The two Tribolium E(spl) genes show evolutionarily conserved expression and function during embryonic neurogenesis

Abstract

Tribolium castaneum is a well-characterised model insect, whose short germ-band mode of embryonic development is characteristic of many insect species and differs from the exhaustively studied Drosophila. Mechanisms of early neurogenesis, however, show significant conservation with Drosophila, as a characteristic pattern of neuroblasts arises from neuroectoderm proneural clusters in response to the bHLH activator Ash, a homologue of Achaete-Scute. Here we study the expression and function of two other bHLH proteins, the bHLH-O repressors E(spl)1 and E(spl)3. Their Drosophila homologues are expressed in response to Notch signalling and antagonize the activity of Achaete-Scute proteins, thus restricting the number of nascent neuroblasts. E(spl)1 and 3 are the only E(spl) homologues in Tribolium and both show expression in the cephalic and ventral neuroectoderm during embryonic neurogenesis, as well as a dynamic pattern of expression in other tissues. Their expression starts early, soon after Ash expression and is dependent on both Ash and Notch activities. They act redundantly, since a double E(spl) knockdown (but not single knockdowns) results in neurogenesis defects similar to those caused by Notch loss-of-function. A number of other activities have been evolutionarily conserved, most notably their ability to interact with proneural proteins Scute and Daughterless.