Evolutionary relationship of Porifera within the eukaryotes

Abstract

Molecular data, gathered in our laboratory, strongly support the view that all metazoan phyla, including Porifera, are of monophyletic origin. More recently, a hypothetical common ancestor for all Metazoa, the Urmetazoa, had be formulated. The Urmetazoa possessed besides the key structures of an extracellular matrix also immune molecules and signal transduction molecules that are characteristic to Metazoa. In addition, we could establish that all Metazoa share a common ancestry with the Fungi and Plantae, while the unicellular eukaryotes are only distantly related. With respect to the origin of freshwater sponges, especially the species found in the Lake Baikal as a representative for an “old” lake, have been investigated. Sequence comparisons were performed with the ubiquitously distributed freshwater sponge Spongilla lacustris (family Spongillidae) as well as with one marine sponge, Suberites domuncula. The sequence comparison of the mitochondrial COI gene revealed a monophyletic grouping of the endemic baikalian sponges with S. lacustris as the most related species to the common ancestor. The sequences from Baikalospongia recta, B. intermedia, B. bacillifera and Lubomirskia baicalensis were found to be identical and separated from that of Swartschewskia lacustris and S. papyracea. In a further approach the exon/intron sequences framing the intron-2 of the sponge tubulin gene were chosen for the phylogenetic analysis. Data analyses revealed again a monophyletic grouping with S. lacustris as the closest related species to the common ancestor. It is concluded that the baikalian sponges, which have been studied here, are of monophyletic origin. Furthermore, the data suggest that the endemic species S. papyracea is the phylogenetic oldest still extant endemic baikalian sponge species.