A number of Chlorophyte algae have lost the ability to photosynthesize, and live as obligate heterotrophs identified by their lack of chlorophyll. Among these are the species called Polytoma which are very similar to the genus Chlamydomonas except for the lack of chlorophyll. Nonphotosynthetic mutants of facultative heterotrophic Chlamydomonadaceae are easily obtained, and Polytoma species presumably descended from one or more such mutants. Polytoma retains a plastid with DNA and functional ribosomal RNA (rRNA) genes. We have undertaken a long-range study to determine the molecular phylogenetic relationships of Polytoma species to green algae, determine how frequently obligate heterotroph species arise, and estimate how long heterotrophic clades survive and how often they produce new clades. Our nuclear and a plastid rRNA gene sequences from P. uvella show that this species lies within one branch of Chlamydomonas, closely related to C. humicola. Now we propose to sequence the small nuclear and large plastid rRNA genes from the strains of Polytoma now in culture, determine the phylogenetic relationships of these stocks to the species of Chlamydomonadaceae for which rRNA genes are being sequenced in other laboratories, and begin collecting new stocks of Polytoma from nature for future studies.
