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Sexual selection and the evolution of sex-role reversal in honeylocust beetles
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Animal ecology. (Animal Ecology)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sexual selection is the prime evolutionary force that makes males and females different. This process has long been viewed as one where male compete with one another and where females choose. However, since the discovery that multiple mating by females is common in animals, sexual selection theory has been expanded to include mate competition between females and mate choice by males. However, empirical studies addressing these themes are scarce. In my thesis, I explore the evolution of sex role reversed mating systems using the honey locust beetles (Megabruchidius dorsalis and M. tonkineus). I used these species to shed light on (1) how closely sexual selection in females resembles its better‑studied male counterpart, (2) the implications of male mating costs for mating system evolution and (3) the effects of reproductive competition between females on the evolution of female courtship behaviour. By manipulating male mating rate, I found that males that mated more lived shorter lives, showing that mating is costly for males. I also demonstrated that males are choosy about whom they mate with and prefer vigorously courting females (Paper II). In contrast to males, previous studies suggested that female honey locust beetles benefit nutritionally from mating due to the large ejaculates provided by males. I manipulated male condition to show that male adult feeding had significant effects on female reproduction. Females that mated with males of good condition lived longer and produced more offspring than females whose mates were in poor condition (Paper III). When mating is costly for males, theory predicts that sexual selection in females can be strong. I compared sexual selection in honey locust beetles to that in two other species of seed beetles with conventional sex roles. I found substantial sexual selection in honey locust beetle females, which was comparable in strength to that in males (Paper I). I also measured the evolutionary effects of altered sex ratios on mating system parameters in both honey locust beetle species, using an experimental evolution design. Under female-biased sex ratios, representing strong sexual selection in females, females of M. dorsalis rapidly evolved elevated courtship intensity, thereby intensifying the reversal of sex roles (Paper V). In M. tonkineus, males evolved under male-biased sex ratios to transfer larger ejaculates, demonstrating the role of male-male reproductive competition for the evolution of male provisioning (Paper IV). My thesis highlights the essential, and often overlooked, role that females play in mating system evolution and that their contribution cannot simply be reduced to mate choice.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 46 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1240
National Category
Evolutionary Biology
Research subject
Animal Ecology
Identifiers
URN: urn:nbn:se:uu:diva-246715ISBN: 978-91-554-9209-0 (print)OAI: oai:DiVA.org:uu-246715DiVA: diva2:797038
Public defence
2015-05-13, Zootissalen, Evolutionary Biology Centre, Norbyvägen, Gamla Zoologen (Hus 1), Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2015-04-22 Created: 2015-03-09 Last updated: 2015-07-07
List of papers
1. Homage To Bateman: Sex Roles Predict Sex Differences In Sexual Selection
Open this publication in new window or tab >>Homage To Bateman: Sex Roles Predict Sex Differences In Sexual Selection
2013 (English)In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 67, no 7, 1926-1936 p.Article in journal (Refereed) Published
Abstract [en]

Classic sex role theory predicts that sexual selection should be stronger in males in taxa showing conventional sex roles and stronger in females in role reversed mating systems. To test this very central prediction and to assess the utility of different measures of sexual selection, we estimated sexual selection in both sexes in four seed beetle species with divergent sex roles using a novel experimental design. We found that sexual selection was sizeable in females and the strength of sexual selection was similar in females and males in role-reversed species. Sexual selection was overall significantly stronger in males than in females and residual selection formed a substantial component of net selection in both sexes. Furthermore, sexual selection in females was stronger in role-reversed species compared to species with conventional sex roles. Variance-based measures of sexual selection (the Bateman gradient and selection opportunities) were better predictors of sexual dimorphism in reproductive behavior and morphology across species compared to trait-based measures (selection differentials). Our results highlight the importance of using assays that incorporate components of fitness manifested after mating. We suggest that the Bateman gradient is generally the most informative measure of the strength of sexual selection in comparisons across sexes and/or species.

Keyword
Bateman gradient, mating system, sexual dimorphism, sperm competition
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-204985 (URN)10.1111/evo.12086 (DOI)000321184500009 ()
Available from: 2013-08-16 Created: 2013-08-13 Last updated: 2017-12-06Bibliographically approved
2. The cost of mating and mutual mate choice in 2 role-reversed honey locust beetles
Open this publication in new window or tab >>The cost of mating and mutual mate choice in 2 role-reversed honey locust beetles
2011 (English)In: Behavioral Ecology, ISSN 1045-2249, E-ISSN 1465-7279, Vol. 22, no 5, 1104-1113 p.Article in journal (Refereed) Published
Abstract [en]

Situations where both males and females simultaneously exercise mate choice may be much more common than previously believed. Yet, experimental studies of mutual mate choice are rare as is information on the types of female traits that are favored by male mate choice. We first assessed the cost of mating to males under different feeding regimes in 2 honey locust beetles (Bruchidae, Megabruchidius spp.) where females actively search for and court males. Further, in a series of mate choice trials, we manipulated female mating status and male food provisioning to assess how male and female characteristics affected the outcome of male-female interactions. Mating carried substantial costs to males, but these costs were independent of food availability. Males generally showed a preference for large females but also for females that delivered a more vigorous courtship display. Moreover, males preferred virgin females in one species but nonvirgin females in the other species, and we provide data suggesting that this choice is adaptive. Female choice was restricted to a lower rate of female mate rejection of larger males in one of the species. Our results reveal a striking interspecific variation in mutual mate choice, even between these closely related species, and show that sexual selection in females can act on much the same types of traits that are commonly considered sexually selected in males, such as size-related traits and courtship vigor. This suggests that a preference for condition-dependent traits may be a commonality that is shared between mate choice by both sexes.

Keyword
allometry, Bruchidae, mate choice, nuptial gifts, sex-role reversal, sexual selection
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-158884 (URN)10.1093/beheco/arr097 (DOI)000294358400029 ()
Available from: 2011-09-20 Created: 2011-09-19 Last updated: 2017-12-08Bibliographically approved
3. The effects of male phenotypic condition on reproductive output in a sex role-reversed beetle
Open this publication in new window or tab >>The effects of male phenotypic condition on reproductive output in a sex role-reversed beetle
2015 (English)In: Animal Behaviour, ISSN 0003-3472, E-ISSN 1095-8282, Vol. 102, 209-215 p.Article in journal (Refereed) Published
Abstract [en]

In insects with sex role reversal in mating, in which females actively court males, large and nutritious ejaculates are a common direct benefit to females. Such ejaculates are costly for males to produce and their size and composition can depend on male condition. However, the fitness effects to males and females of such condition-dependent provisioning are less clear. Here, we studied the effects of phenotypic condition on mating behaviour, ejaculate size and reproductive output in honeylocust beetles, Megabruchidius dorsalis. Our experimental design allowed us to disentangle the independent effects of juvenile resource acquisition in both sexes (as reflected by body size) and resource acquisition by adult males (feeding). We show that phenotypic condition of both sexes had sizeable independent and interactive effects on mating and reproductive output. In males, resources accrued during the juvenile phase had significant but relatively marginal effects on male mating and reproduction. Male adult feeding, in contrast, had sizeable effects on almost all aspects of male and female reproduction, through the nutritional effects of ejaculates in females. We discuss our findings in light of the reversal of both sex roles and sexual size dimorphism exhibited by this species, relative to related species. Our results highlight the importance of testing the interaction of male and female condition on components of fitness to understand the evolution and maintenance of mating systems.

Keyword
condition dependence, fecundity selection, nuptial gift, nutritious ejaculates, sex role reversal, sexual selection
National Category
Evolutionary Biology
Identifiers
urn:nbn:se:uu:diva-246714 (URN)10.1016/j.anbehav.2015.01.025 (DOI)000351058700021 ()
Available from: 2015-03-09 Created: 2015-03-09 Last updated: 2017-12-04Bibliographically approved
4. Sperm competition generates evolution of increased paternal investment in a sex role-reversed seed beetle
Open this publication in new window or tab >>Sperm competition generates evolution of increased paternal investment in a sex role-reversed seed beetle
2014 (English)In: Journal of Evolutionary Biology, ISSN 1010-061X, E-ISSN 1420-9101, Vol. 27, no 12, 2841-2849 p.Article in journal (Refereed) Published
Abstract [en]

When males provide females with resources at mating, they can become the limiting sex in reproduction, in extreme cases leading to the reversal of typical courtship roles. The evolution of male provisioning is thought to be driven by male reproductive competition and selection for female fecundity enhancement. We used experimental evolution under male- or female-biased sex ratios and limited or unlimited food regimes to investigate the relative roles of these routes to male provisioning in a sex role-reversed beetle, Megabruchidius tonkineus, where males provide females with nutritious ejaculates. Males evolving under male-biased sex ratios transferred larger ejaculates than did males from female-biased populations, demonstrating a sizeable role for reproductive competition in the evolution of male provisioning. Although larger ejaculates elevated female lifetime offspring production, we found little evidence of selection for larger ejaculates via fecundity enhancement: males evolving under resource-limited and unlimited conditions did not differ in mean ejaculate size. Resource limitation did, however, affect the evolution of conditional ejaculate allocation. Our results suggest that the resource provisioning that underpins sex role reversal in this system is the result of male-male reproductive competition rather than of direct selection for males to enhance female fecundity.

Keyword
experimental evolution, fecundity enhancement, Megabruchidius tonkineus, paternal investment, reproductive competition, sex role reversal
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-242014 (URN)10.1111/jeb.12549 (DOI)000346280100025 ()25394675 (PubMedID)
Available from: 2015-01-20 Created: 2015-01-20 Last updated: 2017-12-05Bibliographically approved
5. The experimental evolution of sex roles in beetles
Open this publication in new window or tab >>The experimental evolution of sex roles in beetles
(English)Article in journal (Other academic) Submitted
Keyword
sexual selection, female courtship, sex role reversal, mating system evolution
National Category
Evolutionary Biology
Research subject
Biology with specialization in Animal Ecology
Identifiers
urn:nbn:se:uu:diva-247577 (URN)
Available from: 2015-03-22 Created: 2015-03-22 Last updated: 2015-07-07

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