Biogeography and systematics of Hildenbrandia (Rhodophyta, Hildenbrandiales) in North America: inferences from morphometrics and rbcL and 18S rRNA gene sequence analyses

Morphometric analysis and phylogenetic analysis of sequences of the rbcL ch loroplast gene (which codes for the large subunit of the ribulose-1,5-bisph osphate carboxylase/oxygenase enzyme) and the nuclear 18S ribosomal RNA (rR NA) gene were carried out on 26 specimens of marine and freshwater Hildenbr andia from North America. Nineteen marine specimens were collected from Ala ska to Costa Rica on the Pacific coast and from Newfoundland to Connecticut on the Atlantic coast. Seven freshwater samples were collected from Texas, Costa Rica, St Lucia and Puerto Rico. Three groups of samples were disting uished by morphometric analysis: one containing all freshwater samples (H. angolensis Werwitsch ex West et West), one consisting of a marine sample wi th parallel tetrasporangial divisions (H. occidentalis Setchell ex Gardner) and one group with non-parallel tetrasporangial divisions (H, rubra (Somme rfelt) Meneghini. These groupings were partially incongruent with those obt ained by analyses of the molecular data. Parsimony and distance analyses of the rbcL gene resulted in trees in which Atlantic and Pacific clades were largely resolved. However, an Alaskan sample was included in the Atlantic g roup, which may indicate a trans-Arctic invasion event. The freshwater samp les were paraphyletic for the rbcL gene, among the marine collections, whic h supports the concept of multiple invasions establishing the freshwater po pulations in North America. The ISS rRNA gene sequence data indicate that t he freshwater samples are monophyletic with the exception of the unresolved position of the H. occidentalis sample. The freshwater samples form a mono phyletic clade when multiple outgroups are used. The rbcL data appear to be mutationally saturated above approximately 17% divergence, which makes int erpretation of phylogenetic signal among distant groups difficult. This may be a result of the asexual reproduction of the alga.