Comparative studies on gametangial distribution and structure in species of Zonaria and Homoeostrichus (Dictyotales, Phaeophyceae) from Australia

Abstract

A comparative study of fertile gametophytes has been carried out on six species of the Dictyotales from Australia. Patterns of gametangial distribution and structure are similar in Zonaria angustata, Zonaria diesingiana, Zonaria spiralis and Zonaria turneriana, but there are some marked differences apparent between both Homoeostrichus olsenii and Homoeostrichus sinclairii and the Zonaria species. Sterile cortical cells and mucilage between the cylindrical oogonia are characteristic of the indusiate female sori of Zonaria, which differ markedly from the comparable structures of H. olsenii, in which all cortical cells have been transformed into oogonia and the walls of adjacent oogonia are contiguous. In contrast, the large spherical oogonia of H. sinclairii are unique in the Dictyotales, lacking a stalk cell and occurring among brown paraphyses in a large non-indusiate sorus on each terminal branch segment. Antheridial sori of the six species exhibit far less structural variation than the oogonial sori. Male sori are surrounded by undifferentiated cortical cells in H. olsenii, by tall brown paraphyses and short paraphyses in H. sinclairii and by sterile cells in Zonaria spp. Short paraphyses form only around the male sori of H. sinclairii in the Dictyotales; they differ in structure from the brown paraphyses but are similar to the antheridia, from which they may have been derived. Gametangial characters have proved invaluable in defining the genus Zonaria and in separating H. olsenii from H. sinclairii, but they cannot be used to delimit the species of Zonaria. Furthermore, the differences observed in the gametangia have provided new characters for phylogenetic analyses, as well as providing further supporting evidence for the recognition of Zonaria and Homoeostrichus as distinct entities.