Intralocus sexual conflict can maintain alternative reproductive tactics

Alternative reproductive tactics (ARTs) are ubiquitous throughout the animal kingdom and widely regarded as an outcome of high variance in reproductive success. Proximate mechanisms underlying ARTs include genetically based polymorphisms, environmentally induced polymorphisms, and those mediated by a combination of genetic and environmental factors. However, few ultimate mechanisms have been proposed to explain the maintenance of ARTs over time, the most important of which have been disruptive and negative frequency-dependent selection. Here we explore the role that intralocus sexual conflict may play in the maintenance of sex-specific ARTs. We use a genetically explicit individual-based model in which body size influences both female fecundity and male tactic through a shared genetic architecture. By modeling ART maintenance under varying selection regimes and levels of sex-specific gene expression, we explore the conditions under which intralocus sexual conflict can maintain a hypothetical ART defined by larger (alpha) and smaller (beta) tactics. Our models consistently revealed that sexual conflict can result in the persistence of a sex-specific polymorphism over hundreds of generations, even in the absence of negative frequency-dependent selection. ARTs were maintained through correlated selection when one male ART has lower fitness but produces daughters with higher fitness. These results highlight the importance of understanding selection on both sexes when attempting to explain the maintenance of ARTs. Our results are consistent with a growing literature documenting genetic correlations between male ARTs and female fitness, suggesting that the maintenance of sex-specific ARTs through intralocus sexual conflict may be common and widespread in nature.

Evolution of Drosophila buzzatii wings: Modular genetic organization, sex-biased integrative selection and intralocus sexual conflict

The Drosophila wing is a structure shared by males and females with the main function of flight. However, in males, wings are also used to produce songs, or visual displays during courtship. Thus, observed changes in wing phenotype depend on the interaction between sex-specific selective pressures and the genetic and ontogenetic restrictions imposed by a common genetic architecture. Here, we investigate these issues by studying how the wing has evolved in twelve populations of Drosophila buzzatii raised in common-garden conditions and using an isofemale line design. The between-population divergence shows that sexual dimorphism is greater when sex evolves in different directions. Multivariate Qst-Fst analyses confirm that male wing shape is the target for multiple selective pressures, leading males' wings to diverge more than females' wings. While the wing blade and the wing base appear to be valid modules at the genetic (G matrix) and among-population (D matrix) levels, the reconstruction of between-population adaptive landscapes (Ω matrix) shows selection as an integrative force. Also, cross-sex covariances reduced the predicted response to selection in the direction of the extant sexual dimorphism, suggesting that selection had to be intensified in order to circumvent the limitations imposed by G. However, such intensity of selection was not able to break the modularity pattern of the wing. The results obtained here show that the evolution of D. buzzatii wing shape is the product of a complex interplay between ontogenetic constraints and conflicting sexual and natural selections.

Female-limited X-chromosome evolution effects on male pre- and post-copulatory success

Intralocus sexual conflict arises when the expression of shared alleles at a single locus generates opposite fitness effects in each sex (i.e. sexually antagonistic alleles), preventing each sex from reaching its sex-specific optimum. Despite its importance to reproductive success, the relative contribution of intralocus sexual conflict to male pre- and post-copulatory success is not well-understood. Here, we used a female-limited X-chromosome (FLX) evolution experiment in Drosophila melanogaster to limit the inheritance of the X-chromosome to the matriline, eliminating possible counter-selection in males and allowing the X-chromosome to accumulate female-benefit alleles. After more than 100 generations of FLX evolution, we studied the effect of the evolved X-chromosome on male attractiveness and sperm competitiveness. We found a non-significant increase in attractiveness and decrease in sperm offence ability in males expressing the evolved X-chromosomes, but a significant increase in their ability to avoid displacement by other males' sperm. This is consistent with a trade-off between these traits, perhaps mediated by differences in body size, causing a small net reduction in overall male fitness in the FLX lines. These results indicate that the X-chromosome in D. melanogaster is subject to selection via intralocus sexual conflict in males.

Genomic signatures of globally enhanced gene duplicate accumulation in the megadiverse higher Diptera fueling intralocus sexual conflict resolution

Gene duplication is an important source of evolutionary innovation. To explore the relative impact of gene duplication during the diversification of major insect model system lineages, we performed a comparative analysis of lineage-specific gene duplications in the fruit fly Drosophila melanogaster (Diptera: Brachycera), the mosquito Anopheles gambiae (Diptera: Culicomorpha), the red flour beetle Tribolium castaneum (Coleoptera), and the honeybee Apis mellifera (Hymenoptera). Focusing on close to 6,000 insect core gene families containing maximally six paralogs, we detected a conspicuously higher number of lineage-specific duplications in Drosophila (689) compared to Anopheles (315), Tribolium (386), and Apis (223). Based on analyses of sequence divergence, phylogenetic distribution, and gene ontology information, we present evidence that an increased background rate of gene duplicate accumulation played an exceptional role during the diversification of the higher Diptera (Brachycera), in part by providing enriched opportunities for intralocus sexual conflict resolution, which may have boosted speciation rates during the early radiation of the megadiverse brachyceran subclade Schizophora.