Ilyanassa Notch signaling implicated in dynamic signaling between all three germ layers

Two cells (3D and 4d) in the mud snail Ilyanassa obsolete function to induce proper cell fate. In this study, we provide support for the hypothesis that Notch signaling in Ilyanassa obsolete functions in inductive signaling at multiple developmental stages.The expression patterns of Notch, Delta and Suppressor of Hairless (SuH) are consistent with a function for Notch signaling in endoderm formation, the function of 3D/4d and the sublineages of 4d. Veligers treated with DAPT show a range of defects that include a loss of endodermal structures, and varying degrees of loss of targets of 4d inductive signaling. Veligers that result from injection of Ilyanassa Delta siRNAi in general mimic the defects observed in the DAPT treated larvae.The most severe DAPT phenotypes mimic early ablations of 4d. However, the early specification of 4d itself appears normal and MAPK activation in both 3D/4d and the micromeres, which are known to activate MAPK as a result of 3D/4d induction, are normal in DAPT treated larvae. Treating larvae at successively later timepoints with DAPT suggests that Notch/Delta signaling is not only required during early 4d inductive signaling, but during subsequent stages of cell fate determination as well. Based on our results, combined with previous reports implicating the endoderm in maintaining induced fate specification in Ilyanassa, we propose a speculative model that Notch signaling is required to specify endoderm fates and 4d sublineages, as well as to maintain cell fates induced by 4d.