Phylogenetic reconstructions of the Haptophyta inferred from 18S ribosomal DNA sequences and available morphological data

Most haptophytes are unicellular, photosynthetic flagellates, although some have coccoid, colonial, amoeboid, or filamentous stages. Nearly all have a characteristic filamentous appendage, the haptonema, arising between the two flagella. The small subunit ribosomal RNA gene (18S ribosomal DNA) from 18 haptophyte species has been sequenced, and the sequences aligned with those of more than 300 published and unpublished chlorophyll a + c algae. Phylogenies were constructed using maximum likelihood, neighbor-joining, and weighted maximum parsimony analyses. The high divergence (6%) between members of Pavlova and the remaining haptophytes supports the division of Haptophyta into two classes: Prymnesiophyceae and Pavlovophyceae. Three major clades that correspond to known taxa within the Prymnesiophyceae were identified: one clade embraces Phaeocystis spp.; the second includes members of the genera Chrysochromulina, Prymnesium, and Imantonia; and the third includes coccolithophorid genera and the genus Isochrysis. Two other clades contain taxa whose sequences were derived from a gene clone library. These taxa are not strongly related to any of the cultured taxa included in this study. Based on 18S ribosomal DNA sequence data and available information on morphological structure and ultrastructure, we propose that the class Prymnesiophyceae be divided into four orders: Phaeocystales ord, nov., Prymnesiales, Isochrysidales, and Coccolithales. A total of 1-2% divergence at this level in the 18S ribosomal RNA gene analysis warrants a separation above the level of family. Within the Pavlovophyceae, a new genus is established, Rebecca J.C. Green gen. nov., into which Pavlova salina and Pavlova helicata are moved.