Phylogenetic analysis of "Volvocacae" for comparative genetic studies

Sequence analysis based on multiple isolates representing essentially all g enera and species of the classic family Volvocaeae has clarified their phyl ogenetic relationships. Cloned internal transcribed spacer sequences (ITS-1 . and ITS-2, flanking the 5.8S gene of the nuclear ribosomal gene cistrons) were aligned, guided by ITS transcript secondary structural features, and subjected to parsimony and neighbor joining distance analysis. Results conf irm the notion of a single common ancestor, and Chlamydomonas reinharditii alone among all sequenced green unicells is most similar. interbreeding iso lates were nearest neighbors on the evolutionary tree in all cases. Some ta xa, at whatever level, prove to be clades by sequence comparisons, but othe rs provide striking exceptions. The morphological species Pandorina morum, known to be widespread and diverse in mating pairs, was found to encompass all of the isolates of the four species of Volvulina. Platydorina appears t o have originated early and not to fall within the genus Eudorina, with whi ch it can sometimes be confused by morphology. The four species of Pleodori na appear variously associated with Eudorina examples. Although the species of Volvox are each clades, the genus Volvox is not The conclusions confirm and extend prior, more limited, studies on nuclear SSU and LSU rDNA genes and plastid-encoded rbcL and atpB. The phylogenetic tree suggests which cla ssical taxonomic characters are most misleading and provides a framework fo r molecular studies of the cell cycle-related and other alterations that ha ve engendered diversity in both vegetative and sexual colony patterns in th is classical family.