Phylogeny of the Sphaeropleaceae: Inferences from the nuclear rDNA internal transcribed spacer regions

Traditional morphological approaches to sphaeropleacean taxonomy have emphasized the singular importance of comparative zygote ornamentation in establishing species boundaries. However, cladistic analysis of isozyme evidence within the genus Sphaeroplea suggests that zygote wall ornamentation may be a homoplastic character. An independent molecular data set derived from the internal transcribed spacer (ITS, ca. 500 bases per taxon) regions of the nuclear rDNA array was used to test competing phylogenetic hypotheses based on analyses of morpho-logical and isozyme data. The ITS data corroborate the general inferences from the isozyme evidence in suggesting that zygote wall ornamentation is either plastic or that distinct wall ornamentation types have evolved independently. Moreover, the ITS data indicate that the presence of elaborated septa in vegetative cells of some Sphaeroplea taxa (e.g., S. soleirolii var. crassisepta and S. wilmani) is a character that diagnoses a monophyletic group.