all females contributed similarly to the next generation with one male and one female offspring. This experimental design allowed potential sexual conflicts to increase in the polygamous populations because selection could not operate on adult-female traits. Clutch size and offspring survival showed a weak decline from generation to generation but did not differ among treatments. Offspring size, however, declined across generations, but more in monogamous than polygamous populations. By generation eight, orange- and black-spot areas were larger in males from the polygamous treatment, but these differences were not statistically significant. Overall, these results suggest that neither sexual conflict nor the purging of deleterious mutation had important effects on the fitness of our experimental populations. However, only few generations of enforced monogamy in a benign environment were sufficient to negatively affect offspring size, a trait potentially crucial for survival in the wild. Sexual selection may therefore, under certain circumstances, be beneficial over enforced monogamy during captive breeding.

Rapid adaptation to mammalian sociality via sexually selected traits

BACKGROUND

Laboratory studies show that the components of sexual selection (e.g., mate choice and intrasexual competition) can profoundly affect the development and fitness of offspring. Less is known, however, about the total effects of sexual selection on offspring in normal social conditions. Complex social networks, such as dominance hierarchies, regulate the opportunity for mating success, and are often missing from laboratory studies. Social selection is an extended view of sexual selection that incorporates competition during sexual and nonsexual interactions, and predicts complex evolutionary dynamics. Whether social selection improves or constrains offspring fitness is controversial.

RESULTS

To identify fitness consequences of social selection, wild-derived mice that had bred under laboratory conditions for eight generations were re-introduced to naturalistic competition in enclosures for three consecutive generations (promiscuous line). In parallel, a control lineage bred in cages under random mate assignment (monogamous line). A direct competition experiment using second-generation animals revealed that promiscuous line males had greater reproductive success than monogamous line males (particularly during extra-territorial matings), in spite of higher mortality and equivalent success in social dominance and sperm competition. There were no major female fitness effects (though promiscuous line females had fewer litters than monogamous line females). This result suggested that selection primarily acted upon a sexually attractive male phenotype in the promiscuous line, a hypothesis we confirmed in female odor and mating preference trials.

CONCLUSIONS

We present novel evidence for the strength of sexual selection under normal social conditions, and show rapid male adaptation driven largely by sexual trait expression, with tradeoffs in survivorship and female fecundity. Re-introducing wild-derived mice to competition quickly uncovers sexually selected phenotypes otherwise lost in normal colony breeding.

Mating system affects population performance and extinction risk under environmental challenge

Failure of organisms to adapt to sudden environmental changes may lead to extinction. The type of mating system, by affecting fertility and the strength of sexual selection, may have a major impact on a population's chances to adapt and survive. Here, we use experimental evolution in bulb mites (Rhizoglyphus robini) to examine the effects of the mating system on population performance under environmental change. We demonstrate that populations in which monogamy was enforced suffered a dramatic fitness decline when evolving at an increased temperature, whereas the negative effects of change in a thermal environment were alleviated in polygamous populations. Strikingly, within 17 generations, all monogamous populations experiencing higher temperature went extinct, whereas all polygamous populations survived. Our results show that the mating system may have dramatic effects on the risk of extinction under environmental change.

Populations with elevated mutation load do not benefit from the operation of sexual selection

Theory predicts that if most mutations are deleterious to both overall fitness and condition-dependent traits affecting mating success, sexual selection will purge mutation load and increase nonsexual fitness. We explored this possibility with populations of mutagenized Drosophila melanogaster exhibiting elevated levels of deleterious variation and evolving in the presence or absence of male-male competition and female choice. After 60 generations of experimental evolution, monogamous populations exhibited higher total reproductive output than polygamous populations. Parental environment also affected fitness measures - flies that evolved in the presence of sexual conflict showed reduced nonsexual fitness when their parents experienced a polygamous environment, indicating trans-generational effects of male harassment and highlighting the importance of a common garden design. This cost of parental promiscuity was nearly absent in monogamous lines, providing evidence for the evolution of reduced sexual antagonism. There was no overall difference in egg-to-adult viability between selection regimes. If mutation load was reduced by the action of sexual selection in this experiment, the resultant gain in fitness was not sufficient to overcome the costs of sexual antagonism.

The influence of maternal effects on indirect benefits associated with polyandry

Despite numerous and diverse theoretical models for the indirect benefits of polyandry, empirical support is mixed. One reason for the difficulty in detecting indirect benefits of polyandry may be that these are subtle and are mediated by environmental effects, such as maternal effects. Maternal effects may be especially important if females allocate resources to their offspring depending on the characteristics of their mating partners. We test this hypothesis in the burying beetle Nicrophorus vespilloides, a species that provides extensive and direct parental care to offspring. We used a fully factorial design and mated females to one, two, three, four or five different males and manipulated conditions so that their offspring received reduced (12 h) or full (ca 72 h) maternal care. We found that average offspring fitness increased with full maternal care but there was no significant effect of polyandry or the interaction between the duration of maternal care and the level of polyandry on offspring fitness. Thus, although polyandry could provide a mechanism for biasing paternity towards high quality or compatible males, and variation in parental care matters, we found no evidence that female N. vespilloides gain indirect benefits by using parental care to bias the allocation of resources under different mating conditions.

Offspring viability benefits but no apparent costs of mating with high quality males

Traditional models of sexual selection posit that male courtship signals evolve as indicators of underlying male genetic quality. An alternative hypothesis is that sexual conflict over mating generates antagonistic coevolution between male courtship persistence and female resistance. In the scarabaeine dung beetle Onthophagus taurus, females are more likely to mate with males that have high courtship rates. Here, we examine the effects of exposing females to males with either high or low courtship rates on female lifetime productivity and offspring viability. Females exposed to males with high courtship rates mated more often and produced offspring with greater egg-adult viability. Female productivity and lifespan were unaffected by exposure to males with high courtship rates. The data are consistent with models of sexual selection based on indirect genetic benefits, and provide little evidence for sexual conflict in this system.

Fertilization success and the estimation of genetic variance in sperm competitiveness

A key question in sexual selection is whether the ability of males to fertilize eggs under sperm competition exhibits heritable genetic variation. Addressing this question poses a significant problem, however, because a male's ability to win fertilizations ultimately depends on the competitive ability of rival males. Attempts to partition genetic variance in sperm competitiveness, as estimated from measures of fertilization success, must therefore account for stochastic effects due to the random sampling of rival sperm competitors. In this contribution, we suggest a practical solution to this problem. We advocate the use of simple cross-classified breeding designs for partitioning sources of genetic variance in sperm competitiveness and fertilization success and show how these designs can be used to avoid stochastic effects due to the random sampling of rival sperm competitors. We illustrate the utility of these approaches by simulating various scenarios for estimating genetic parameters in sperm competitiveness, and show that the probability of detecting additive genetic variance in this trait is restored when stochastic effects due to the random sampling of rival sperm competitors are controlled. Our findings have important implications for the study of the evolutionary maintenance of polyandry.

Anthropogenic disturbance and evolutionary parameters: a lemon shark population experiencing habitat loss

The level of genetic variation in natural populations influences evolutionary potential, and may therefore influence responses to selection in the face of future environmental changes. By combining long-term monitoring of marked individuals with genetic pedigree reconstruction, we assessed whether habitat loss influenced genetic variation in a lemon shark (Negaprion brevirostris) population at an isolated nursery lagoon (Bimini, Bahamas). We also tracked changes in the strength and direction of natural selection. Contrary to initial expectations, we found that after the habitat loss neutral genetic variation increased, as did additive genetic variance for juvenile morphological traits (body length and mass). We hypothesize that these effects might result from philopatric behavior in females coupled with a possible influx of male genotypes from other nursery sites. We also found changes in the strength of selection on morphological traits, which weakened considerably after the disturbance; habitat loss therefore changed the phenotypes favored by natural selection. Because such human-induced shifts in the adaptive landscape may be common, we suggest that conservation biologists should not simply focus on neutral genetic variation per se, but also on assessing and preserving evolutionary parameters, such as additive genetic variation and selection.

Sexual selection counteracts extinction of small populations of the bulb mites

Genetic drift in small populations can increase frequency of deleterious recessives and consequently lead to inbreeding depression and population extinction. On the other hand, as homozygosity at deleterious recessives increases, they should be purged from populations more effectively by selection. Sexual selection has been postulated to strengthen selection against deleterious mutations, and should thus decrease extinction rate and intensify purging of inbreeding depression. We tested these predictions in the bulb mite Rhizoglyphus robini. We created 100 replicate lines of small populations (five males and five females) and in half of them experimentally removed sexual selection by enforcing monogamy. The lines were propagated for eight generations and then assayed for purging of inbreeding depression. We found that proportion of lines which went extinct was lower with sexual selection than without. We also found evidence for purging of inbreeding depression in the lines with sexual selection, but not in lines without sexual selection. Our results suggest that purging of inbreeding depression was more effective against mutations with relatively large deleterious effects. Thus, although our data clearly indicate a positive impact of sexual selection on short-term survival of bottlenecked populations, long-term consequences are less clear as they may be negatively impacted by accumulation of deleterious mutations of small effect.

Mating frequency and inclusive fitness in Drosophila melanogaster

In many species, increased mating frequency reduces maternal survival and reproduction. In order to understand the evolution of mating frequency, we need to determine the consequences of increased mating frequency for offspring. We conducted an experiment in Drosophila melanogaster in which we manipulated the mating frequency of mothers and examined the survival and fecundity of the mothers and their daughters. We found that mothers with the highest mating frequency had accelerated mortality and more rapid reproductive senescence. On average, they had 50% shorter lives and 30% lower lifetime reproductive success (LRS) than did mothers with the lowest mating frequency. However, mothers with the highest mating frequency produced daughters with 28% greater LRS. This finding implies that frequent mating stimulates cross-generational fitness trade-offs such that maternal fitness is reduced while offspring fitness is enhanced. We evaluate these results using a demographic metric of inclusive fitness. We show that the costs and benefits of mating frequency depend on the growth rate of the population. In an inclusive fitness context, there was no evidence that increased mating frequency results in fitness costs for mothers. These results indicate that cross-generational fitness trade-offs have an important role in sexual selection and life-history evolution.

Maintenance of genetic variation in sexual ornaments: a review of the mechanisms

Female preferences for elaborate male sexual traits have been documented in a number of species in which males contribute only genes to the next generation. In such systems, mate choice has been hypothesised to benefit females genetically. For the genetic benefits to be possible there must be additive genetic variation (V A) for sexual ornaments, such that highly ornamented males can pass fitter genes on to the progeny of choosy females. Here, I review the mechanisms that can contribute to the maintenance of this variation. The variation may be limited to sexual ornaments, resulting in Fisherian benefits in terms of the increased reproductive success of male progeny produced by choosy females. Alternatively, ornaments may capture V A in other life-history traits. In the latter case, "good genes" benefits may apply in terms of improved performance of the progeny of either sex. Some mechanisms, however, such as negative pleiotropy, sexually antagonistic variation or overdominance, can maintain V A in ornaments and other life-history traits with little variation in total fitness, leaving little room for any genetic benefits of mate choice. Distinguishing between these mechanisms has consequences not only for the theory of sexual selection, but also for evolution of sex and for biological conservation. I discuss how the traditional ways of testing for genetic benefits can usefully be supplemented by tests detecting benefits resulting from specific mechanisms maintaining V A in sexual ornaments.

Do female black field crickets Teleogryllus commodus benefit from polyandry?

Female insects that mate multiply tend to have increased lifetime fitness, apparently because of greater access to male-derived resources (e.g. sperm, nuptial gifts) that elevate fertility/fecundity. Experiments that standardize the number of matings per female also show that polyandry can improve aspects of offspring performance, most notably early embryo survival (egg hatching success). This improvement is widely attributed to genetic benefits which would arise if polyandrous females skew paternity to produce fitter offspring. In two separate experiments with field crickets (Teleogryllus commodus) polyandrous females (two, three or four mates) did not have higher egg hatching success than monandrous females (effect sizes: r = 0.03 and 0.08 for the respective experiments), which is consistent with our finding of no sire effect on hatching success. Polyandry also had no effect on post-hatching offspring survival. Polyandrous females' offspring took significantly longer to mature but their sons were not heavier and their daughters were actually significantly smaller than those of monandrous females. Finally, after controlling for relative male size, monandrous females' sons were more successful when directly competing for a mate.

MULTIPLE MATING IN THE GLANVILLE FRITILLARY BUTTERFLY: A CASE OF WITHIN-GENERATION BET HEDGING?

Many hypotheses have been proposed to explain multiple mating in females. One of them is bet hedging, that is avoiding having no or very few offspring in any given generation, rather than maximizing the expected number of offspring. However, within-generation bet hedging is generally believed to be an unimportant evolutionary force, except in very small populations. In this study, we derive predictions of the bet-hedging hypothesis for a case in which local insect populations are often small, offspring performance varies, for example, due to inbreeding depression, and the groups of gregarious larvae have to exceed a threshold size before they are likely to survive throughout the larval stage. These conditions exist for populations of the Glanville fritillary butterfly (Melitaea cinxia), potentially making bet-hedging benefits larger than usual. We observed matings in a field cage, which allowed detailed observations under practically natural conditions, and analyzed genetic paternity of egg clutches laid by females under direct observation. The egg-laying and survival patterns are in line with the predictions, supporting the hypothesis that multiple mating in M. cinxia presents a rare case of within-generation bet hedging.

Sexual selection and conflict in the bulb mite, Rhizoglyphus robini (Astigmata: Acaridae)

Whether benefits of mate choice accrued by females outweigh costs associated with sexual selection remains largely unresolved. The 'good genes' perspective, posing that mate choice benefits females genetically has been challenged by the arguments that sexual selection is driven mostly by direct costs and inter-sexual conflict. Here, I present an overview of experimental tests of predictions of good genes and sexual conflict mechanisms in the bulb mite Rhizoglyphus robini.

Postcopulatory inbreeding avoidance by female crickets only revealed by molecular markers

Multiple mating is thought to provide an opportunity for females to avoid the costs of genetic incompatibility by postcopulatory selection of compatible sperm haplotypes. Few studies have tested the genetic incompatibility hypothesis directly. Here we experimentally manipulated the compatibility of females with their mates using the gryllid cricket Teleogryllus oceanicus. We recorded the hatching success of eggs laid by females mated with two nonsibling males, two siblings, or one nonsibling male and one sibling. In contrast with two previous studies on crickets that have adopted this approach, the hatching success of eggs did not differ between females mated with two full siblings and females mated with two unrelated males, indicating that embryo viability was not a cost of inbreeding in this species. We assigned paternity to offspring produced by females mated to both a sibling and a nonsibling male using microsatellite markers. As in previous studies of this species, we were unable to detect any difference in the proportion of offspring sired by the 1st and the 2nd male to mate with a female when females were unrelated to their mates. However, in our experimental matings the proportion of offspring sired by the nonsibling male depended on his sequence position. Paternity was biased toward the nonsibling male when he mated first. Our data show that molecular analyses of paternity are essential to detect subtle mechanisms of postcopulatory sexual selection.

Evolution under relaxed sexual conflict in the bulb mite Rhizoglyphus robini

The experimental evolution under different levels of sexual conflict have been used to demonstrate antagonistic coevolution in muscids, but among other taxa a similar approach has not been employed. Here, we describe the results of 37 generations of evolution under either experimentally enforced monogamy or polygamy in the bulb mite Rhizoglyphus robini. Three replicates were maintained for each treatment. Monogamy makes male and female interests congruent; thus selection is expected to decrease harmfulness of males to their partners. Our results were consistent with this prediction in that females from monogamous lines achieved lower fecundity when housed with males from polygamous lines. Fecundity of polygamous females was not affected by mating system under which their partners evolved, which suggests that they were more resistant to male-induced harm. As predicted by the antagonistic coevolution hypothesis, the decrease in harmfulness of monogamous males was accompanied by a decline in reproductive competitiveness. In contrast, female fecundity and embryonic viability, which were not expected to be correlated with male harmfulness, did not differ between monogamous and polygamous lines. None of the fitness components assayed differed between individuals obtained from crosses between parents from the same line and those obtained from crosses between parents from different lines within the same mating system. This indicates that inbreeding depression did not confound our results. However, interpretation of our results is complicated by the fact that both males and females from monogamous lines evolved smaller body size compared to individuals from polygamous lines. Although a decrease in reproductive performance of males from monogamous lines was still significant when body size was taken into account, we were not able to separate the effects of male body size and mating system in their influence on fecundity of their female partners.

Post-mating sexual selection increases lifetime fitness of polyandrous females in the wild

Females often mate with several males before producing offspring. Field studies of vertebrates suggest, and laboratory experiments on invertebrates confirm, that even when males provide no material benefits, polyandry can enhance offspring survival. This enhancement is widely attributed to genetic benefits that arise whenever paternity is biased towards males that sire more viable offspring. Field studies suggest that post-mating sexual selection biases fertilization towards genetically more compatible males and one controlled experiment has shown that, when females mate with close kin, polyandry reduces the relative number of inbred offspring. Another potential genetic benefit of polyandry is that it increases offspring survival because males with more competitive ejaculates sire more viable offspring. Surprisingly, however, there is no unequivocal evidence for this process. Here, by experimentally assigning mates to females, we show that polyandry greatly increases offspring survival in the Australian marsupial Antechinus stuartii. DNA profiling shows that males that gain high paternity under sperm competition sire offspring that are more viable. This beneficial effect occurs in both the laboratory and the wild. Crucially, there are no confounding non-genetic maternal effects that could arise if polyandry increases female investment in a particular reproductive event because A. stuartii is effectively semelparous. Our results therefore show that polyandry improves female lifetime fitness in nature. The threefold increase in offspring survival is not negated by a decline in maternal lifespan and is too large to be offset by an equivalent decline in the reproductive performance of surviving offspring.

Maternal effects due to male attractiveness affect offspring development in the zebra finch

Maternal effects occur when offspring phenotype is influenced by environmental factors experienced by the mother. Mothers are predicted to invest differentially in offspring in ways that will maximize offspring fitness depending on the environment she expects them to encounter. Here, we test for maternal effects in response to mate attractiveness on offspring developmental traits in the zebra finch Taeniopygia guttata. We controlled for parental genetic quality by manipulating male attractiveness using coloured leg rings and by randomly assigning mating pairs. The potential confounding effect of differential nestling care was controlled for by cross-fostering clutches and by allowing for variance due to foster father attractiveness in general linear models. We found a difference in egg mass investment between attractiveness groups and, importantly, we found that all of the offspring traits we measured varied with the attractiveness of the father. This provides strong evidence for maternal effects in response to mate attractiveness. Furthermore, due to the experiment design, we can conclude that these effects were mediated by differential investment of egg resources and not due to genetic differences or differences in nestling care.

Strong association between a single gene and fertilization efficiency of males and fecundity of their mates in the bulb mite

Although variation in male fertilization efficiency has been shown to have a genetic basis in several species, the genes responsible for the effect are generally unknown. Here, we show a strong association between the fertilization success of males and their phosphogluconate dehydrogenase (Pgdh) genotype in the bulb mite Rhizoglyphus robini. Males homozygous for the slow (S) allele fathered a significantly greater proportion of offspring when competing with males homozygous for the fast (F) allele. There was no evidence that female fecundity was influenced by their Pgdh genotype. The fecundity of FF females did not differ significantly from the fecundity of SS females but female fecundity was significantly influenced by the genotype of their mate. Females paired with SS males laid significantly fewer eggs than females paired with FF males. Altogether these data show a trade-off, with the male SS genotype associated with their higher fertilization efficiency but at the cost of a negative impact on the fecundity of females mating with them.

Sexual selection and mate choice

The past two decades have seen extensive growth of sexual selection research. Theoretical and empirical work has clarified many components of pre- and postcopulatory sexual selection, such as aggressive competition, mate choice, sperm utilization and sexual conflict. Genetic mechanisms of mate choice evolution have been less amenable to empirical testing, but molecular genetic analyses can now be used for incisive experimentation. Here, we highlight some of the currently debated areas in pre- and postcopulatory sexual selection. We identify where new techniques can help estimate the relative roles of the various selection mechanisms that might work together in the evolution of mating preferences and attractive traits, and in sperm-egg interactions.

Effect of inbreeding and heritability of sperm competition success in the bulb mite Rhizoglyphus robini

Sperm competition is a potent evolutionary force shaping the reproductive biology of most animal species. Here, we estimated the heritability of sperm competition success in the promiscuous bulb mite Rhizoglyphus robini. Sperm competition success was measured with the sterile male technique as the proportion of eggs fertilised by the second of three males mated with a single female. Sperm competition success responded significantly to selection. The heritability estimated from the response to five generations of selection was 0.13. We also estimated the effect of inbreeding on sperm competition success. Males produced by sib-mating (F=0.25) had a significantly lower sperm competition success than outbred males. The estimated coefficient of inbreeding depression was 0.53. Such high inbreeding depression together with moderately low heritability is consistent with the view that sperm competitive ability is under strong directional selection and strongly influences the reproductive success of males.

Remating in Drosophila melanogaster: an examination of the trading-up and intrinsic male-quality hypotheses

Female Drosophila melanogaster remate more frequently than necessary to ensure fertilization. We tested whether polyandrous females gain genetic benefits for their offspring by (1) selecting secondary sires of higher genetic-quality than original partners or (2) because post-copulatory mechanisms bias fertilizations towards genetically superior males. We screened 119 hemiclones of males for lifetime fitness then selected eight hemiclones (four of extreme high fitness and four of extreme low fitness) and mated them to virgin females. Females were then given the opportunity to remate with males of benchmark-genetic quality and their propensity to remate (fidelity) and sperm displacement scored. A female's fidelity and her level of sperm displacement varied depending on which hemiclone she mated first, but not on male-genetic quality. These findings indicate that female remating and sperm displacement are strongly influenced by male genotype, but provide no evidence that these traits contribute to adaptive female choice to obtain superior genes for offspring.

Genetic quality and sexual selection: an integrated framework for good genes and compatible genes

Why are females so choosy when it comes to mating? This question has puzzled and marveled evolutionary and behavioral ecologists for decades. In mating systems in which males provide direct benefits to the female or her offspring, such as food or shelter, the answer seems straightforward--females should prefer to mate with males that are able to provide more resources. The answer is less clear in other mating systems in which males provide no resources (other than sperm) to females. Theoretical models that account for the evolution of mate choice in such nonresource-based mating systems require that females obtain a genetic benefit through increased offspring fitness from their choice. Empirical studies of nonresource-based mating systems that are characterized by strong female choice for males with elaborate sexual traits (like the large tail of peacocks) suggest that additive genetic benefits can explain only a small percentage of the variation in fitness. Other research on genetic benefits has examined nonadditive effects as another source of genetic variation in fitness and a potential benefit to female mate choice. In this paper, we review the sexual selection literature on genetic quality to address five objectives. First, we attempt to provide an integrated framework for discussing genetic quality. We propose that the term 'good gene' be used exclusively to refer to additive genetic variation in fitness, 'compatible gene' be used to refer to nonadditive genetic variation in fitness, and 'genetic quality' be defined as the sum of the two effects. Second, we review empirical approaches used to calculate the effect size of genetic quality and discuss these approaches in the context of measuring benefits from good genes, compatible genes and both types of genes. Third, we discuss biological mechanisms for acquiring and promoting offspring genetic quality and categorize these into three stages during breeding: (i) precopulatory (mate choice); (ii) postcopulatory, prefertilization (sperm utilization); and (iii) postcopulatory, postfertilization (differential investment). Fourth, we present a verbal model of the effect of good genes sexual selection and compatible genes sexual selection on population genetic variation in fitness, and discuss the potential trade-offs that might exist between mate choice for good genes and mate choice for compatible genes. Fifth, we discuss some future directions for research on genetic quality and sexual selection.

No evidence for inbreeding avoidance through postcopulatory mechanisms in the black field cricket, Teleogryllus commodus

Several studies suggest that females mate multiply so that they can preferentially fertilize eggs with the sperm of genetically more compatible males. Unrelated males are expected to be genetically more compatible with a female than her close relatives. We tested whether black field crickets, Teleogryllus commodus, can bias sperm usage toward unrelated males by comparing egg hatching success of females mated to two of their siblings (SS), two sibling males unrelated to the female (NN) or to one unrelated male and a sibling male (NS or SN). Egg hatching success was highly repeatable. Hatching success varied significantly among females of the three mating types (P = 0.011, n = 245 females). The estimated mean hatching success of 36.8% for SS females was significantly less that the 43.4% of NN females, indicating an effect of inbreeding on hatching success. If females preferentially use the sperm of a less closely related male, the hatching success of NS/SN females should be closer to 43.4% than 36.8%. It was, in fact, only 34.9%. This does not differ significantly from the value expected if the two males contributed an equal amount of sperm that was then used randomly. Although polyandry may confer indirect genetic benefits, our results provide no evidence that female T. commodus gain these benefits by biasing paternity toward genetically more compatible males through postcopulatory mechanisms.

Sire attractiveness influences offspring performance in guppies

According to the good-genes hypothesis, females choose among males to ensure the inheritance of superior paternal genes by their offspring. Despite increasing support for this prediction, in some cases differential (non-genetic) maternal effects may obscure or amplify the relationship between paternal attractiveness and offspring quality. Artificial insemination controls such effects because it uncouples mate choice from copulation, therefore denying females the opportunity of assessing male attractiveness. We adopted this technique in the live-bearing fish Poecilia reticulata and examined whether paternal coloration was associated with the behavioural performance of newborn offspring. Sexually receptive virgin females were inseminated with sperm taken individually from donor males that exhibited high variation in the area of orange pigmentation, a trait known to influence female choice in the study population. Our analysis of offspring performance focused on the anti-predator behaviour of newborn fish, including schooling by sibling pairs, the response (swimming speed) of these fishes to a simulated avian predator, and the time taken for a naive investigator to capture the offspring. Although we found no significant effect of sire coloration on either schooling or swimming speed, our analysis revealed a significant positive association between sire coloration and the ability of newborn offspring to evade capture. This finding supports the view that at least one aspect of anti-predator behaviour in newborn offspring is influenced by sire genotype, which in turn is revealed by the expression of secondary sexual traits.

Egg size and offspring performance in the collared flycatcher (Ficedula albicollis): a within-clutch approach

Adaptive within-clutch allocation of resources by laying females is an important focus of evolutionary studies. However, the critical assumption of these studies, namely that within-clutch egg-size deviations affect offspring performance, has been properly tested only rarely. In this study, we investigated effects of within-clutch deviations in egg size on nestling survival, weight, fledgling condition, structural size and offspring recruitment to the breeding population in the collared flycatcher ( Ficedula albicollis). Besides egg-size effects, we also followed effects of hatching asynchrony, laying sequence, offspring sex and paternity. There was no influence of egg size on nestling survival, tarsus length, condition or recruitment. Initially significant effect on nestling mass disappeared as nestlings approached fledging. Thus, there seems to be limited potential for a laying female to exploit within-clutch egg-size variation adaptively in the collared flycatcher, which agrees with the majority of earlier studies on other bird species. Instead, we suggest that within-clutch egg-size variation originates from the effects of proximate constraints on laying females. If true, adaptive explanations for within-clutch patterns in egg size should be invoked with caution.