Determining the affinities of salt marsh fucoids using microsatellite markers: Evidence of hybridization and introgression between two species of fucus (Phaeophyta) in a Maine estuary

The high degree of morphological plasticity displayed by species of the brown algal genus Fucus L. is well documented. Such variation is especially pronounced for those estuarine taxa lacking holdfasts (termed ecads) that often bear little resemblance to the attached species from which they are derived. To better understand the systematics of salt marsh fucoids, we developed a suite of four microsatellite-containing loci capable of distinguishing between F. vesiculosus L. and F. spiralis L. The genetic markers were used to determine the relationships of the fucoid ecads F. vesiculosus ecad volubilis (Hudson) Turner and a muscoides-like Fucus in the Brave Boat Harbor (ME, USA) estuary. Ecad populations had 2- to 3-fold higher levels of heterozygosity than attached populations of F. vesiculosus and F. spiralis. Further, ecads were intermediate between F. vesiculosus and F. spiralis in their allele frequencies and genotype composition. Our data indicate that populations of muscoides-like Fucus in Brave Boat Harbor mainly consist of F-1 hybrids between F. vesiculosus and F. spiralis, whereas F. vesiculosus ecad volubilis may arise through backcrosses between F. vesiculosus and other fertile hybrids. Finally, our data support the hypothesis that introgression has occurred between attached populations of F. vesiculosus and F. spiralis