First genetic evidence of interspecific hybridization in the genus Sargassum (Fucales, Phaeophyceae)

In Fucales, naturally occurring interspecific hybrids have mostly been observed in the genus Fucus, with no reports involving the genus Sargassum. In the present study, we found two suspected hybrids of Sargassum siliquastrum and Sargassum yezoense (KTS39 and ESS20) that exhibited intermixed morphological traits between the parental species. To verify these suspected hybridizations, we collected 115 samples (57 S. siliquastrum individuals, two suspected hybrids and 56 S. yezoense individuals) from two localities for genetic analyses using the mitochondrial marker nad6 and newly developed 21 microsatellite markers. A clustering analysis using STRUCTURE revealed two clusters corresponding to S. siliquastrum and S. yezoense, among which 12 individuals (KTS35–39, KTS75, KTS76, ESS10 and ESS17–20) had low assignment probabilities (P < 0.9). Of these 12 individuals, ten (KTS36–39, KTS75, KTS76and ESS17–20) were identified as hybrids based on NewHybrids analysis, which calculates assignment probabilities for F1, F2 and backcross categories. KTS39, ESS18 and ESS19 had a high probability of being backcrossed to S. siliquastrum (P > 0.895), KTS36, KTS76 and ESS20 were classified as backcrosses with S. yezoense (P > 0.750), KTS37 was classified as F1 hybrid (P = 0.733), and the remaining three individuals (KTS38, KTS75 and ESS17) were assigned to multiple categories with probabilities ranging from 0.25 to 0.60. Although holdfast height and the branching patterns of the main branches were intermediate in suspected hybrids (KTS39 and ESS20), morphological analysis showed that all hybrids were difficult to distinguish from pure species. These results provide the first genetic evidence of naturally occurring interspecific hybrids in Sargassum, although we lacked detailed hybrid categorization. The observed hybridization may be attributed to the low genetic differentiation within the section Halochloa. Further taxonomic reassessment is needed for this group.