Evolution of Nervous System Centralization: Role of BMP Signaling in Neural Fate and Dorsal-Ventral Axis Specification in Capitella Teleta
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- Evolution & development; Developmental biology; Biology; Neurosciences
thesis / dissertation description
Bone Morphogenetic Protein (BMP) signaling plays an important role in establishing the dorsal-ventral (D-V) axis across Bilateria. BMP signaling has also been implicated in neural fate specification in several bilaterians that possess centralized nervous systems. The classic examples are found in vertebrates and Drosophila melanogaster, where opposing gradients of BMP2/4/Dpp and Chordin/Sog specify the D-V axis and the nervous system arises on the Chordin/Sog side of the early embryo due to the anti-neural role of BMPs. These similarities have been used as support for the hypothesis that the bilaterian ancestor possessed a centralized nervous system and that BMP signaling delimited its neural tissue to one side of this axis. However, recent studies in hemichordates, cnidarians, and annelids suggest that BMPs are not similarly involved in neural fate specification. This challenges the idea that BMP signaling played a role in neural fate specification in the early bilaterian ancestor. The third major clade of Bilateria, Spiralia, is understudied and could help answer this question. Here, we assess the role of BMP signaling in the spiralian annelid Capitella teleta, which has a clear D-V axis and a centralized nervous system consisting of a dorsoanterior brain and a ventral nerve cord. Expression of the BMP ligands Ct-BMP2/4 and Ct-BMP5-8 and the antagonists Ct-noggin and Ct-chordin-like was analyzed at embryonic and larval stages in C. teleta. To investigate BMP function, early cleavage-stage embryos were incubated in the drug dorsomorphin dihydrochloride, which has been shown to block BMP signaling in other animals. Our results suggest that BMP signaling is involved in specifying certain tissues along the D-V axis; however, the overall D-V axis of the larvae remained intact following BMP inhibition. Dorsomorphin treatment also resulted in the decrease of neural tissue, which is opposite to the anti-neural role of BMPs in insects and vertebrates. This suggests that BMPs may be involved in neural induction in C. teleta, but in a novel way compared to insects and vertebrates. Based on these findings, it seems unlikely that BMPs specified D-V axis and blocked neural fate specification in the ancestor of Bilateria.