DEGNAN, BM*; ADAMSKA, M; University of Queensland; University of Queensland: Pre-mesoderm: the expression of Wnts during sponge gastrulation and their ancestral role in metazoan development

While the original body plan from which all living metazoans evolved remains obscure, recent insights into the genome and development of basal animals, such as cnidarians, ctenophores and sponges, suggest that the metazoan cenancestor (last common ancestor) was complex, employing many of the developmental mechanisms used by extant bilaterians. The demosponge Reniera is a representative of the most basal metazoan lineage and uses asymmetric cell divisions and morphogen gradients to specify and pattern cells during embryogenesis. Cell movements at gastrulation lead to the formation of a trilayered larva with a variety of cell types patterned along the anterior-posterior axis. While a blastopore is not obvious in Reniera, a number of cell types migrate to the (larval) posterior pole during gastrulation. Upon reaching the pole, some (i.e. primary sclerocytes) ingress into the inner cell mass. Wnt is expressed at this pole during and prior to the initiation of cell movements at gastrulation. Other components of the canonical Wnt pathway also are developmentally expressed in Reniera, suggesting this pathway is playing a key role in organising sponge embryonic cells in a manner akin to that observed in eumetazoans. From these and other observations, we can infer that the cells of the metazoan cenancestor had the capacity to secrete long- and short-range signalling molecules that could induce different responses in other cells. Expansion and co-option of Wnt and other metazoan-specific signalling pathways and transcription factors in the eumetazoan lineage allowed for the invention and elaboration of new cell types, germ layers and tissues, including the mesoderm.