EVOLUTIONARY CONSEQUENCES OF THE LOSS OF PHOTOSYNTHESIS IN CHLAMYDOMONADACEAE: PHYLOGENETIC ANALYSIS OFRrn18(18S rDNA) IN 13POLYTOMASTRAINS (CHLOROPHYTA)1

Complete sequences of theRrn 18genes were obtained from 13 strains of the nonphotosynthetic algal genusPolytoma.Phylogenetic analyses showed that these strains formed two clades. One clade shows only modest sequence diversity but is represented by strains collected at widely dispersed sites in Europe and America. The other clade consists of a single isolate from the Canary Islands. Both clades lie well within the extended clade that includes all species ofChlamydomonasfor which sequence data are available. The twoPolytomaclades are separated from each other by several green species, suggesting that the extant nonphotosynthetic 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 higher probability of extinction than their photosynthetic congeners.