Early Development and Diversity of Gryllus Appendages

Abstract

Among insects, orthopterans such as Gryllus bimaculatus display an extraordinary diversity regarding the arrangement and morphology of their appendages. In the head region, previous studies have shown that despite the superficial similarities in the morphology of mandibulate mouthparts between holometabolous and hemimetabolous species, the development of these appendages may be regulated in different ways. At present, a comprehensive analysis in any hemimetabolous mandibulate species is lacking; therefore studying the mouthparts in Gryllus will significantly improve the current understanding of the evolution of mouthparts in insects. Orthopteran wings are also quite distinct, featuring the hardened, leathery protective forewings (FWs) and the membranous flying hind wings (HWs). Furthermore, the FWs in Gryllus are characterized by a complex vein-intervein arrangement, similar to the ancestral hardened wings observed in fossils, providing a unique opportunity to understand the evolution of wing sclerotization in basal insects. Finally, orthopterans feature one of the best-known examples of appendage modification in insects – the presence of the greatly enlarged jumping hind leg. Studies of gene expression and functional analyses suggest that this enlargement is controlled by the Hox gene Ultrabithorax (Ubx), which acts as a “trigger” for differential leg growth. Furthermore, rather than acting on all genes in the leg development network, Ubx seems to selectively upregulate growth factors such as decapentaplegic (dpp) and Lowfat in Gryllus. Hence, cricket hind leg can serve as an exceptional model for combined studies of both tissue growth and segmental patterning during embryonic leg development. Overall, this review formulates a general framework that can be used for future studies on the development and diversification of insect appendages.