Phylogenetic study of Gymnodinium dorsalisulcum comb. nov. from tropical Australian coastal waters (Dinophyceae) Shauna Murray, 1 * Miguel de Salas, 2 Jim Luong-Van 3 and Gustaaf Hallegraeff 2 1 School of Biological Sciences AO8, University of Sydney, New South Wales, Australia, 2 School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania, Australia and 3 School of Science and Primary Industries, Casuarina Campus B.40, Charles Darwin University, Darwin, Northern Territory, Australia SUMMARY A new species of the dinoflagellate genus Gymnodinium Stein, previously considered a member of Katodinium Fott, is characterized from two marine benthic habitats in tropical northern Australia. Gymnodinium dorsalisul- cum comb. nov. was found to be very abundant at times, and in culture produced large quantities of mucus. We analyzed two regions of ribosomal DNA from this species (partial large subunit and complete small subunit sequences), using Bayesian analysis and phy- logenetic models appropriate to alignments of riboso- mal RNA genes. We compared it to eight species of the ‘true’ Gymnodinium clade and to other dinoflagellates. The results show that it is a member of the Gymnod- inium clade, and is closely related to Gymnodinium impudicum and G. chlorophorum. Katodinium was originally defined as having cells with an epitheca that is much larger than the hypotheca. However, this char- acter is clearly inadequate, and the genus requires a re-investigation to determine the apomorphies of the type species. Key words: Bayesian inference, dinoflagellates, Gymno- dinium, Katodinium, phylogeny, protists, ribosomal RNA. INTRODUCTION Gymnodinium (sensu Daugbjerg et al. 2000) is an extremely diverse clade of dinoflagellates, encompass- ing a variety of species with different lifestyles, inhab- iting different habitats and with different nutritional strategies. Gymnodinium catenatum is well-known as a source of toxins causing paralytic shellfish poisoning (Hallegraeff et al. 1988). Species of Gymnodinium sensu stricto are found in freshwater, marine plankton and sediment habitats, species may or may not possess plastids, may possess plastids with atypical pigments, and may or may not form chains. Two examples of the diversity present in this genus are the type species Gymnodinium fuscum (Ehrenberg) Stein, a freshwater planktonic species with a peridinin-containing plastid which does not form chains, and Gymnodinium venator Flø Jørgensen and Murray, a marine benthic species that is entirely heterotrophic (Flø Jørgensen et al. 2004). Katodinium is a relatively little-known genus of unarmoured, heterotrophic and autotrophic, marine and freshwater dinoflagellates. It was created by Fott (1957) who transferred species previously assigned to Massartia Conrad, and described a number of new species. A number of common species of Katodinium, K. rotundatum (Lohmann) Loeblich, K. asymmetricum (Massart) Loeblich, K. glandulum (Herdman) Loeblich, K. fungiforme Anissimowa (Loeblich) and an unnamed Katodinium species, have now been found to possess thecal plates in an apparently peridinioid-like pattern (Hansen 1995; Hoppenrath 2000; Murray 2003). The full plate patterns of most have not yet been described, while K. rotundatum has been found to belong to the genus Heterocapsa (Hansen 1995). Using scanning electron microscopy (SEM), many small species previ- ously considered to belong to Gymnodinium, Katod- inium or Gyrodinium have been found to possess thin thecal plates that were not visible with the light micro- scope (Steidinger et al. 1996). The apomorphy that defines membership of Gymno- dinium is the possession of a horse-shoe shaped apical groove, that encircles the apical region of the cell (Daugbjerg et al. 2000). In addition, several ultrastruc- tural features are thought to be common to species of Gymnodinium (Daugbjerg et al. 2000), although these have not been examined in all species transferred to the redefined Gymnodinium sensu stricto (Daugbjerg et al. 2000). These include a nuclear fibrous connector, or dorsal connective, which is a distinctive fiber that con- nects the longitudinal microtubular root 1 with the nucleus. In addition, many species are thought to