Download According to a study published on the journal Molecular Biology and

According to a study published on the journal Molecular Biology and Evolution, most
genes involved in complex processes are present in sponges. The first author of the
article is Ana Riesgo, Juan de la Cierva researcher at the Department of Animal
Biology and the Biodiversity Research Institute (IRBio) of the University of
Barcelona. Sponges or Porifera -- there are over 8,000 species currently recognised
-- are the most basal phylum of metazoans. They represent a rich animal diversity
found throughout the world, from tropical climates to the arctic poles, and they are an
excellent model to study metazoan 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 wereAphrocallistes 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 ofAmphimedon
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.