Molecular phylogeny of labyrinthulids and thraustochytrids based on the sequencing of 18S ribosomal RNA gene

Labyrinthulids and thraustochytrids are unicellular heterotrophs, formerly considered as fungi, but presently are recognized as members in the stramen opiles of the kingdom Protista sensu late. We determined the 18S ribosomal RNA gene sequences of 14 strains from different species of the six genera a nd analyzed the molecular phylogenetic relationships. The results conflict with the current classification based on morphology, at the genus and speci es levels. These organisms are separated, based on signature sequences and unique inserted sequences, into two major groups, which were named the laby rinthulid phylogenetic group and the thraustochytrid phylogenetic group. Al though these groupings are in disagreement with many conventional taxonomic characters, they correlated better with the sugar composition of the cell wall. Thus, the currently used taxonomic criteria need serious reconsiderat ion.