The Molecular Machinery of Gametogenesis in Geodia Demosponges (Porifera): Evolutionary Origins of a Conserved Toolkit across Animals.

All animals are capable of undergoing gametogenesis. The ability of forming haploid cells from diploid cells through meiosis and recombination appeared early in eukaryotes, whereas further gamete differentiation is mostly a metazoan signature. Morphologically, the gametogenic process presents many similarities across animal taxa, but little is known about its conservation at the molecular level. Porifera are the earliest divergent animals and therefore are an ideal phylum to understand evolution of the gametogenic toolkits. Although sponge gametogenesis is well known at the histological level, the molecular toolkits for gamete production are largely unknown. Our goal was to identify the genes and their expression levels which regulate oogenesis and spermatogenesis in five gonochoristic and oviparous species of the genus Geodia, using both RNAseq and proteomic analyses. In the early stages of both female and male gametogenesis, genes involved in germ cell fate and cell-renewal were upregulated. Then, molecular signals involved in retinoic acid pathway could trigger the meiotic processes. During later stages of oogenesis, female sponges expressed genes involved in cell growth, vitellogenesis, and extracellular matrix reassembly, which are conserved elements of oocyte maturation in Metazoa. Likewise, in spermatogenesis, genes regulating the whole meiotic cycle, chromatin compaction, and flagellum axoneme formation, that are common across Metazoa were overexpressed in the sponges. Finally, molecular signals possibly related to sperm capacitation were identified during late stages of spermatogenesis for the first time in Porifera. In conclusion, the activated molecular toolkit during gametogenesis in sponges was remarkably similar to that deployed during gametogenesis in vertebrates.

Analysis of the axial filament in spicules of the demosponge Geodia cydonium: different silicatein composition in microscleres (asters) and megascleres (oxeas and triaenes).

The skeleton of the siliceous sponges (Porifera: Hexactinellida and Demospongiae) is supported by spicules composed of bio-silica. In the axial canals of megascleres, harboring the axial filaments, three isoforms of the enzyme silicatein (-alpha, -beta and -gamma) have been identified until now, using the demosponges Tethya aurantium and Suberites domuncula. Here we describe the composition of the proteinaceous components of the axial filament from small spicules, the microscleres, in the demosponge Geodia cydonium that possesses megascleres and microscleres. The morphology of the different spicule types is described. Also in G. cydonium the synthesis of the spicules starts intracellularly and they are subsequently extruded to the extracellular space. In contrast to the composition of the silicateins in the megascleres (isoforms: -alpha, -beta and -gamma), the axial filaments of the microscleres contain only one form of silicatein, termed silicatein-alpha/beta, with a size of 25kDa. Silicatein-alpha/beta undergoes three phosphorylation steps. The gene encoding silicatein-alpha/beta was identified and found to comprise the same characteristic sites, described previously for silicateins-alpha or -beta. It is hypothesized, that the different composition of the axial filaments, with respect to silicateins, contributes to the morphology of the different types of spicules.