EVIDENCE FOR PARALLEL EVOLUTION OF FRUSTULE SHAPE IN TWO LINES OF PENNATE DIATOMS FROM THE EPIPHYTON

Abstract

The biraphid pennate genus, Rhoikoneis, originally included species flexed in girdle view with a central nodule on each valve. Recently, this genus was shown to contain taxa belonging to three distinct genera: Rhoikoneis, Campylopyxis, and Rhoicosphenia. Rhoikoneis stands apart from the other two in possessing slit-like external openings to the areolae, chambers above the helictoglossae, non-perforate heteromoiphic copulae, and an axial rib accompanying a laterally deflected raphe system. Results from a study of marine gomphonemoid taxa showed that the new cuneate genus, Pseudogomphonema, shared frustule characters with the genuflexed taxon, Rhoikoneis, as well as with Trachyneis. A second cuneate genus, Cuneolus, was clearly related to the other genuflexed taxa, Campylopyxis and Rhoicosphenia. The latter group shares frustule features of rounded areolae, perforate isomorphic copulae and a raphe system opening vertically at the top of a small ridge. Thus, two distinct lineages appear to have evolved in these marine taxa, most of which are found in the epiphyton. Frustulee shape appears to most susceptible to selective pressure in these diatoms as both cuneate and genuflexed taxa are present in both groups. Raphe, cingulum, and areola structure are more static and may be used to define higher taxonomie status, perhaps at the family or order level. Possible adaptive functions for these shapes in the epiphyton are discussed.