A molecular investigation of the systematic relationships of Sirodotia species (Batrachospermales, Rhodophyta) in North America

Six collections of the freshwater red algal genus Sirodotia from North Amer ica were sampled as follows: S. tenuissima (Collins) Skuja ex Flint from Rh ode Island, S. suecica Kylin from Rhode Island and Labrador, and S. huillen sis (Welwitsch ex West et G.S.West) Skuja from Texas, Mexico, and Costa Ric a. To investigate relationships among these taxa, a fragment of the rbcL ge ne was sequenced. The ITS1-5.8S rDNA-ITS2 region, the rbcL-S spacer, and pa rt of the rbcS gene were sequenced for all taxa and all but one collection. The rbcL gene sequence analysis showed S. suecica and S. tenuissima to be paraphyletic. The S. huillensis samples formed a monophyletic clade, with t he Texas and Mexico collections being most closely related. The rbcL-S spac er and partial bcS gene sequence data showed the same trend as the rbcL gen e data, with S. suecica and S. tenuissima having identical sequence and S. huillensis samples forming a distinct, monophyletic clade. The rbcL-S space r region may provide evidence for a closer phylogenetic relationship betwee n Sirodotia and Batrachospermum gelatinosum (Linnaeus) DeCandolle than with the Lemaneaceae. In comparing the ITS1-5.8S rDNA-ITS2 region from S. sueci ca and S. tenuissima, little sequence divergence (similar to 2%) was observ ed. Previous work on these two taxa revealed little morphological different iation. Therefore, we conclude that S. suecica and S. tenuissima should be synonymized; S. suecica has priority. Based on the sequence variation among S. huillensis collections, further phylogeographic studies are recommended . The utility of ITS1-5.8S rDNA-ITS2 region data for phylogenetic and phylo geographic studies is discussed.