Differentiating hybridizing species in the Fucus complex

Intertidal rocky shores provide classical examples of habitat-driven divergent selection. We show that the species complex Fucus vesiculosus L./ F. spiralis L. is composed of three distinct genetic entities that have evolved along different time scales. Using assignment tests based on microsatellite markers and performed on randomly sampled individuals in two separate geographic regions (Portugal and France) we reveal that F. spiralis consists of two 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, bushy with dichotomous ramifications and no sterile rim 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 selfing hermaphroditism. Later, in the hermaphroditic lineage, parallel evolution of two co-occurring genetic clusters may have been driven by natural selection and facilitated by high selfing rates in the F. spiralis complex.

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