Final Report Summary - AGEINGSEXUALCONFLICT (Senescence and sexual conflict in a model insect species (Tribolium castaneum): a test of evolutionary ideas)
The fundamental question 'why do organisms age?' remains among the most enduring challenges in evolutionary biology. One of the most complex aspects of ageing is sex-specific selection on ageing rates and, more generally, the dynamic link between ageing and sexual conflict. Although sexual conflict was recognized over 30 years ago, its role in adaptive evolution has only recently received theoretical and empirical attention. Sexual conflict is currently recognized as a key evolutionary process, and recent evidence suggests that it can drastically influence patterns of ageing by acting as both a catalyst of and a constraint to the evolution of sex-specific life histories. Considerations of sexual conflict are therefore fundamental to understand why males and females age at different rates, the costs of sex and, ultimately, the evolution of ageing. Unfortunately, ageing theory has largely neglected sexual conflict, while theory of sexual conflict has only begun to consider ageing. The integration of these topical areas of evolutionary biology is in its infancy and, as a consequence, comprehensive work investigating the relationship between sex-specific patterns of ageing and – simultaneously- interlocus sexual conflict (IRSC) and intralocus sexual conflict (IASC; the two fundamental mechanisms of sexual conflict) has been extremely rare. Using a model insect species with intense sexual conflict and sex-specific senescence rates, the current project adopted a strongly experimental approach aimed to address some of the fundamental evolutionary questions concerning ageing and sexual conflict. In particular, the main original questions set out to address by this project were: a) Does the intensity of interlocus conflict (IRSC) influence ageing rates and, does senescence foster IRSC?, and b) Does intralocus conflict (IASC) constrain the evolution of sex-specific ageing rates?. However, little before the beginning of this project a couple of theoretical papers were published on applications of kin selection theory to sexual conflict. Briefly, these studies suggested that kin selection could be an ignored but crucial driving factor in the evolution of both sexual conflict intensity and its effects on sex-specific ageing. Given the potential impact of these predictions on the original aims of this project and its novelty, we set out to address the following questions (see mid-term report for detailed justification of deviations from original project plan):
Does kin selection modulate sexual conflict intensity and reproductive ageing?
On a set of experiments recently published in Nature, we provide the first empirical ‘proof of concept’ of the idea that kin selection may modulate sexual conflict intensity and reproductive ageing idea in Drosophila melanogaster (Carazo, Tan, Allen, Wigby, & Pizzari, 2014). Briefly, we experimentally manipulated the relatedness of groups of male D. melanogaster competing over females to demonstrate that, as expected, within-group male relatedness inhibits male competition and female harm. Females exposed to groups of brothers (unrelated to her) had higher lifetime reproductive success and slower reproductive ageing compared to females exposed to groups of males that were unrelated to each other.
How widespread and important might kin selected modulation of sexual conflict be?
On a theoretical paper submitted to BioEssays (currently in second round of review), we review the potentially widespread implications of the finding above by exploring how population structure may generally affect sexual conflict intensity. In this paper, we: a) call into question a widespread assumption in sexual selection research, that populations are well-mixed (panmictic), b) analyze how population structure may affect sexual conflict while taking into account the role of females, population demography and the types of underlying proximate mechanisms, c) provide an overview of relevant case samples, and d) model the influence of population structure (i.e. group size and local relatedness) on the evolution of male-male competition, when such competition is mediated by traits that either directly harm females or not. We conclude by identifying some new lines of research at the interface of sexual selection and social evolution.
Does modulation of sexual conflict intensity and reproductive ageing have trans-generational effects?
Third, we conducted an experiment to investigate the potential existence of trans-generational effects in kin selected modulation of sexual conflict. We replicated the AAA (related males) vs. ABC (unrelated males) environments that females were subject to in Carazo et al. 2014 (see above) and then studied the effects on the lifespan of offspring from those females. While the results are still being analyzed, preliminary data: a) replicated the effect of male relatedness on male-male aggression described in Carazo et al. 2014, and b) show that male relatedness not only benefits females intra-generationally by decreasing female reproductive senescence, but also affect the lifespan of their offspring, hence providing evidence that kin selected modulation of sexual conflict might have a more important net impact that suspected.
Does kin selection modulate sexual conflict intensity and reproductive ageing?
On a set of experiments recently published in Nature, we provide the first empirical ‘proof of concept’ of the idea that kin selection may modulate sexual conflict intensity and reproductive ageing idea in Drosophila melanogaster (Carazo, Tan, Allen, Wigby, & Pizzari, 2014). Briefly, we experimentally manipulated the relatedness of groups of male D. melanogaster competing over females to demonstrate that, as expected, within-group male relatedness inhibits male competition and female harm. Females exposed to groups of brothers (unrelated to her) had higher lifetime reproductive success and slower reproductive ageing compared to females exposed to groups of males that were unrelated to each other.
How widespread and important might kin selected modulation of sexual conflict be?
On a theoretical paper submitted to BioEssays (currently in second round of review), we review the potentially widespread implications of the finding above by exploring how population structure may generally affect sexual conflict intensity. In this paper, we: a) call into question a widespread assumption in sexual selection research, that populations are well-mixed (panmictic), b) analyze how population structure may affect sexual conflict while taking into account the role of females, population demography and the types of underlying proximate mechanisms, c) provide an overview of relevant case samples, and d) model the influence of population structure (i.e. group size and local relatedness) on the evolution of male-male competition, when such competition is mediated by traits that either directly harm females or not. We conclude by identifying some new lines of research at the interface of sexual selection and social evolution.
Does modulation of sexual conflict intensity and reproductive ageing have trans-generational effects?
Third, we conducted an experiment to investigate the potential existence of trans-generational effects in kin selected modulation of sexual conflict. We replicated the AAA (related males) vs. ABC (unrelated males) environments that females were subject to in Carazo et al. 2014 (see above) and then studied the effects on the lifespan of offspring from those females. While the results are still being analyzed, preliminary data: a) replicated the effect of male relatedness on male-male aggression described in Carazo et al. 2014, and b) show that male relatedness not only benefits females intra-generationally by decreasing female reproductive senescence, but also affect the lifespan of their offspring, hence providing evidence that kin selected modulation of sexual conflict might have a more important net impact that suspected.