Within the class Chlorophyceae, the family Hydrodictyaceae (order Sphaeropleales) consists of taxa that form flat or net-like coenobia and reproduce asexually by way of biflagellated zoospores. Two taxa within this family, Pediastrum and Hydrodictyon, share many features during development, especially the manner of daughter colony formation. The colonies of Pediastrum differ from Hydrodictyon in that growth is planar resulting in two-dimensional colonies, while three-dimensional nets are formed in Hydrodictyon. Studies of the Chlorophyceae using morphological and ultrastructural data, as well as molecular sequence data, have supported the close relationship of Pediastrum and Hydrodictyon and they have remained in the family Hydrodictyaceae throughout major classification revisions. However, in these studies only single species of Pediastrum (P. duplex) and Hydrodictyon (H. reticulatum) were included, therefore the exact relationship of these two taxa could not be explored. Preliminary molecular data from a second species of Pediastrum, P. boryanum, indicate that Hydrodictyon may be derived from Pediastrum. In this analysis, H. reticulatum resolves as sister taxon to P. duplex with P. boryanum the ancestral taxon. Further molecular studies of the family Hydrodictyaceae allow exploration of the relationships between Pediastrum spp. and Hydrodictyon spp., and assist in determining whether they are monophyletic. Preliminary analyses include H. reticulatum and H. africanum and several species of Pediastrum, P. duplex, P. boryanum, P. tetras, P. biradiatum and P. simplex. An understanding of the phylogenetic relationships among the species of Pediastrum and Hydrodictyon will allow a more complete comparison of colony formation.

Key words: Chlorophyceae, Hydrodictyaceae, Hydrodictyon, Pediastrum