Phylogeny of Prymnesiales (Haptophyta) inferred from rDNA sequences and morphological data

Summary

The Prymnesiales is one of presently four orders within the haptophyte class Prymnesiophyceae and includes members of the genera Chrysochromulina, Corymbellus, Imantonia, Hyalolithus, Plathychrysis and Prymnesium. They are unicellular biflagellates with a third appendage called haptonema, and possess non-calcified, organic body scales. These haptophytes are important components of the marine and brackish water nanoplankton, and some species form blooms that may cause fish kills. Previous studies suggest that the genus Chrysochromulina is paraphyletic and that a taxonomic revision of this group is needed. The nucleotide sequence of the SSU rDNA was determined in 37 members of the Prymnesiales. Phylogenies based on these and 96 other haptophyte SSU rDNA sequences have been reconstructed and compared with available morphological and ultrastructural data. Members of Prymnesiales formed a monophyletic clade that diverged into two major clades, B1 and B2 with high support. Clade B1 included members of the genera Prymnesium, Platychrysis, Imantonia and some Chrysochromulina species, as well as a newly described species, Hyalolithus neolepis. All members of clade B1 have irregular to sphaeroid cell shape, with a haptonema that is usually equal or shorter in length than the flagella. Members of clade B2, in contrast, are saddle shaped and have a long and coiling haptonema. This group include the type species C. parva and has priority to the name Chrysochromulina. We suggest a revised taxonomy of Prymnesiales in accordance with available molecular evidence and morphological data.