Male parentage does not vary with colony kin structure in a multiple-queen ant

Kin selection theory predicts that, in social Hymenoptera, the parentage of males should be determined by within-colony relatedness. We present a model showing that, when sex ratios are split (bimodal) as a function of colony kin structure, the predictions of kin selection theory regarding the occurrence of worker reproduction and policing (prevention of worker reproduction) require modification. To test the predictions of kin selection theory and our model, we estimated using microsatellites the frequency of worker-produced male eggs and adults in the facultatively polygynous (multiple-queen) ant Leptothorax acervorum. Analysis of 210 male eggs and 328 adult males from 13 monogynous (single-queen) and nine polygynous colonies demonstrated that the frequency of worker-produced males was low (2.3-4.6% of all males) and did not differ significantly between colony classes or between eggs and adults. This suggested workers' self-restraint as the cause of infrequent worker reproduction in both colony classes. Such an outcome is not predicted either by comparing relatedness values or by our model. Therefore, it appears that factors other than colony kin structure and sex ratio effects determine the pattern of male parentage in the study population. A likely factor is a colony-level cost of worker reproduction.     

Intralocus sexual conflict and offspring sex ratio

Males and females frequently have different fitness optima for shared traits, and as a result, genotypes that are high fitness as males are low fitness as females, and vice versa. When this occurs, biasing of offspring sex-ratio to reduce the production of the lower-fitness sex would be advantageous, so that for example, broods produced by high-fitness females should contain fewer sons. We tested for offspring sex-ratio biasing consistent with these predictions in broad-horned flour beetles. We found that in both wild-type beetles and populations subject to artificial selection for high- and low-fitness males, offspring sex ratios were biased in the predicted direction: low-fitness females produced an excess of sons, whereas high-fitness females produced an excess of daughters. Thus, these beetles are able to adaptively bias sex ratio and recoup indirect fitness benefits of mate choice.     

Sex allocation conflict in insect societies: who wins?

Sex allocation in colonies of eusocial Hymenoptera is one of the best studied social conflicts. We outline a framework for analysing conflict outcome through power and the costs of manipulation and suggest that the conflict will often be unresolved because both major parties of interest, the queen and the workers, should manipulate allocation even at considerable costs to the colony. We suggest future work for analysing power in the conflict between queen and workers over sex allocation and discuss the extent of male power.     

Parental sex discrimination and intralocus sexual conflict

Intralocus sexual conflict occurs when populations segregate for alleles with opposing fitness consequences in the two sexes. This form of selection is known to be capable of maintaining genetic and fitness variation in nature, the extent of which is sensitive to the underlying genetics. We present a one-locus model of a haploid maternal effect that has sexually antagonistic consequences for offspring. The evolutionary dynamics of these maternal effects are distinct from those of haploid direct effects under sexual antagonism because the relevant genes are expressed only in females. Despite this, we find the same opportunity for sexually antagonistic polymorphism at the maternal effect locus as at a direct effect locus. Thus, sexually antagonistic maternal effects may underlie some natural genetic variation. The model we present permits alternative interpretations of how the genes are expressed and how the fitness variation is assigned, which invites a theoretical comparison to models of both imprinted genes and sex allocation.     

The mismeasurement of sexual selection

Sexual selection can explain major micro‐ and macro‐evolutionary patterns. Much of current theory predicts that the strength of sexual selection (i) is driven by the relative abundance of males and females prepared to mate (i.e. the operational sex ratio, OSR) and (ii) can be generally estimated by calculating intra‐sexual variation in mating success (e.g. the opportunity for sexual selection, I_s). Here, we demonstrate the problematic nature of these predictions. The OSR and I_s only accurately predict sexual selection under a limited set of circumstances, and more specifically, only when mate monopolization is extremely strong. If mate monopolization is not strong, using OSR or I_s as proxies or measures of sexual selection is expected to produce spurious results that lead to the false conclusion that sexual selection is strong when it is actually weak. These findings call into question the validity of empirical conclusions based on these measures of sexual selection.     

Sex allocation and sexual conflict in simultaneously hermaphroditic animals

Links between sex allocation (SA) and sexual conflict in simultaneous hermaphrodites have been evident since Charnov''s landmark paper published 30 years ago. We discuss two links, namely the potential for sexual conflict over SA between sperm donor and recipient, and the importance of post-copulatory sexual selection and the resulting sexual conflict for the evolution of SA. We cover the little empirical and theoretical work exploring these links, and present an experimental test of one theoretical prediction. The link between SA and sexual conflict is an interesting field for future empirical and theoretical research.     

The evolution of haplodiploidy by male‐killing endosymbionts: importance of population structure and endosymbiont mutualisms

Haplodiploid inheritance systems, characterized by male transmission of only their maternally inherited genomic elements, have evolved more than 20 times within the animal kingdom. A number of theoretical studies have argued that infection with certain male‐killing endosymbionts can potentially lead to the evolution of haplodiploidy. By explicitly investigating the coevolutionary dynamics between host and endosymbiont, we show that the assumptions of current models cannot explain the evolution of haplodiploidy very well, as the endosymbiont will often go extinct in the long term. Here, we provide two additional mechanisms that can explain the stable evolution of haplodiploidy by male‐killing endosymbionts. First of all, a spatially structured population can facilitate the long‐term persistence of haplodiploidy, but this applies only when levels of inbreeding are very high. By contrast, endosymbionts that are mutualistic with their hosts provide a much more general and promising route to the stable evolution of haplodiploidy. This model is the first to provide a formal explanation of the supposed association between the evolution of haplodiploidy and the highly inbred lifestyles of some ancestors, while it also provides a hypothesis for the evolution of haplodiploidy in more outbred ancestors.     

A theoretical basis for measures of kin selection in subdivided populations: finite populations and localized dispersal

The analysis of kin selection in subdivided populations has been hampered by the lack of well‐defined measures of genealogical relatedness in the presence of localized dispersal. Furthermore, the usual arguments underlying the definition of game‐theoretical measures of inclusive fitness are not exact under localized dispersal. We define such measures to give the first‐order effects of selection on the probability of fixation of an allele. The derived measures of kin selection and relatedness are valid in finite populations and under localized dispersal. For the infinite island model, the resulting measure of kin selection is equivalent to a previously used measure. In other cases its definition is based on definitions of relatedness which are different from the usual ones. To illustrate the approach, we reanalyse a model with localized dispersal. We consider sex ratio evolution under sex‐specific dispersal behaviour, and the results confirm the earlier conclusion that the sex ratio is biased towards the sex with the dispersal rate closer to the optimal dispersal rate in the absence of sex‐specific dispersal behaviour.     

Direct fitness for dynamic kin selection

The direct-fitness approach to modelling the evolution of social traits is an alternative to the classical inclusive-fitness-based approach. Despite both its utility and popularity, the direct-fitness approach has not yet been extended to include the analysis of dynamic traits, i.e. traits whose level of expression may vary over time. In this article, I apply the direct-fitness approach to cope with the evolution of a dynamic resource-allocation behaviour when this behaviour influences the fitness of relatives. I am able to implement the direct-fitness approach using components (reproductive value, fitness changes and measures of relatedness) found in standard, social-evolutionary models. I illustrate the modified direct-fitness model with an example studied by previous authors, and I show how the direct-fitness perspective can aid the validation of analytical results by means of a genetic algorithm.     

Sex-biased survival predicts adult sex ratio variation in wild birds

Adult sex ratio (ASR) is a central concept in population demography and breeding system evolution, and has implications for population viability and biodiversity conservation. ASR exhibits immense interspecific variation in wild populations, although the causes of this variation have remained elusive. Using phylogenetic analyses of 187 avian species from 59 families, we show that neither hatching sex ratios nor fledging sex ratios correlate with ASR. However, sex-biased adult mortality is a significant predictor of ASR, and this relationship is robust to 100 alternative phylogenetic hypotheses, and potential ecological and life-history confounds. A significant component of adult mortality bias is sexual selection acting on males, whereas increased reproductive output predicts higher mortality in females. These results provide the most comprehensive insights into ASR variation to date, and suggest that ASR is an outcome of selective processes operating differentially on adult males and females. Therefore, revealing the causes of ASR variation in wild populations is essential for understanding breeding systems and population dynamics.     

Adult sex ratio,sexual dimorphism and sexual selection in a Mesozoic reptile

The evolutionary history of sexual selection in the geologic past is poorly documented based on quantification, largely because of difficulty in sexing fossil specimens. Even such essential ecological parameters as adult sex ratio (ASR) and sexual size dimorphism (SSD) are rarely quantified, despite their implications for sexual selection. To enable their estimation, we propose a method for unbiased sex identification based on sexual shape dimorphism, using size-independent principal components of phenotypic data. We applied the method to test sexual selection in Keichousaurus hui, a Middle Triassic (about 237 Ma) sauropterygian with an unusually large sample size for a fossil reptile. Keichousaurus hui exhibited SSD biased towards males, as in the majority of extant reptiles, to a minor degree (sexual dimorphism index −0.087). The ASR is about 60% females, suggesting higher mortality of males over females. Both values support sexual selection of males in this species. The method may be applied to other fossil species. We also used the Gompertz allometric equation to study the sexual shape dimorphism of K. hui and found that two sexes had largely homogeneous phenotypes at birth except in the humeral width, contrary to previous suggestions derived from the standard allometric equation.     

Queen control over reproductive decisions--no sexual deception in the ant Lasius niger

Queen-worker conflicts in social insect societies have received much attention in the past decade. In many species workers modify the colony sex ratio to their own advantage or produce their own male offspring. In some other species, however, queens seem to be able to prevent workers from making selfish reproductive decisions. So far, little effort has been made to find out how queens may keep control over sex ratio and male parentage. In this study we use a Lasius niger population under apparent queen control to show that sexual deception cannot explain queen dominance in this population. The sexual deception hypothesis postulates that queens should prevent workers from discriminating against males by disguising male brood as females. Contrary to the predictions of this hypothesis, we found that workers are able to distinguish male and female larvae early in their development: in early spring workers generally placed only either female or male larvae in the uppermost chambers of the nest, although both types of larvae must have been present. At this time males were only at 11% of their final dry weight, a developmental stage at which (according to two models) workers would still have benefited from replacing queen-produced males by females or worker-produced males. This study thus demonstrates that sexual deception cannot account for the apparent queen control over colony sex ratio and male parentage in L. niger.     

Anisogamy,sexual selection,and the evolution and maintenance of sex

Summary In the present paper we distinguish between two aspects of sexual reproduction. Genetic recombination is a universal features of the sexual process. It is a primitive condition found in simple, single-celled organisms, as well as in higher plants and animals. Its function is primarily to repair genetic damage and eliminate deleterious mutations. Recombination also produces new variation, however, and this can provide the basis for adaptive evolutionary change in spatially and temporally variable environments.The other feature usually associated with sexual reproduction, differentiated male and female roles, is a derived condition, largely restricted to complex, diploid, multicellular organisms. The evolution of anisogamous gametes (small, mobile male gametes containing only genetic material, and large, relatively immobile female gametes containing both genetic material and resources for the developing offspring) not only established the fundamental basis for maleness and femaleness, it also led to an asymmetry between the sexes in the allocation of resources to mating and offspring. Whereas females allocate their resources primarily to offspring, the existence of many male gametes for each female one results in sexual selection on males to allocate their resources to traits that enhance success in competition for fertilizations. A consequence of this reproductive competition, higher variance in male than female reproductive success, results in more intense selection on males.The greater response of males to both stabilizing and directional selection constitutes an evolutionary advantage of males that partially compensates for the cost of producing them. The increased fitness contributed by sexual selection on males will complement the advantages of genetic recombination for DNA repair and elimination of deleterious mutations in any outcrossing breeding system in which males contribute only genetic material to their offspring. Higher plants and animals tend to maintain sexual reproduction in part because of the enhanced fitness of offspring resulting from sexual selection at the level of individual organisms, and in part because of the superiority of sexual populations in competition with asexual clones.