GENETIC DIFFERENTIATION AMONG 3 COLONY-FORMING SPECIES OF PHAEOCYSTIS- FURTHER EVIDENCE FOR THE PHYLOGENY OF THE PRYMNESIOPHYTA

Sequence data from the 18S small subunit ribosomal RNA gene have been used to support the species status of three colony-forming species of Phaeocystis Lagerheim (Prymnesiophyta). Two of these correspond to Pha eocystis globosa Scherffel and Phaeocystis pouchetii (Hariot) Lagerhei m. The third species originates from antarctic waters and is referred to Phaeocystis antarctica, described by Karsten at the turn of the cen tury. Morphological and physiological data supporting the separation o f the three species is compiled from the literature. Phylogenetic tree s generated from the sequence data suggest that the warm-water species , Phaeocystis globosa diverged prior to the separation of the two cold -water forms. Tectonic events and climatic changes during the middle t o late Cenozoic provide mechanisms by which speciation events could ha ve occurred as both polar oceans were being formed.