The evolution and function of the Pax/Six regulatory network in sponges

Abstract

Examining the origins of highly conserved gene regulatory networks (GRNs) will inform our understanding of the evolution of animal body plans. Sponges are believed to be the most ancient extant metazoan lineage, and as such, hold clues about the evolution of genetic programs deployed in animal development. We used the emerging freshwater sponge model, Ephydatia muelleri, to study the evolutionary origins of the Pax/Six/Eya/Dac (PSED) GRN. Orthologs to Pax and Six family members are present in E. muelleri and are expressed in endothelial cells lining the canal system as well as cells in the choanoderm. Knockdown of EmPaxB and EmSix1/2 by RNAi resulted in defects to the canal systems. We further show that PaxB may be in a regulatory relationship with Six1/2 in E. muelleri, thus demonstrating that a component of the PSED network was present early in metazoan evolution.

Comparative analyses of developmental transcription factor repertoires in sponges reveal unexpected complexity of the earliest animals

“…Demosponges, including Ephydatia and Amphimedon, have only one SINE class gene each, belonging to the Six1/2 subfamily Hill et al, 2010;Rivera et al, 2013). We have recently reported that calcisponges have three Six genes each, although despite performing several phylogenetic analyses we were not able to confidently classify these genes into recognized bilaterian subfamilies (Fortunato et al, 2014a).…”

“…The Pax family has been included in several recent studies investigating the origins of the Pax-Six-Eya-Dach gene regulatory network (Hoshiyama et al, 2007;Hill et al, 2010;Rivera et al, 2013;Fortunato et al, 2014a). Demosponges have a single Pax gene with clear affinity to the PaxB (Pax2/5/8) subfamily (Larroux et al, 2006;Hill et al, 2010).…”

Vertebrate Cranial Placodes as Evolutionary Innovations—The Ancestor's Tale

“…An Atonal like gene is expressed in globular cells, an unciliated cell with potential sensory functions (Richards et al, 2008), while Six1/2 and PaxB are expressed in contractile pinacocytes (Hill et al, 2010;Rivera et al, 2013). Unless sponges have secondarily lost neurons (see above), this suggests that sensory cells in the eumetazoans may have evolved by co-option and diversification of several molecular components expressed in different sponge cells.…”

RNA interference in early branching metazoans

“…The feeding of dsRNA (betaactin) expressing bacteria produced significant but modest mRNA down regulation and as above, the phenotype that was observed resembled general ill health. In a separate study on E. muelleri, successful knockdown by feeding was concluded following the qRT-PCR of a downstream target (EmSix1/2) of the RNAi-targeted gene (EmPaxB) (Rivera et al 2013).…”

“…Two papers have been published on experimental RNAi in two other sponge species however their success is questionable due to a lack of evidence confirming both RNA delivery and successful knockdown of the target (Rivera et al 2011(Rivera et al , 2013. The lack of functional tools for interrogating gene function is a significant limiting factor in the further development of A. queenslandica as a model sponge species.…”