Taxonomy and biogeography of Agarum and Thalassiophyllum (Laminariales, Phaeophyceae) based on sequences of nuclear, mitochondrial, and plastid markers

Agarum is a perennial kelp genus that has a broad distribution from the northwest Pacific to the northwest Atlantic Ocean. In order to address questions regarding phylogenetic relationships and present-day distribution patterns of species within this genus, we analyzed the nuclear ITS2, mitochondrial cox1 and cox3, and plastid RuBisCO spacer from a subset of 130 samples of the genus plus Thalassiophyllum and Costarla. All analyses of the individual markers and the ITS2+cox1+cox3 dataset consistently produced a clade of Agarum that included Thalassiophyllum. The clade consisted of two well-resolved subclades: one composed of A. fimbriatum and A. oharaense, and the other of A. clathratum, A. turneri, A. yakishiriense, and T. clathrus (the type of the generic name Thalassiophyllum). On the basis of these results, we treat Thalassiophyllum as a synonym of Agarum, reinstating the combination A. clathrus made by Greville in 1830. Despite its wide distribution from Japan through Alaska to the east coast of North America, A. clathratum had low genetic diversity, suggesting a relatively recent long-distance dispersal of the species. The low degree of substitutions between A. clathratum and A. yakishiriense suggests their recent divergence from a common ancestor, and we recognize the latter as a subspecies of the former. Because type material of A. clathratum has not been found, we designate as neotype a specimen collected in Kamchatka, one of two syntype localities cited by Gmelin for Fucus agarum.