COMPARISONS OF NUCLEAR-ENCODED SMALL-SUBUNIT RIBOSOMAL-RNAS REVEAL THE EVOLUTIONARY POSITION OF THE GLAUCOCYSTOPHYTA

The Glaucocystophyta (e.g., Cyanophora paradoxa) form a morphologicall y distinct group of photosynthetic protists that is primarily distingu ished by its cyanelles(= plastids). To elucidate their evolutionary re lationships, we determined nuclear-encoded small-subunit ribosomal RNA (SSU rRNA) coding regions for four taxa classified in the Glaucocysto phyta( C. paradoxa, Glaucocystis nostochinearum, Glaucosphaera vacuola ta, Gloeochaete wittrockiana; sensu Kies and Kremer), and these sequen ces were positioned within the eukaryotic phylogeny. Maximum likelihoo d, maximum-parsimony, and neighbor-joining phylogenetic analyses show that the Glaucocystophyta is a relatively late-diverging monophyletic assemblage within the ''crown'' group radiation that forms a sister gr oup to cryptophyte algae. Glaucosphaera vacuolata is a red alga and la cks some cyanelle (e.g., bounding peptidoglycan wall) and host cell (e .g., cruciate flagellar roots) characters typical of glaucocystophytes . Our data are consistent with a monophyletic origin of the cyanelle i n the glaucocystophytes. The distribution of photosynthetic taxa withi n the glaucocystophytes/cryptophytes and other lineages such as the fi lose amoebae/chlorarachniophytes and heterokont protists provide clues to the origin of plastids with four bounding membranes. We speculate that multiple, likely independent, secondary endosymbioses gave rise t o these plastids.