Making a mouth: elucidating morphogenetic events of mouth development in the sea urchin Lytechinus variegatus

Deuterostomes are bilaterian animals in which the blastopore, the site of gastrulation, becomes the anus and the mouth develops secondarily. Deuterostome phyla include Chordata, such as vertebrates like ourselves, and Echinodermata, such as sea urchins. The extent of homology among deuterostome mouths is unknown. To address this question, this thesis compares three aspects of mouth morphogenesis between frogs (Xenopous laevis), a vertebrate, and sea urchins (Lytechinus variegatus), an echinoderm and basally branching deuterostome: 1) if mouth formation requires signaling from the gut endoderm, 2) if Wnt signaling regulates basement membrane dissolution during mouth development, and 3) if the mouth perforates by apoptosis. It was found through gut removal and isolation experiments and by inducing exogastrulation that sea urchins may not require signaling from the gut for mouth formation. Treating sea urchin embryos with C59, a Wnt signal-inhibiting drug, developed smaller mouths as the level of Wnt-inhibition increased. Lastly, an apoptosis assay that immunostained embryos for anti-caspase3 antibody revealed that sea urchin mouths may not open by apoptosis. We found that these three aspects of mouth development contrast in the sea urchin and frog, supporting less homology among deuterostome mouths.