Fucus vesiculosus and spiralis species complex: a nested model of local adaptation at the shore level

Intertidal rocky shores provide classic examples of habitat-driven divergent selection. We show that the species complex Fucus vesiculosus L./F. spiralis L. is composed of 3 distinct genetic entities that have evolved along different time scales. Using assignment tests based on microsatellite markers and performed on randomly sampled individuals in 2 separate geographic regions (Portugal and France), we reveal that F. spiralis consists of 2 genetic entities that have distinct vertical distributional patterns along the intertidal gradient of selective pressures. Individuals assigned to the cluster found higher on the shore are also morphologically different. They are smaller and bushy, with dichotomous ramifications and no sterile rime around receptacles. Patterns of genetic divergence suggest different times and pathways to reproductive isolation. Divergence between F. vesiculosus and the F. spiralis complex seems to have occurred first, coinciding with divergence in reproductive mode; dioecy versus selling hermaphroditism. Later, in the hermaphroditic lineage, parallel evolution of 2 co-occurring genetic clusters may have been driven by natural selection and facilitated by high selfing rates in the F. spiralis complex.