Molecular phylogeny of glass sponges (Porifera, Hexactinellida): increased taxon sampling and inclusion of the mitochondrial protein-coding gene, cytochrome oxidase subunit I

Marine sponges of the class Hexactinellida (glass sponges) are among the most understudied groups of Porifera, and molecular approaches to investigating their evolution have only recently emerged. Although these first results appeared reliable as they largely corroborated morphology-based hypotheses, they were almost exclusively based on ribosomal RNA genes (rDNA) and should, therefore, be further tested with independent types of genetic data, such as protein-coding genes. To this end, we established the mitochondrial-encoded cytochrome oxidase subunit I gene (COI) as an additional marker, and conducted phylogenetic analyses on DNA- and amino-acid level, as well as a supermatrix analysis based on combined COI DNA and rDNA alignments. Furthermore, we increased taxon sampling compared to previous studies by adding seven additional species. The COI-based phylogenies were largely congruent with the rDNA-based phylogeny but suffered from poor bootstrap support for many nodes. However, addition of the COI sequences to the rDNA data set increased resolution of the overall molecular phylogeny. Thus, although obtaining COI sequences from glass sponges turned out to be quite challenging, this gene appears to be a valuable supplement to rDNA data for molecular evolutionary studies of this group. Some implications of our extended phylogeny for the evolution and systematics of Hexactinellida are discussed.