Phylogenetic systematics of the canal raphe bearing orders Surirellales and Rhopalodiales (Bacillariophyta)

Abstract

The class Bacillariophyceae, encompasses all diatoms that possess a raphe. Members of the orders Bacillariales, Rhopalodiales and Surirellales have a canal raphe system. Historically, the possession of this character was considered support for a close evolutionary relationship. To investigate phylogenetic relationships, a three gene dataset was constructed for 49 strains representing 9 of the 11 orders within the Bacillariophyceae. We provide the first formal tests of homology of the canal raphe system and the first demonstration that the Rhopalodiales are nested within the Surirellales. Results strongly reject the monophyly of Bacillariales, Rhopalodiales, and Surirellales thereby discounting the canal raphe as a homologous character. The Surirellales include three families: Surirellaceae, Entomoneideaceae, and Auriculaceae, while Rhopalodiales has only three genera in one family, the Rhopalodiaceae. In order to test familial and generic concepts, I expanded taxon representation and collected DNA sequence data for 125 strains. Taken together, only 5 of the 12 genera (Entomoneis, Stenopterobia, Cymatopleura, Petrodictyon and Epithemia) were found to be monophyletic. Our current concepts of the two most species-rich genera, Surirella and Campylodiscus, are too broad as the analyses resolved taxa from these two genera into multiple independent lineages. The “Robustoid” lineage, comprised of Surirella Robustae, Campylodiscus Robusti, and Stenopterobia, exhibits a high degree of endemism within ancient Lake Ohrid, with 17 species considered endemic or relict taxa. A dataset of three molecular markers and 71 Robustoid taxa from Lake Ohrid, Europe, and North America was used to construct a preliminary phylogeny. The aim was to investigate phylogenetic relationships and test hypotheses of speciation and morphological evolution. The recovered paraphyly of Lake Ohrid taxa eliminates the possibility that the Ohrid Robustoids as a whole are a product of intralacustrine speciation. However, sister relationships between putative Ohrid endemics and unexplored morphological diversity within some species complexes (e.g. Campylodiscus marginatus, Scoliodiscus spp.) leave open the possibility that these lineages may be the result of intralacustrine speciation.