Meeting Abstract
The early branching metazoan phylum, Ctenophora, represents an important group for understanding how coordinated developmental patterning of discrete germ layers evolved into higher-order organ systems in the early metazoans. In this study we investigated gut patterning in the ctenophores Mnemiopsis and Pleurobrachia through a combination of morphological observations and gene expression analysis. The animal gut is typically composed of ectodermal (mouth and anus) and endodermal (midgut) derivatives. The endoderm is thought to be the oldest germ layer among metazoans, and many of the genes expressed during endoderm patterning predate multicellularity. The ectoderm is thought to have originated as an additional germ layer after the endoderm. Among bilaterian animals, genes that regulate endoderm and ectoderm development are well conserved, and these animals share similar structural and morphological gut traits. For example, most bilaterians have a unidirectional alimentary canal with two openings. However, few studies have examined gut patterning at the molecular or morphological level in basally branching metazoan lineages, with most having been done in cnidarian species. It has been previously shown that during gastrulation in ctenophores, endodermal precursors are internalized in the embryo and eventually give rise to the midgut analog (the endodermal canal system), whereas ectodermal precursors have been shown to give rise to a majority of the stomodeum (mouth) and pharynx. We describe the morphological and functional organization of the ctenophore gut system as well as gene expression patterns of conserved endodermal and ectodermal markers. Our findings highlight an elaborately organized gut with an unexpected degree of differentiation.
