Summary: Adaptive Traits Are Maintained on Steep Selective
Gradients despite Gene Flow and Hybridization in the
Gerardo I. Zardi.
, Katy R. Nicastro*.
, Fernando Canovas, Joana Ferreira Costa, Ester A. Serra~o, Gareth A.
CCMAR-CIMAR Laborato´rio Associado, Universidade do Algarve, Gambelas, Faro, Portugal
Gene flow among hybridizing species with incomplete reproductive barriers blurs species boundaries, while selection under
heterogeneous local ecological conditions or along strong gradients may counteract this tendency. Congeneric, externally-
fertilizing fucoid brown algae occur as distinct morphotypes along intertidal exposure gradients despite gene flow.
Combining analyses of genetic and phenotypic traits, we investigate the potential for physiological resilience to emersion
stressors to act as an isolating mechanism in the face of gene flow. Along vertical exposure gradients in the intertidal zone
of Northern Portugal and Northwest France, the mid-low shore species Fucus vesiculosus, the upper shore species Fucus
spiralis, and an intermediate distinctive morphotype of F. spiralis var. platycarpus were morphologically characterized. Two
diagnostic microsatellite loci recovered 3 genetic clusters consistent with prior morphological assignment. Phylogenetic
analysis based on single nucleotide polymorphisms in 14 protein coding regions unambiguously resolved 3 clades;
sympatric F. vesiculosus, F. spiralis, and the allopatric (in southern Iberia) population of F. spiralis var. platycarpus. In contrast,
the sympatric F. spiralis var. platycarpus (from Northern Portugal) was distributed across the 3 clades, strongly suggesting