Meeting Abstract
A fundamental challenge in the field of evo-devo is to understand the evolution of animal body plan diversity. The body plan of sponges (phylum Porifera) is an outlier among modern animals and is thought to have special evolutionary significance. Sponges lack muscles, nerves and a gut. Instead, they are composed of few cell types and simple tissues that function to pump water through an internal canal network where bacterial prey are filtered by a specialized tissue called the choanoderm. Not only is the choanoderm the defining feature of the sponge body plan, it is composed of cells with striking similarity to choanoflagellates, the unicellular relatives of animals. Thus, the traditional view is that the sponge choanoderm is a useful model of the first animal epithelial tissues. Using the freshwater sponge model, Ephydatia muelleri, we have performed comprehensive gene expression analysis of the choanoderm tissue and have begun to experimentally validate and characterize the function of candidate choanoderm genes. An early insight from these data is that the sponge choanoderm may be the only metazoan tissue not reliant upon the classical cadherin/catenin complex for cell-cell adhesion. In contrast, we find evidence for conserved developmental mechanisms and other structural features, such as a possible role for tolloid-like 1 and hensin in choanoderm morphogenesis, and a possibly role for usherin, VLGR1, and cadherin 23 in regulating microvillar organization. Moreover, both planar and apical polarity markers are differentially expressed in the choanoderm, suggesting homology with other animal epithelia, despite the absence of a conserved cadherin/catenin complex. Finally, we will explore the possibility that genes unique to choanoflagellates and sponges, have conserved functions in the choanoderm tissue. This prediction derives from the hypothesized homology of these putatively ancient cell types.
