Innexin7a forms junctions that stabilize the basal membrane during cellularization of the blastoderm in Tribolium castaneum

Abstract

In insects thefertilized egg undergoes a series of rapid nuclear divisions before the syncytial blastoderm starts to cellularize. Cellularization has been extensively studied in Drosophila melanogaster but its thick columnar blastoderm is unusual among insects.We therefore set out to describe cellularization in the beetle Triboliumcastaneum the embryos of which exhibit a thin blastoderm of cuboidal cells like most insects. Using immunohistochemistry live imaging and transmission electron microscopy we describe several striking differences to cellularization in Drosophila including the formation of junctions between the forming basal membrane and the yolk plasmalemma. To identify the nature of this novel junction we used the parental RNAi technique for a smallscale screen of junction proteins. We find that maternal knockdown of Tribolium innexin7a (Tc-inx7a) an ortholog of the Drosophila gap junction gene Innexin 7 leads to failure of cellularization. In Inx7adepleted eggs the invaginated plasma membrane retracts when basal cell closure normally begins.Furthermore transiently expressed tagged Inx7a localizes to the nascent basal membrane of the forming cells in wild-type eggs. We propose that Inx7a forms the newly identified junctions that stabilize the forming basal membrane and enable basal cell closure. We put forward Tribolium as a model for studying a more ancestral mode of cellularization in insects.