The Consequences of Sex Roles for Molecular Evolution in Birds

: (Alternative Format Thesis)

Abstract

Human-induced environmental changes – including climate change and the degradation of natural habitats – deliver new challenges for wild populations. Some of these populations will adapt and thrive, whilst others will not survive without conservation intervention. Understanding the drivers of adaptation is therefore not only a major goal in population genetics, but also helps to identify populations at risk from biodiversity conservation perspectives. Population resilience to changing environmental conditions depends on core traits affecting molecular evolution, in particular the mutation rate, the migration rate, the effective population size and the strength of selection. All of these traits are hypothesised to be influenced by sex role divergence – the extent to which males and females differ in their reproductive behaviour. However, the combined outcome of these various interactions and selective pressures is far from clear.

In this thesis, I aim to address this knowledge gap and provide some clarity to the relationship between divergent sex roles and molecular evolution, using birds, especially shorebirds (sandpipers, plovers and allies), as model organisms.

First, I build on the literature seeking to understand the drivers of sex role divergence, and show that latitude can drive variation in sex role divergence in incubation routines in the Common Ringed Plover. Second, I use whole genome resequencing data to reveal previously hidden genetic structure in the Ringed Plover, and show that despite female-biased dispersal, the Z chromosome remains stubbornly undifferentiated between populations. Third, I take a comparative approach and investigate the distribution of polymorphisms across the genomes of 150 bird species, revealing that the alignment of sexual selection and natural selection leads to the particularly efficient removal of harmful alleles in polygamous species. Fourth, I investigate the impact of role-reversed polyandry on the rate of evolution on the Z chromosome, and find that positive selection on recessive alleles in females may be particularly effective under this mating system.

Taken together, these analyses suggest that polygamy, which correlates highly with sex role divergence, leads to more efficient selection, and improves the genetic health of natural bird populations. The overall implications of divergent sex roles for population resilience depend also on other factors, such as inter-locus conflict and the extent to which sexually selected traits drag populations away from their naturally-selected optimum. Altogether, the research presented here paves the way for more accurate predictions of population resilience in natural populations – a major goal in both evolutionary genetics and biodiversity conservation.

Date of Award	17 Jan 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Tamas Szekely (Supervisor), Araxi Urrutia (Supervisor), Mike Bruford (Supervisor) & Guojie Zhang (Supervisor)