Phylogeny and Biogeography of Chara (Charophyta) Inferred from 18S rDNA Sequences

Abstract

Competing hypotheses concerning the relationships among subsections of the genus Chara were tested with parsimony and maximum likelihood analysis of small subunit (18S) ribosomal DNA gene (rDNA) sequences. The traditional hypothesis as stated by Wood and Imahori (1965) separates Chara into subgenera and subsections based primarily on stipulode and cortication characters, whereas Proctor’s (1980) hypothesis proposed subgenera based on the break-up of Pangea into Laurasia and Gondwana, and subsections based on the subsequent break-up of these two supercontinents. Both parsimony and maximum likelihood analyses strongly refuted Wood and Imahori’s hypothesis, as well as the importance traditionally placed on stipulode and cortication characters. Partial or total loss of cortication may occur within a subsection, as demonstrated by C. imperfecta, C. braunii, C. muelleri and C. brittonii. Although the nodal stem cell gives rise to the branchlets, stipulodes and axial cortex, stipulodes can be retained despite the loss of cortication as demonstrated by C. hydropitys, which is placed solidly within Willdenowia by both analyses. Proctor’s hypothesis was supported with Gondwanan subsections basal (except for C. tomentosa) and Laurasian subsections placed together in a monophyletic clade in both analyses, although the placement of subsection Grovesia is uncertain. Monophyly of subsections Wallmania and Willdenowia and Lamprothamnium are strongly supported. Lamprothamnium is placed within the genus Chara, which disagrees with previous (McCourt et al. 1996) rbcL analyses. Further studies are needed to identify phylogenetically informative morphological characters for extant taxa, and a method is needed to link extant species to the extensive fossil data for a fuller understanding of the phylogeny and biogeography of this distinctive genus.