Largest evolutionary study of sponges sheds new light on animal evolution

The article is also authored by experts from the University of Alberta (Canada) and Harvard University (United States).

Transcriptome: the study of gene expression

The article constitutes the largest sequencing study to date on the genes of representatives from eight sponge genera covering all four currently recognized sponge classes. To be exact, analysed species were Aphrocallistes vastus (Class Hexactinellida), Chondrilla nucula, Ircinia fasciculata, Petrosia ficiformis, Spongilla lacustris, Pseudospongosorites suberitoides (Class Demospongiae), Sycon coactum (Class Calcarea) y Corticium candelabrum (Class Homoscleromorpha). These species represent diverse habitats: from the deep sea to freshwater lakes and rivers.

The research group has analysed the transcriptome of these species, in other words, the set of all mRNA molecules that reflect gene expression profiles of the genome. Ana Riesgo explains that "to date, we only knew the genome and transcriptome of Amphimedon queenslandica and Oscarella carmela." "The study analysed the genome of these eight species and performed comparative analysis of animal genes important for signalling, neuronal and ionic conduction, epithelia, immunity and reproduction, which are the basic functions associated to multicellularity," she adds.

Surprising genetic complexity

The study identifies a wide set of genes -- previously associated with complex structures in higher animals -- which were supposedly absent in sponges. For example, the genus Corticium also uses the gene DMRT1 for sex determination; this gene was thought to be present only in more complex metazoans (reptiles, birds, mammals, etc.).

Homoscleromorph sponges have always been considered more similar to metazoans than to sponges. Ana Riesgo states that "they have more complex morphological characteristics than other sponges and, in the case of Corticium, they follow the same process of gametogenesis than cnidarians." Moreover, the presence of DMRT1 gene, also present in cnidarians, confirms "the great heterogeneity of sponges despite their simple morphology," affirms the expert.

Another noteworthy finding is the detection of glutamate receptor genes (iGluRs) -- involved in rapid physiological response transmission -- in sponges Corticium, Sycon, and Iricinia. Although it has always been believed that sponges are so simple than their responses to stimuli are minimal, the study identifies some genes involved in nerve conduction in more complex animals: "This proves that sponges' responses to environmental changes or physical stimuli may be more complex than it was thought," underlines Ana Riesgo, who also participates in the project Actiquim, focused on chemical ecology in Antarctica.

Novel gene functions throughout evolution

Nearly all genes identified in sponges have more complex functions in metazoans. Nevertheless, gene combination and regulation is more relevant than gene presence in the time-scale of evolution. In sponges, genes seem to be involved in more basic structures. Ana Riesgo points out that "the function of many of these genes remains unknown. Probably, the genes we know from other animals have a different function in sponges and were co-opted later for the function we are familiar with." Determining gene function in sponges is the next challenge.

The article published on the journal Molecular Biology and Evolution provides the framework for posing new hypotheses on the phylogenetic relations among poriferans, determining gene function in sponges and the early evolution of molecular complexity in metazoans.