Evolution of the polyphyletic genus Pleurastrum (Chlorophyta):: inferences from nuclear-encoded ribosomal DNA sequences and motile cell ultrastructure

To assess the taxonomic position of members of the genus Pleurastrum Chodat, which was shown in previous studies to be polyphyletic, small subunit (18S) ribosomal RNA coding regions were determined for additional taxa from this group,. P. erumpens Deason et Bold, P. paucicellulare Vischer, P. sarcinoideum Groover et Bold, Leptosira obovata Vischer, and for the ulvophytes, Pseudendoclonium basiliense Vischer and Ulothrix zonata (Weber et Mohr) Kutzing. In addition, motile cell ultrastructure was investigated for P. paucicellulare and P. insigne Chodat (the type species of Pleurastrum). Both types of characters demonstrate P. paucicellulare as a member of the Ulvophyceae/Ulotrichales, and a close relationship of P. insigne with the genus Chlamydopodium Ettl et Komarek within the Chlorophyceae. A very close relationship of P. P.acicellulare and P. sorcinoideum with Gloeotilopsis planctonica Iyengar et Philipose within the Ulvophyceae/Ulotrichales is clearly resolved by analyses of the 18S and internal transcribed spacer (ITS) rDNA sequences. This is also corroborated by the examination of group I introns that interrupt the 18S rDNA sequences of these algae. The 18S rRNA coding regions of L. obovata, P. erumpens, and P. terrestre Fritsch et John are identical; these taxa belong to the Trebouxiophyceae. Based on the rDNA sequence analyses, P. paucicellulare and P. sarcinoideum are transferred to the genus Gloeotilopsis Iyengar et Philipose, and the previous assignments of P. erumpens and P. terrestre to the genus Leptosira Borzi are substantiated. The rDNA phylogenies provide strong evidence that the Ulvophyceae/Ulotrichales and the Trebouxiophyceae are evolutionarily far removed from each other and represent two independent lineages of green algae.