XVI International Botanical Congess

Mesostigma is generally classified in the Prasinophyta, a group of scaly green flagellates thought to occupy a basal phylogenetic position relative to the Streptophyta and Chlorophyta. However, recent studies of rDNA sequences by Nakayama et al. demonstrated that Prasinophyta are not monophyletic, instead, the taxa studied formed four independent lineages within the Chlorophyta. Our phylogenetic analyses of the 18S rDNA sequence of Mesostigma (which was referred to, but not included in the Nakayama study) showed that it is part of the streptophyte clade (in support of actin sequence comparisons) and is a sister group of Chaetosphaeridium which occupies a basal position in the Streptophyta 18S rDNA tree. Hormidiella is the sister group of Klebsormidium, and may be the closest known algal relative of the embryophyte land plants.