EVOLUTIONARY CONSEQUENCES OF THE LOSS OF PHOTOSYNTHESIS IN CHLAMYDOMONADACEAE - PHYLOGENETIC ANALYSIS OF RRN18 (18S RDNA) IN 13 POLYTOMA STRAINS (CHLOROPHYTA)

Complete sequences of the Rrn18 genes were obtained from 13 strains of the nonphotosynthetic algal genus Polytoma. Phylogenetic analyses sho wed that these strains formed two clades. One clade shows only modest sequence diversity but is represented by strains collected at widely d ispersed sites in Europe and America. The other clade consists of a si ngle isolate from the Canary islands. Both clades lie well within the extended clade that includes all species of Chlamydomonas for which se quence data are available. The two Polytoma clades are separated from each other by several green species, suggesting that the extant nonpho tosynthetic Chlamydomonadaceae arose from photosynthetic ancestors at least twice. These results suggest that nonphotosynthetic mutants are capable of establishing lineages that can spread widely but have a hig her probability of extinction than their photosynthetic congeners.