The influence of environmental heterogeneity and sexual selection on the genome of the Brown Booby (Sula leucogaster)

Abstract

Sexual selection has been a central theme in evolutionary biology since the time of Darwin, but even though field studies have shown that sexual selection on focal traits can vary across populations and over time, we rarely understand why. In some mating systems, ornamental traits (e.g., colorful displays) are thought to be condition-dependent signals that reveal the genetic characteristics and the current state of the bearer, acting as honest phenotypic cues through which the quality of potential mates can be assessed. In the present thesis, blood samples of brown boobies (Sula leucogaster brewsteri) were collected from males, females and offspring in three colonies with different environmental conditions in the Pacific Coast of Mexico. I used a customized ddRAD sequencing approach to generate genome-wide SNPs to investigate the genetic variability associated with phenotypes subject to sexual selection in brown boobies, and how selection of such traits might vary in the presence of environmental heterogeneity.
In chapter 2, I examined the mate choice and extra-pair paternity of brown boobies under different environments. A parentage analysis was performed to determine the rate of extra-pair paternity (EPP), and measurements of heterozygosity and genetic similarity were used to find correlations of genetic quality and ornaments used during courtship. Additionally, the genetic quality of the individuals was compared against measurements of colour and body mass which are traits that could be subject to sexual selection. The results suggest that variation in skin colour is an honest indicator that reflect the heterozygosity and dissimilarity of brown boobies, and that levels of EPP are very low and that it is potentially influenced by local environment.
In chapter 3, I investigated the distribution of the genetic diversity will and the correlation with geographic distance and to the environmental variables like sea surface temperature (SST) and primary productivity (PP) in each colony of brown boobies. I also measured the genomic structure, gene flow and demographic history of brown boobies and explore the possible implications in the context of mate choice. I found that brown booby populations in the Eastern Pacific seem to be highly differentiated and genetically isolated regardless of their relatively proximity. Additionally, a small but significant correlation between environmental variables and the genomic variation was found, which could be subject to natural or sexual selection.
In chapter 4, a specific sampling scheme of my genomic data was applied to characterize different signatures of selection (genome-wide selection component analysis framework). Two genome-wide selection scans were used to detect outlier loci under putative positive selection by comparing allele frequencies between males, females and offspring. The genomic regions near to such outlier loci were extracted to investigate the biological function (using Gene Ontology terms) associated with such regions. By comparing different groups of individuals based on their sex, living stage and geographical location, different types of selection were identified in brown boobies, though the caveats of the used framework are discussed. Various biological functions appear to be associated with different forms of selection simultaneously, like in the case of sex-specific viability, gametic and natural selection. However, the biological functions like growth, rhythmic processes and locomotion seems to be associated specifically to sexual selection.
Finally, I revisited the main results of my thesis in chapter 5 and discuss impact of my findings in a broader context.