Collinsiella (Ulvophyceae, Chlorophyta) and other ulotrichalean taxa with shell-boring sporophytes form a monophyletic clade

Four genera of Ulotrichales, Collinsiella, Eugomontia, Gomontia, and Monostroma, contain species with shell-boring codiolum phases in their life histories. The codiolum phases of these species are difficult to distinguish from each other, although the alternate phases are readily separated on the basis of morphology and development. The monotypic Eugomontia differs from the other genera in that both phases of the life history are shell-boring and multicellular. Phylogenetic analyses of 18S rDNA sequences from Collinsiella tuberculata, Eugomontia sacculata and Gomontia polyrhiza indicate that these species, together with Monostroma grevillei, form a distinct and well supported clade within the Ulotrichales. Codiolum phases were produced in culture from two populations of C. tuberculata, the type species of Collinsiella, the first record of such phases from North America. Zoospores from one population of these codiolum phases reproduced the thalloid phase. The ITS sequences from thalloid and codiolum phases of C. tuberculata are identical.