The nervous system is present in all but two animal phyla – one of them being Porifera, sponges. Sponges have no neurons and yet have organized behavior and finely tuned sensation. Furthermore, sponges have genes involved in the nervous system of other animals (informally called ‘neural’ genes). Do these genes impart a sensory capacity in sponges and does their presence suggest that the sponge sensory system is homologous to the nervous system? There are few manipulative genetic techniques for sponges; instead approaches include looking for the expression of ‘neural’ genes in sponge structures. A review of sponge gene expression studies found little correlation between the expression of ‘neural’ genes and sponge sensory structures. Instead non-sensory cells in sponges expressed just as many, if not more, ‘neural’ genes.

A scientist from the University of Alberta recently carried out an RNA-seq study to determine whether candidate ‘neural’ genes might be differentially upregulated in the osculum, a demonstrated sensory structure that is the excurrent vent of the sponge filtration system. Four candidate ‘neural’ genes – mGluR, GABAR, Kir and Bsh – were significantly upregulated in sponges with oscula compared to those in which oscula were still developing or in sponge body tissues. While glutamate (L-Glu) and GABA have been shown to trigger and arrest (respectively) sponge contraction behavior, glutamate and GABA receptors themselves may have roles in normal metabolic processes and therefore their upregulation in tissues may reflect differential activity of other activities that occur in the osculum.

Taken together, the data presented in this thesis suggest that genes involved in the nervous system of bilaterians are ineffective markers for sensory/coordinating systems in sponges. Instead, studying ‘neural’ genes without the assumption that they hold sensory or coordinating functions may provide a less biased way of investigating sensory-neural origins.

Differential gene expression during oscular development and in the osculum

Heatmaps of differentially expressed genes in (A) the pre-oscular (PON) and juvenile stages of Spongilla lacustris and (B) the body and oscular samples of Aphrocallistes vastus. Each row is a differentially expressed gene and each column is a biological replicate. Expression levels are indicated in the legend as median centered log2(TMM-normalized FPKM). Higher gene expression is indicated in yellow while lower gene expression is represented in purple.

 

Reference
J. L. Mah (2017) Gene expression and sensory structures in sponges: Explorations of sensory-neural origins in a non-bilaterian context. Thesis, University of Alberta [article]

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