Nucleotide sequences from the nuclear rDNA internal transcribed spacers (ITS1 and ITS2) and seven morphological characters were compared among 10 isolates of siphoncladalean algae representing six species in Boodlea, Chamaedoris, Cladophoropsis and Struvea. Parsimony analysis of both datasets revealed that Struvea is not monophyletic, Struvea elegans Børgesen being more closely related to Chamaedoris peniculum(Ellis et Solander) Kuntze and Struvea anastomosans (Harvey) Piccone et Grunow being more closely related to Cladophoropsis membranacea (Hofman Bang ex C. Agardh) Børgesen. Parsimony analysis of the ITS data further indicates that Cladophoropsis is not monophyletic and that Cladophoropsis and Boodlea may be paraphyletic. The influences of life history strategies, overlapping gene pools and the possibility of introgression are discussed. Finally, the point is made that although the results help to clarify phylogenetic relationships among species within these genera, they also illustrate the hazard of a priori assumptions about generic monophyly in which a single species (often the most commonly recognized one) is used to represent that genus in a biogeographic and/or phylogenetic study.
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Kooistra et al. (1993) studied this question.