Background
Rocky shores usually comprise a heterogeneous environment with steep gradients of several physical factors and biological interactions, providing a natural laboratory to study local adaptation and ecological speciation. The marine gastropods of the genus Littorina, in particular L. saxatilis, have been increasingly recognised as one of the most interesting model systems to study these processes in the intertidal zone. Multiple ecotype pairs (‘Crab’ and ‘Wave’) of this species have independently diverged across the intertidal gradient of different geographic regions (Sweden, UK and Spain), as a response to similar selective pressures (crab predation and wave exposure, respectively). Despite the evidence in favour of parallel phenotypic divergence and reproductive isolation between L. saxatilis ecotypes, until the beginning of this project the main genomic regions influenced by natural selection and their distribution across the genome were largely unknown. Moreover, although the importance of chromosomal rearrangements (e.g. inversions) in adaptation and speciation is widely recognised, knowledge about their role in diversification was still limited to some taxa and had not yet been thoroughly investigated in molluscs. Benefiting from genomic resources that became available for L. saxatilis (e.g. reference genome and linkage map), INVERTIDAL tried to fill this gap by assessing the impact of inversions on the origin of phenotypic diversity observed in L. saxatilis.
Goals
Leveraging on the potential of the L. saxatilis system and of emerging genomic tools, four main goals were defined: i) test for the presence of chromosomal rearrangements, inversions in particular, in this system; ii) evaluate the role of inversions in local adaptation, ecotype evolution and reproductive isolation between L. saxatilis ecotypes; iii) quantify the level of sharedness of inversions involved in adaptation and diversification among countries; and iv) assess the main processes contributing for a sharedness of inversions (standing genetic variation in the ancestral population that expanded versus gene flow).
