Genetic architecture of the shell characteristics in the marine snail Littorina saxatilis

Speciation is a key process underlying biodiversity. This process is facilitated by local adaptation, when divergent selection overcomes gene flow, resulting in the accumulation of reproductive barriers. Theory suggests that this accumulation is strongly dependent on the genetic architecture of the traits underlying local adaptation. The aim of this project was to investigate the genetic architecture of locally adaptive traits in the marine snail Littorina saxatilis. This marine snail (Littorina saxatilis) is an excellent model to study speciation and local adaptation. Two diverging ecotypes live a few metres apart in distinct habitats and face divergent selection pressures dominated by crab predation and wave action. The ecotypes have evolved traits to adapt locally that make them behaviourally and structurally distinct. The most observable differences are seen in the shell size, shape, colours and patterns. Despite the differences, the two ecotypes meet in narrow contact zones and hybridize. Intermediates between the two parental ecotypes are observed in a crab-wave environmental gradient across the hybrid zones. This situation provides an excellent opportunity to exploit the power of association mapping in the hybrid zone to elucidate the genetic architectures of the locally adaptive traits. However, a prerequisite for the application of evolutionary genetic approaches is a genomic toolbox. In Chapters 2 and 3, I describe the construction of a transcriptome assembly and high-density linkage map for this species. These genetic resources were utilized in the subsequent analyses and other studies in this system. In Chapter 4, I investigate the genetic architecture of the adaptive shell traits. Theory suggests that the ground colours or banding patterns possess Mendelian inheritance and may respond directly to selection or may be linked with genes that respond to the physical environment and may thus be affected by selection. Shell morphometric characters (size and shape) may have a more complex pattern of inheritance and tend to be responsive to the environmental conditions. Thus, shell characteristics are excellent to study divergent selection pressures and local adaptation while making it imperative to understand their underlying genetic architecture. In the current study, we applied association analysis to a single hybrid zone in Sweden to elucidate the genes underlying six shell phenotypic traits (size, shape, banding pattern, ground colours – beige, black and dark beige). We sampled individuals from the hybrid zone and implemented targeted capture-sequencing to obtain genotypic data. We identified loci associated with the black and beige ground colours and banding pattern of the shell. No significant associations with the shell shape and size were found which may suggest polygenic and complex architecture, consistent with the theoretical expectation. In addition, our analysis suggests a possible role for chromosomal inversion underlying locally adaptive traits. This thesis addressed longstanding questions regarding the genetic architecture of the adaptive shell traits in this organism and provides directions for the future follow-up studies. The genetic resources described in this thesis will assist the future studies that may address a wide-range of evolutionary questions in this species.