The fitness cost to females of exposure to males does not depend on water availability in seed beetles

Monogamy removes constraints on reproductive tissue investment imposed by intense sexual selection

A key factor influencing the reproductive success and fitness of an individual is the allocation of resources to reproductive tissue. Previous research has documented positive influences of sexual selection on the evolution of male traits such as testes size. However, few studies have focused on whether sexual selection shapes overall reproductive tissue investment patterns, and even fewer have studied it in both males and females. Furthermore, whether the ecological/demographic context modulates these effects is not known. We used experimental evolution on an insect model system to assess the independent and interactive effects of sexual selection intensity (intense [polygamy] vs. relaxed [monogamy]) and metapopulation structure (absent vs. present) on reproductive tissue investment in both sexes. Counterintuitively, we found that both males and females evolving under monogamy had higher relative reproductive tissue investment than individuals evolving under polygamy. We found no independent effect of metapopulation structure or any interactive effect with sexual selection on reproductive tissue investment of either sex. Our findings suggest that trade-offs between pre- and post-copulatory sexual selection may constrain reproductive tissue investment under intense sexual selection, and that selection under monogamy removes this constraint. Generally, our study underscores the complexities underpinning the evolution of reproductive allocation strategies.

Do more food choices lead to bad decisions? A case study in predaceous ladybird beetle, Propylea dissecta

Understanding why animals choose one food over another is one of the key questions underlying the fields of behaviour ecology. This study aims to test if ladybird beetle, Propylea dissecta Mulsant (Coleoptera: Coccinellidae) can forage selectively for nutrients in order to redress specific nutritional imbalances to maximise their fitness. We hypothesised that the presence of more food choices leads to bad decisions in terms of their food selection which ultimately negatively affects the mating and reproductive parameters of P. dissecta. To test this, we first manipulated the predator's nutritional status by rearing them in five separate dietary groups from first instar larvae to newly emerged adult stage. Thereafter, we tested their food choice between five different foods, i.e. Aphis craccivora Koch, Aphis nerii Boyer de Fonsclombe, conspecific eggs, heterospecific eggs and mixed pollen grains, equidistantly placed in a Petri dish. Based on the food choice of the newly emerged adults, they were reared on the chosen diet for 10 days. Thereafter, adults were paired with their opposite sex (collected from stock culture reared on A. craccivora) and mating and reproductive parameters were recorded. Our results suggested that the variety of food did not affect the food choice of ladybird beetle, P. dissecta. They tend to choose their natural diet, i.e. aphid in each dietary regime. We found that previous dietary regime, i.e. larval dietary regime, significantly influences the mating and reproductive parameters of both the male and female except for the time to commence mating by the male. Food choices of adult beetles were found to significantly influence the time to commence mating, average fecundity and per cent egg viability in males and only mating duration in females. Our findings suggest that P. dissecta consistently made optimal decisions when facing various food choices. They consistently preferred their natural and preferred food choice over others, indicating a strong food selection behaviour.

Omnivorous Notoxus trinotatus Pic (Coleoptera: Anthicidae) is a newly recognized vector of northern leaf blight in maize

The adaptations of omnivorous insects to food are manifested in a multifaceted manner, and the availability of food resources directly determines insect feeding tendencies, which contribute to a complex insect-food relationship and impact insect functionality in the environment. Stable isotope analysis was applied to test the feeding preference and further define the functional role of omnivorous beetles in cropland. Our results confirmed that as an omnivorous beetle, the fungivorous nature of Notoxus trinotatus accounted for a prominent proportion food selection at the adult stage, and more importantly, this dietary feature contributed to the dispersal of the northern corn leaf blight in maize (NLB) during the feeding trials. In addition to the preference for fungi, water supplementation was an essential element extending adult longevity, which directly prolonged the contact time of adults with pathogenic fungi in agricultural fields. Consistent with the herbivorous characteristics of beetles, before the emergence of NLB fungal pathogens, corn tissues served as the main food, which provided the beetles with more opportunities to transmit fungal pathogen propagules. We conclude that the role of N. trinotatus in carrying NLB pathogen is due to its feeding on this plant mycopathogen, and an increased abundance of beetles carrying the pathogen may increase the rate of NLB disease infestation. More focus should be concentrated on the functions of fungivorous beetles, not only as pathogen-transmitting pests, but also as an element among the balanced biotic factors in farmland.

How does access to water at different life-stages affect male investment in reproduction and survival?

Holometabolous insects have four distinct life-stages – eggs, larvae, pupae and adults. Active resource acquisition generally occurs during either or both the larval and adult stages. Previous research on the acquisition of food resources in holometabolous insects, has shown that resources acquired during each of these life-stages can differ in how they are allocated to different traits, and how they affect fitness. In addition to food, water is also an essential resource needed for a range of biological processes that enhance fitness. Yet, how water acquired at different life-stages affects key fitness traits is still not clear. Here, we manipulated both developmental and adult water availability of male seed beetles (Callosobruchus maculatus) to explore how water acquired at each life-stage affects a range of fitness related traits, including body mass, ejaculate weight, mating behavior and lifespan. Our results indicate that the juvenile environment had little effect on body weight, ejaculate weight, ejaculate replenishment or lifespan, but did influence male mating behavior in both early and late adult life. This contrasts with the adult environment, which had strong effects on weight loss, ejaculate size and lifespan but little effect on mating behavior. Males with access to water during adulthood transferred larger ejaculates during their first mating, lost less weight as they aged, and lived longer. These results suggest that water acquired during development and adulthood are allocated differently to a variety of fitness traits across life.

Does developmental environment affect sexual conflict? An experimental test in the seed beetle

Sexual conflict and sexually antagonistic coevolution are driven by differences in reproductive interests between the sexes. There have been numerous studies focused on how both the social and physical environment that individuals experience as adults, or where mating occurs, mediate the intensity of sexual conflict. However, how the physical environment that juveniles experience, mediates their later mating interactions, is still poorly understood. In seed beetles, Callosobruchus maculatus, water is an important resource that can impact fitness and reproduction. Here, we manipulated the water content of beans that beetles were reared in and explored how this environmental variation affects mating interactions and subsequent male and female fitness. We measured the mass of ejaculate transferred, mating behavior, female fecundity, and offspring production as well as male and female lifespan. We found that males reared in wet environments transferred a larger ejaculate to females, but only when females were reared in dry environments. We also found that females mated to males reared in dry environments laid more eggs than those mated to males from wet environments. Additionally, eggs laid by females reared in dry conditions had greater survival when they had mated to males reared in dry than wet environments. Overall, however, there were no treatment effects on the number of adult offspring females produced nor male or female adult lifespan, thus it is difficult to determine the evolutionary implications of these results. Our research provides evidence for the importance of developmental environment for determining the expression of adult mating and fitness traits.

No evidence for divergence in male harmfulness or female resistance in response to changes in the opportunity for dispersal

The outcome of sexual conflict can depend on the social environment, as males respond to changes in the inclusive fitness payoffs of harmfulness and harm females less when they compete with familiar relatives. Theoretical models also predict that if limited male dispersal predictably enhances local relatedness while maintaining global competition, kin selection can produce evolutionary divergences in male harmfulness among populations. Experimental tests of these predictions, however, are rare. We assessed rates of dispersal in female and male seed beetles Callosobruchus maculatus, a model species for studies of sexual conflict, in an experimental setting. Females dispersed significantly more often than males, but dispersing males travelled just as far as dispersing females. Next, we used experimental evolution to test whether limiting dispersal allowed the action of kin selection to affect divergence in male harmfulness and female resistance. Populations of C. maculatus were evolved for 20 and 25 generations under one of three dispersal regimens: completely free dispersal, limited dispersal and no dispersal. There was no divergence among treatments in female reproductive tract scarring, ejaculate size, mating behaviour, fitness of experimental females mated to stock males or fitness of stock females mated to experimental males. We suggest that this is likely due to insufficient strength of kin selection rather than a lack of genetic variation or time for selection. Limited dispersal alone is therefore not sufficient for kin selection to reduce male harmfulness in this species, consistent with general predictions that limited dispersal will only allow kin selection if local relatedness is independent of the intensity of competition among kin.

Sexual selection, phenotypic plasticity and female reproductive output

In a rapidly changing environment, does sexual selection on males elevate a population's reproductive output? If so, does phenotypic plasticity enhance or diminish any such effect? We outline two routes by which sexual selection can influence the reproductive output of a population: a genetic correlation between male sexual competitiveness and female lifetime reproductive success; and direct effects of males on females' breeding success. We then discuss how phenotypic plasticity of sexually selected male traits and/or female responses (e.g. plasticity in mate choice), as the environment changes, might influence how sexual selection affects a population's reproductive output. Two key points emerge. First, condition-dependent expression of male sexual traits makes it likely that sexual selection increases female fitness if reproductively successful males disproportionately transfer genes that are under natural selection in both sexes, such as genes for foraging efficiency. Condition-dependence is a form of phenotypic plasticity if some of the variation in net resource acquisition and assimilation is attributable to the environment rather than solely genetic in origin. Second, the optimal allocation of resources into different condition-dependent traits depends on their marginal fitness gains. As male condition improves, this can therefore increase or, though rarely highlighted, actually decrease the expression of sexually selected traits. It is therefore crucial to understand how condition determines male allocation of resources to different sexually selected traits that vary in their immediate effects on female reproductive output (e.g. ornaments versus coercive behaviour). In addition, changes in the distribution of condition among males as the environment shifts could reduce phenotypic variance in certain male traits, thereby reducing the strength of sexual selection imposed by females. Studies of adaptive evolution under rapid environmental change should consider the possibility that phenotypic plasticity of sexually selected male traits, even if it elevates male fitness, could have a negative effect on female reproductive output, thereby increasing the risk of population extinction. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Male responses to sperm competition when rivals vary in number and familiarity

Males of many species adjust their reproductive investment to the number of rivals present simultaneously. However, few studies have investigated whether males sum previous encounters with rivals, and the total level of competition has never been explicitly separated from social familiarity. Social familiarity can be an important component of kin recognition and has been suggested as a cue that males use to avoid harming females when competing with relatives. Previous work has succeeded in independently manipulating social familiarity and relatedness among rivals, but experimental manipulations of familiarity are confounded with manipulations of the total number of rivals that males encounter. Using the seed beetle Callosobruchus maculatus, we manipulated three factors: familiarity among rival males, the maximum number of rivals encountered simultaneously and the total number of rivals encountered over a 48 h period. Males produced smaller ejaculates when exposed to more rivals in total, regardless of the maximum number of rivals they encountered simultaneously. Males did not respond to familiarity. Our results demonstrate that males of this species can sum the number of rivals encountered over separate days, and therefore the confounding of familiarity with the total level of competition in previous studies should not be ignored.