Molecular phylogeny of the Haptophyta based on the rbcL gene and sequence variation in the spacer region of the RUBISCO operon

A phylogeny of 21 haptophyte algae was inferred by maximum parsimony, neigh bor-joining, and maximum likelihood analyses of sequences of the plastid-en coded gene, rbcL. Sequence variation in the spacer region of the RUBISCO op eron was also investigated. In all the rbcL trees constructed, the haptophy tes form two distinct clades: one includes the Pavlovales and the other inc ludes the Prymnesiales, Coccosphaerales, and Isochrysidales (sensu Parke an d Green 1976). This relationship coincides with the recent taxonomic treatm ent splitting the division into two subclasses, the Prymnesidae and Pavlovi dae (Cavalier-Smith 1989) or the Prymnesiophycidae and the Pavlovophycidae using botanical suffixes (Jordan and Green 1994), or into two classes, the Patelliferea and the Pavlovea (Cavalier-Smith 1993). In the Prymnesiophycid ae, all the coccolithophorids examined are placed in a single clade, which suggests a single origin of the coccolithophorids and the ability of coccol ith formation in the haptophytes. The genus Chrysochromulina is polyphyleti c. Species of Chrysochromulina with a very long haptonema and a compressed cell body (typical of species including the type C. parva Lackey) form a cl ade, including Imantonia, that is often classified in the Isochrysidales in the neighbor-joining tree, whereas some species possessing a nontypical ce ll body and cell covering form a clade with Prymnesium and Platychrysis in all trees. It is suggested that loss of the haptonema in Imantonia and the reduction in Prymnesium and Platychrysis occurred secondarily and independe ntly in two different lineages. Within the coccolithophorids, four clades a re recognized: Pleurochrysis, Calyptrosphaera-Cruciplacolithus-Calcidiscus- Umbilicosphaera, Helicosphaera, and Emtliania-Gephyrocapsa. A non-coccolith -bearing haptophyte, Isochrysis, is an ingroup of the Emiliania-Gephyrocaps a clade, suggesting its secondary loss of the ability to form a coccolith. Sequence comparison of the spacer region of RUBISCO operon supports most re sults obtained in the analysis of rbcL sequences. Monophyly of the Prymnesi ales sensu Parke and Green is still unclear because of low (<50%) bootstrap support for this group.