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. Impact StatementThe Drosophila amnioserosa originated as a composite extraembryonic epithelium by loss of epithelial spreading and rather than changes in amnion or serosa tissue differentiation.

Extraembryonic tissues contribute to animal development, which often entails spreading over embryo or yolk. Apart from changes in cell shape, the requirements for this tissue spreading are not well understood. Here, we analyze spreading of the extraembryonic serosa in the scuttle fly Megaselia abdita. The serosa forms from a columnar blastoderm anlage, becomes a squamous epithelium, and eventually spreads over the embryo proper. We describe the dynamics of this process in long-term, whole-embryo time-lapse recordings, demonstrating that free serosa spreading is preceded by a prolonged pause in tissue expansion. Closer examination of this pause reveals mechanical coupling to the underlying yolk sac, which is later released. We find mechanical coupling prolonged and serosa spreading impaired after knockdown of M. abdita Matrix metalloprotease 1. We conclude that tissue–tissue interactions provide a critical functional element to constrain spreading epithelia.

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