Extraembryonic development in insects and the acrobatics of blastokinesis

Extraembryonic development is familiar to mouse researchers, but the term is largely unknown among insect developmental geneticists. This is not surprising, as the model system Drosophila melanogaster has an extremely reduced extraembryonic component, the amnioserosa. In contrast, most insects retain the ancestral complement of two distinct extraembryonic membranes, amnion and serosa. These membranes are involved in several key morphogenetic events at specific developmental stages. The events of anatrepsis and katatrepsis--collectively referred to as blastokinesis--are specific to hemimetabolous insects. Corresponding events in holometabolous insects are simplified and lack formal names. All insects retain dorsal closure, which has been well studied in Drosophila. This review aims to resurrect both the terminology and awareness of insect extraembryonic development--which were last common currency in the late nineteenth and early twentieth centuries--as a number of recent studies have identified essential components of these events, through RNA interference of developmental genes and ectopic hormonal treatments. As much remains unknown, this topic offers opportunities for research on tissue specification, the regulation of cell shape changes and tissue interactions during morphogenesis, tracing the origins and final fates of cell and tissue lineages, and ascertaining the membranes' functions between morphogenetic events.