Phylogenetic reconstructions of the Haptophyta inferred from 18S ribosomalDNA 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 t wo flagella. The small subunit ribosomal RNA gene (18S ribosomal DNA) from 18 haptophyte species has been sequenced, and the sequences aligned with th ose of more than 300 published and unpublished chlorophyll a + c algae. Phy logenies were constructed using maximum likelihood, neighbor-joining, and w eighted maximum parsimony analyses. The high divergence (6%) between member s of Pavlova and the remaining haptophytes supports the division of Haptoph yta into two classes: Prymnesiophyceae and Pavlovophyceae. Three major clad es that correspond to known taxa within the Prymnesiophyceae were identifie d: 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. B ased on 18S ribosomal DNA sequence data and available information on morpho logical structure and ultrastructure, we propose that the class Prymnesioph yceae be divided into four orders: Phaeocystales ord, nov., Prymnesiales, I sochrysidales, and Coccolithales. A total of 1-2% divergence at this level in the 18S ribosomal RNA gene analysis warrants a separation above the leve l of family. Within the Pavlovophyceae, a new genus is established, Rebecca J.C. Green gen. nov., into which Pavlova salina and Pavlova helicata are m oved.