Ultrastructure of the vegetative colonies and systematic position of Basichlamys (Volvocales, Chlorophyta)

Vegetative ultrastructure of the colonial green flagellate Basichlamys sacculifera was examined, with particular regard to the extracellular matrix. Each cell of the colonies was surrounded by a tripartite layer of matrix, the “cellular boundary”. This situation, essentially different from that in the Volvocaceae, is similar to that found in the Goniaceae and the Tetrabaenaceae. Cladistic analysis based on morphological data from B. sacculifera and 25 species of the colonial Volvocales suggested that Basichlamys and Tetrabaena constitute a monophyletic group which is sister to the large monophyletic group composed of the Volvocaceae and Goniaceae. The Tetrabaenaceae thus contains these two genera and has a synapomorphy “reticulate zygote walls”.