Segmental expression of two ecdysone pathway genes during embryogenesis of hemimetabolous insects.

Signaling networks are redeployed across different developmental times and places to generate phenotypic diversity from a limited genetic toolkit. Hormone signaling networks in particular have well-studied roles in multiple developmental processes. In insects, the ecdysone pathway controls critical events in late embryogenesis and throughout post-embryonic development. While this pathway has not been shown to function in the earliest stage of embryonic development in the model insect Drosophila melanogaster, one component of the network, the nuclear receptor E75A, is necessary for proper segment generation in the milkweed bug Oncopeltus fasciatus. Published expression data from several other species suggests possible conservation of this role across hundreds of millions of years of insect evolution. Previous work also demonstrates a second nuclear receptor in the ecdysone pathway, Ftz-F1, plays a role in segmentation in multiple insect species. Here we report tightly linked expression patterns of ftz-F1 and E75A in two hemimetabolous insect species, the German cockroach Blattella germanica and the two-spotted cricket Gryllus bimaculatus. In both species, the genes are expressed segmentally in adjacent cells, but they are never co-expressed. Using parental RNAi, we show the two genes have distinct roles in early embryogenesis. E75A appears necessary for abdominal segmentation in B. germanica, while ftz-F1 is essential for proper germband formation. Our results suggest that the ecdysone network is critical for early embryogenesis in hemimetabolous insects.