In toto live imaging in scuttle fly Megaselia abdita reveals transitions towards a novel extraembryonic architecture.

Evolutionary novelty can be generally traced back to continuous changes rather than disruptive transformations, yet the sudden appearance of novel developmental traits is not well understood. Here we use the extraembryonic amnioserosa in Drosophila melanogaster as example for a suddenly and newly evolved epithelium, and we ask how this tissue originated by gradual transitions from its two ancestors, amnion and serosa. To address this question, we used in toto time-lapse recordings to analyze an intermediate mode of extraembryonic development in the scuttle fly Megaselia abdita. Our results suggest that the amnioserosa evolved by loss of serosa spreading without disrupting the developmental programs of serosa and amnion. Our findings imply that the Drosophila amnioserosa has retained properties of the ancient serosa and, more generally, indicate that non-autonomous interactions between tissues can be a compelling variable for the evolution of epithelial properties.