Studies of sex allocation offer excellent opportunities for examining the constraints and limits on adaptation. A major topic of debate within this field concerns the extent to which the ability of ...individuals to adaptively manipulate their offspring sex ratio is determined by constraints such as the method of sex determination. We address this problem by comparing the extent of sex-ratio adjustment across taxa with different methods of sex determination, under the common selective scenario of interactions between relatives. These interactions comprise the following: local resource competition (LRC), local mate competition (LMC), and local resource enhancement (LRE). We found that: (1) species with supposedly constraining methods of sex determination showed consistent sex-ratio adjustment in the predicted direction; (2) vertebrates with chromosomal sex determination (CSD) showed less adjustment then haplodiploid invertebrates; (3) invertebrates with possibly constraining sex-determination mechanisms (CSD and pseudo-arrhenotoky) did not show less adjustment then haplodiploid invertebrates; (4) greater sex-ratio adjustment was seen in response to LRC and LMC than LRE; (5) greater sex-ratio adjustment was seen in response to interactions between relatives (LRC, LMC, and LRE) compared to responses to other environmental factors. Our results also illustrate the problem that sex-determination mechanism and selective pressure are confounded across taxa because vertebrates with CSD are influenced primarily by LRE whereas invertebrates are influenced by LRC and LMC. Overall, our analyses suggest that sex-allocation theory needs to consider simultaneously the influence of variable selection pressures and variable constraints when applying general theory to specific cases.
Sex ratio theory offers excellent opportunities to examine the extent to which individuals adaptively adjust their behavior in response to local conditions. Hamilton’s theory of local mate ...competition, which predicts female‐biased sex ratios in structured populations, has been extended in numerous directions to predict individual behavior in response to factors such as relative fecundity, time of oviposition, and relatedness between cofoundresses and between mates. These extended models assume that foundresses use different sources of information, and they have generally been untested or have only been tested in the laboratory. We use microsatellite markers to describe the wild oviposition behavior of individual foundresses in natural populations of the parasitoid waspNasonia vitripennis,and we use the data collected to test these various models. The offspring sex ratio produced by a foundress on a particular host reflected the number of eggs that were laid on that host relative to the number of eggs that were laid on that host by other foundresses. In contrast, the offspring sex ratio was not directly influenced by other potentially important factors, such as the number of foundresses laying eggs on that patch, relative fecundity at the patch level, or relatedness to either a mate or other foundresses on the patch.
1. Neonicotinoid insecticides are potent neurotoxins of significant economic importance. However, it is clear that their use can adversely impact beneficial insects in the environment, even at low, ...sub‐lethal doses.
2. It has recently been shown that the neonicotinoid imidacloprid disrupts adaptive sex allocation in the parasitoid wasp Nasonia vitripennis (Walker) by limiting their ability to respond to the presence of other females on oviposition patches. In the present study, that work was extended to explore whether sex allocation when superparasitising – laying eggs on a host that has already been parasitised – is also disrupted by imidacloprid.
3. Under superparasitism, sex allocation theory predicts that females should vary their offspring sex ratio in relation to their relative clutch size. It was found that sex allocation under superparasitism in Nasonia is disrupted in a dose‐dependent manner, with exposed females producing more daughters.
4. Importantly, imidacloprid does not appear to influence the ability of females to estimate the number of eggs already present on a host, suggesting a disassociation between the sex ratio and clutch size cues.
5. The present work highlights the fitness costs to beneficial insects of exposure to neonicotinoids, but also provides clues as to how female Nasonia use information when allocating sex.
The evolution of reproductive isolation lies at the heart of understanding the process of speciation. Of particular interest is the relationship between pre‐ and postzygotic reproductive isolation, ...and the genetic architecture of traits that contribute to one or both forms of reproductive isolation. The sibling species of seed bug Lygaeus equestris and L. simulans show a classic pattern of asymmetric prezygotic reproductive isolation, with female L. equestris hybridizing with male L. simulans, but with no hybridization in the reciprocal direction. We have recently described a mutant pale color form of L. simulans, that inherits as a single Mendelian locus and is pleiotropic for a number of other life history and behavioral traits. Here, we tested whether this locus also influences pre‐ and postzygotic reproductive isolation. Two sets of experimental crosses revealed that behavioral isolation varied with mutant versus wild‐type phenotype for male L. simulans, with the pale form less successful at mating with female L. equestris. In terms of trying to assess postzygotic isolation, levels of hybrid offspring production were uniformly low across the experiments. However, we did obtain, for the first time, hybrid offspring from a pairing between a female L. simulans and a male L. equestris. In this instance, the female was of the pale mutant genotype. Together with evidence for heterozygote advantage in terms of nymph survival, we consider our results in terms of possible mechanisms of reproductive isolation between this species pair, the role of the pale mutation, and the possible genetic architectures underlying the mutation, from a single gene to a supergene.
A pale mutant Lygaeus simulans female copulating with a wild‐type male L. simulans. Here, we show that the pale mutation in L. simulans influences hybridization with its sister species L. equestris. In particular, we found that pale males were less successful at hybridizing than wild‐types. This work confirms the highly pleiotropic nature of the pale locus. Photo credit: Vicki Balfour.
Sexual selection has been shown to be the driving force behind the evolution of the sometimes extreme and elaborate genitalia of many species. Sexual selection may arise before and/or after mating, ...or vary according to other factors such as the social environment. However, bouts of selection are typically considered in isolation. We measured the strength and pattern of selection acting on the length of the male intromittent organ (or processus) in two closely related species of lygaeid seed bug: Lygaeus equestris and Lygaeus simulans. In both species, we measured both pre- and post-copulatory selection. For L. equestris, we also varied the experimental choice design used in mating trials. We found contrasting pre- and post-copulatory selection on processus length in L. equestris. Furthermore, significant pre-copulatory selection was only seen in mating trials in which two males were present. This selection likely arises indirectly due to selection on a correlated trait, as the processus does not interact with the female prior to copulation. In contrast, we were unable to detect significant pre- or post-copulatory selection on processus length in L. simulans. However, a formal meta-analysis of previous estimates of post-copulatory selection on processus length in L. simulans suggests that there is significant stabilising selection across studies, but the strength of selection varies between experiments. Our results emphasise that the strength and direction of sexual selection on genital traits may be multifaceted and can vary across studies, social contexts and different stages of reproduction. SIGNIFICANCE STATEMENT: Animal genitalia vary greatly in size and complexity across species, and selection acting on genital size and shape can be complex. In this study, we show that the length of the penis in two species of seed bug is subject to complex patterns of selection, varying depending on the social context and whether selection is measured before or after mating. In one of the species, we show unexpectedly that penis length is correlated with male mating success, despite the fact that the penis does not interact with the female prior to mating. Our results highlight the fact that genitalia may be subject to both direct and indirect selection at different stages of mating and that to fully understand the evolution of such traits we should combine estimates of selection arising from these multiple episodes.
Research over the past two decades suggests that polyandry is almost ubiquitous in nature. In some cases, females can gain direct and indirect (genetic) fitness benefits from mating with multiple ...males. However, when females accept superfluous matings without gaining any clear benefit, polyandry has been interpreted as a strategy to mitigate the costs of resisting or avoiding matings, a situation known as convenience polyandry. When females mate out of ‘convenience’ the mating rate is expected to be plastic, since females should mate at a higher rate when the costs of resistance or avoidance are high, for instance when males occur in high densities and/or around resources required by females such as oviposition sites. Here we show that remating in Nasonia vitripennis, a species of wasp that is largely monandrous in the wild but that evolves polyandry under laboratory culture, is dependent upon the availability of hosts for oviposition and upon male density. We found that females mated at a higher rate when male density was high but only if a suitable oviposition substrate was available. Outwardly this seems suggestive of convenience polyandry. However, females that remated under these conditions did not gain more time to oviposit than females that resisted superfluous matings. The results of this study highlight the importance of comprehensively assessing the costs and benefits of mating before attributing the observed behaviour to convenience polyandry. Furthermore, these results add to the growing body of evidence that the ecological context under which sexual interactions occur is critical to the economics of mating.
•Nasonia vitripennis females remate when male density is high and they can oviposit.•Females that remate do not gain more time to oviposit when male density is high.•Superficially these findings are suggestive of convenience polyandry.•The absence of cost mitigation contradicts the convenience polyandry hypothesis.
Explaining cooperation is one of the greatest challenges for evolutionary biology 1–3. It is particularly a problem in species such as humans, where there is cooperation between nonrelatives. ...Numerous possible solutions have been suggested for the problem of cooperation between nonrelatives, including punishment, policing, and various forms of reciprocity 3–14. Here, we suggest that local competition for resources can pose a problem for these hypotheses, analogous to how it can select against cooperation between relatives 15–21. We extend the prisoner's dilemma (PD) game to show that local competition between interacting individuals can reduce selection for cooperation between nonrelatives. This is because, with local competition, fitness is relative to social partners, and cooperation benefits social partners. We then test whether nonrelated humans adjust their level of cooperation facultatively in response to the scale of competition when playing the PD for cash prizes. As predicted, we found that individuals were less likely to cooperate when competition was relatively local. Cooperation between humans will therefore be most likely when repeated interactions take place on a local scale between small numbers of people, and competition for resources takes place on a more global scale among large numbers of people.
•Visual detection affects initial shell selection behavior.•Hermit crab shell selection is dependent on both shell and background colour.•Evidence of a competing preference for dark shells against ...background matching.•Shell selection decisions are context-dependent and plastic between long/short-term.
Animals avoid predator attack in different ways; some carry defensive structures to reduce predation, with the classic example being hermit crabs and their use of a mollusc shell as a portable refugium. During shell selection, various shell characteristics are investigated by the crab to determine their suitability. Here we consider the role of visual cues. Previous research suggests that some hermit crabs are more likely to initially choose a conspicuous shell but also to move to backgrounds against which they are less conspicuous, suggesting a short-term/long-term trade-off. Across experiments in which we manipulated shell and background colour, we show initially that Pagurus bernhardus prefer black shells over white but this preference was lost in the absence of visual cues. We then show that the strength of preference was dependent on background colour. We repeated this last experiment with red and yellow shells against red or yellow backgrounds to investigate whether this preference extended to chromatic hues. A preference for darker (red) shells was expressed, but preference alteration with background was not observed. P. bernhardus therefore discriminate between shells in terms of shell and background colour, and discrimination may be rooted in a preference for darker shaded shells.
We previously demonstrated that treatment with the demethylating agent 5-aza-2′-deoxycytidine (5-aza-dC) alters the offspring sex ratios produced by females of the parasitoid wasp Nasonia ...vitripennis. Females allocate offspring sex ratio in line with local mate competition theory, producing more or less female-biased sex ratios as the number of other females laying eggs on a patch varies, thereby reducing competition among their sons for mates. Interestingly, treatment with 5-aza-dC did not ablate the facultative sex allocation response. Instead, sex ratios became less female biased, a shift in the direction of the optimum sex ratio for paternally inherited alleles according to genomic conflict theory. This was the first (albeit indirect) experimental evidence for genomic conflict over sex allocation. In their comment, Ellers and colleagues assayed the effects of 5-aza-dC on DNA methylation in 10 Nasonia genes, finding no evidence of demethylation in these 10 genes, from which they conclude that 5-aza-dC has no demethylating capability in N. vitripennis. Quantifying the efficacy of 5-aza-dC in terms of demethylation is indeed crucial to in-depth interpretation of studies using 5-aza-dC to link phenotypes to epigenetic regulation. Here we outline the mode of action of 5-aza-dC and demonstrate that determining the efficacy of 5-aza-dC in insect systems requires a whole-genome approach.
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