Bibliographic Detail
Zardi, G.I., Nicastro, K.R., Canovas, F., Costa, J.F., Serrão, E.A. & Pearson, G.A., 2011
Reference:
Zardi, G.I., Nicastro, K.R., Canovas, F., Costa, J.F., Serrão, E.A. & Pearson, G.A. (2011). Adaptive traits are maintained on steep selective gradients despite gene flow and hybridization in the intertidal zone. PLoS ONE 6(6): 1-13.
Publication Date:
June 14, 2011 (on line and copies distributed)
Abstract:
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, externallyfertilizing
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
hybridization/introgression with both other entities. Common garden experiments showed that physiological resilience
following exposure to desiccation/heat stress differed significantly between the 3 sympatric genetic taxa; consistent with
their respective vertical distribution on steep environmental clines in exposure time. Phylogenetic analyses indicate that F.
spiralis var. platycarpus is a distinct entity in allopatry, but that extensive gene flow occurs with both higher and lower shore
species in sympatry. Experimental results suggest that strong selection on physiological traits across steep intertidal
exposure gradients acts to maintain the 3 distinct genetic and morphological taxa within their preferred vertical distribution
ranges. On the strength of distributional, genetic, physiological and morphological differences, we propose elevation of F.
spiralis var. platycarpus from variety to species level, as F. guiryi.
