Hox genes limit germ cell formation in the short germ insect Gryllus bimaculatus

Significance

Hox genes are necessary for the proper placement of organs along animal body axes. In insects, Hox genes are used in a “code” of overlapping expression domains to specify body segments. We previously showed that, in the cricket Gryllus bimaculatus, germ cells are specified exclusively in the second through the fourth abdominal segments. Given the role of Hox genes in establishing segmental identity in insects, we tested the hypothesis that Hox genes control the segment-specific development of germ cells in crickets. We found that a subset of Hox genes limit germ cell development in the PGC-bearing segments. These data suggest a role for Hox genes in regulating germ cell placement.

Abstract

Hox genes are conserved transcription factor-encoding genes that specify the identity of body regions in bilaterally symmetrical animals. In the cricket Gryllus bimaculatus, a member of the hemimetabolous insect group Orthoptera, the induction of a subset of mesodermal cells to form the primordial germ cells (PGCs) is restricted to the second through the fourth abdominal segments (A2 to A4). In numerous insect species, the Hox genes Sex-combs reduced ( Scr), Antennapedia ( Antp), Ultrabithorax ( Ubx), and abdominal-A ( abd-A) jointly regulate the identities of middle and posterior body segments, suggesting that these genes may restrict PGC formation to specific abdominal segments in G. bimaculatus. Here we show that reducing transcript levels of some or all of these Hox genes results in supernumerary and/or ectopic PGCs, either individually or in segment-specific combinations, suggesting that the role of these Hox genes is to limit PGC development with respect to their number, segmental location, or both. These data provide evidence of a role for this ancient group of genes in PGC development.