Divergent and conserved roles of extradenticle in body segmentation and appendage formation, respectively, in the cricket Gryllus bimaculatus

Abstract

The cricket Gryllus bimaculatus is a typical hemimetabolous intermediate germ insect, in which the processes of segmentation and appendage formation differ from those in Drosophila, a holometabolous long germ insect. In order to compare their developmental mechanisms, we have focused on Gryllus orthologs of the Drosophila developmental regulatory genes and studied their functions. Here, we report a functional analysis of the Gryllus ortholog of extradenticle (Gb'exd) using embryonic and parental RNA interference (RNAi) techniques. We found the following: (1) RNAi suppression of Gb'exd results in the deletion or fusion of body segments. Especially the head was often very severely affected. This gap-like phenotype may be related to reduced expression of the gap genes hunchback and Krüppel in early RNAi germbands. (2) In the appendages, several segments (podomeres) were fused. (3) Head appendages including the antenna were transformed to a leg-like structure consisting of at least one proximal podomere as well as several tarsomeres. The defects in appendages are reminiscent of the phenotype caused by large exd clones in Drosophila antennal discs. These findings led us to the conclusion that (1) Gb'exd is required for segment patterning in the gnathal to abdominal region, acting in a gap gene-like manner in the anterior region. (2) Gb'exd plays important roles in formation of the appendages and the determination of their identities, acting as a regulatory switch that chooses between the fates of head appendages versus the appendage ground state. Although functions of Gb'exd in appendage patterning appear fundamentally conserved between Gryllus and Drosophila, its role in body segmentation may differ from that of Drosophila exd.

A conserved genetic mechanism specifies deutocerebral appendage identity in insects and arachnids

“…With respect to conservation of gene interaction, knockdown of the hth cofactor exd in G. bimaculatus results in similar homeotic transformation of gnathal appendages towards leg identity, with accompanying loss of gnathal Hox gene expression (Deformed and Sex combs reduced; [25]). The similar phenotypes resulting from knockdown of either hth or exd reflect the requirement of Hth for transport of Exd to the nucleus, and the instability of Hth in the absence of Exd [22,44].…”

Hexapoda: Comparative Aspects of Later Embryogenesis and Metamorphosis

“…This restricts the functional hth / exd domain to the proximal limb region (coxa, trochanter, and sometimes the proximal femur) in all hexapods in which this has been investigated ( Fig. 3.8 ; Jockusch et al 2000Inoue et al 2002a ;Angelini and Kaufman 2004 ;Tanaka and Truman 2007 ;Mito et al 2008 ;Ronco et al 2008 ;Schaeper et al 2013 ). Prior to limb outgrowth, embryos generally express nuclearly localized Exd (n-Exd) in every cell.…”

“…The antennal overlap appears large throughout embryogenesis in Oncopeltus (Angelini and Kaufman 2004 ), consistent with a Drosophila -like antennal identity specifi cation mechanism, although this remains to be tested. By contrast, the expression domains of these genes are very leg-like and either mutually exclusive or narrowly overlapping in the developing antennae of several orthopteran species (Abzhanov and Kaufman 2000 ; Mito et al 2008 ;Ronco et al 2008 ) and Thermobia (Schaeper et al 2013 ). In Tribolium , the overlap is narrow when ss is activated, but broader later in embryogenesis Shippy et al 2009 ).…”

“…For example, the RNAi phenotypes generated by knockdown of hth or exd are similar across appendage types in both Gryllus and Tribolium , but they differ between species ( Fig. 3.18 ; Mito et al 2008 ;Ronco et al 2008 ;Angelini et al 2009Angelini et al , 2012a. Another example of apparent nonindependence across appendage types is the appearance of a sock and ring Dll expression pattern in the palps of some orthopterans.…”

On the border of the homeotic function: Re‐evaluating the controversial role of cofactor‐recruiting motifs

“…This point underlines the importance of identifying the expression profile and function of the Exd and Ftz-F1 cofactors in species where Ftz proteins have diverged. For example, it is interesting to note that the depletion of exd in the cricket Gryllus bimaculatus led to GAPlike, instead of homeotic-like, phenotypes in the embryo [65], which raises the question as to whether homeotic potential still relies on exd in this particular species.…”