Sex determination, adaptive divergence, and the role of inversions in ecotypes of the intertidal snail Littorina saxatilis

Sex determination and sex chromosomes exhibit great diversity within and between species, in their mechanisms and drivers, and over space and time. Why and how this variation has evolved remains poorly understood and empirical evidence remains scarce. Species with young, emerging sex determination systems are useful tools for understanding these processes, in particular in species with labile sex determination between environments. The role of ecology in sex-specific selection is unclear but likely to be central to these processes. Understanding the drivers and mechanisms for adaptive divergence, reproductive isolation and sex determination in the same populations will be crucial and allows the interface between these processes to be tested. In this thesis, I investigate the above processes in the intertidal snail, Littorina saxatilis, utilising a combination of genomic approaches based on previous field sampling and manipulative experiments in natural populations. I uncover evidence for a multigenic female-heterogametic sex determining system in operation in multiple populations that involves genomic regions on two linkage groups and multiple sex-linked inversions. I characterise patterns of sex- and ecotype-specific genomic associations of SNPs and inversions, revealing the role of habitat-dependent sex-specific selection and the concurrent role of inversions in the differentiation of both sexes and ecotypes. Through experimental testing, I quantify differential survival and movement of locally adapted ecotypes across habitats, specifically evidencing the role of divergent selection and habitat choice in reproductive isolation between ecotypes. I further show that arrangement frequencies of two sex-linked inversions are mediated by sex-, ecotype-, and habitat-based selective interactions. Together, my findings shed light on drivers and mechanisms of sex determination, ecotype divergence, and the role of inversions in L. saxatilis. More generally, they offer insight into the relationship between sex- and environment-based selection and transitions in sex- determination systems across heterogeneous environments, uncovering an ideal system for future work.