Adaptation and speciation: what is the role of chromosomal inversions?

The formation of new species via the processes of local adaptation and divergent selection is a key area of research for evolutionary biologists. The divergence of populations remaining connected by gene flow is particularly poorly understood, with debate over the underlying mechanisms. Speciation occurs via the accumulation of barriers to gene flow along the genome until populations are reproductively isolated from each other. Recombination following gene flow is known to have a homogenising effect between populations; therefore processes that prevent recombination can promote speciation by maintaining coadapted complexes of alleles at locally adaptive loci. Evidence supports the role of chromosomal rearrangements in creating recombination suppression. The intertidal snail Littorina saxatilis is emerging as a model system for studies into the genetic basis of local adaptation and speciation for a number of reasons, including the presence of distinct ecotypes adapted to different selection pressures along clines on the seashore linked by hybrid zones. Recent research on this species suggests the presence of a number of chromosomal inversions across the genome that may contribute to differences between these ecotypes. This PhD aims to investigate the roles of these inversions in local adaptation and divergence of the ecotypes using a variety of methods including field sampling and experimental manipulation, and behavioural, morphological and genomic analyses. One of the putative chromosomal inversions additionally displays differences between the sexes as well as between ecotypes. This is suggestive of young sex chromosomes in the initial stages of evolution, and an exciting potential interface between the processes of sex chromosome evolution and divergence with gene flow. Recombination suppression plays an important role in both processes so it is hypothesised that the putative inversion captures both locally adaptive loci and the sex-determining loci. A small number of studies have described similar effects in other species, but this is generally an unknown area with the potential for work on Littorina saxatilis to make a great contribution to knowledge. Therefore, some of the research in this PhD will focus on elucidating how this inversion may create an interaction between these two evolutionary processes.