The neuroblast timer genenubbinexhibits functional redundancy with gap genes to regulate segment identity inTribolium

Abstract

AbstractInDrosophila, segmentation genes of the gap class form a regulatory network that positions segment boundaries and assigns segment identities. This gene network shows striking parallels with another gene network known as the neuroblast timer series. The neuroblast timer geneshunchback, Krüppel, nubbin,andcastorare expressed in temporal sequence in neural stem cells to regulate the fate of their progeny. These same four genes are expressed in corresponding spatial sequence along theDrosophilablastoderm. The first two,hunchbackandKrüppel, are canonical gap genes, butnubbinandcastorhave limited or no roles inDrosophilasegmentation. Whethernubbinandcastorregulate segmentation in insects with the ancestral, sequential mode of segmentation remains largely unexplored.We have investigated the expression and functions ofnubbinandcastorduring segment patterning in the sequentially-segmenting beetleTribolium. Using multiplex fluorescentin situhybridisation, we show thatTc-hunchback,Tc-Krüppel,Tc-nubbinandTc-castorare expressed sequentially in the segment addition zone ofTribolium, in the same order as they are expressed inDrosophilaneuroblasts. Furthermore, simultaneous disruption of multiple genes reveals thatTc-nubbinregulates segment identity, but does so redundantly with two previously described gap/gap-like genes,Tc-giantandTc-knirps. Knockdown of two or more of these genes results in the formation of up to seven pairs of ectopic legs on abdominal segments. We show that this homeotic transformation is caused by loss of abdominal Hox gene expression, likely due to expandedTc-Krüppelexpression. Our findings support the theory that the neuroblast timer series was co-opted for use in insect segment patterning, and contribute to our growing understanding of the evolution and function of the gap gene network outside ofDrosophila.