Antagonistic role of the BTB-zinc finger transcription factors Chinmo and Broad-Complex in the juvenile/pupal transition and in growth control.

Egg, larva, pupa, adult: the life of many insects is structured around these four well-defined stages of development. After hatching, the larva grows until it reaches a certain size; when the right conditions are met, it then becomes a pupa and metamorphoses into an adult. Most larval cells die during metamorphosis; only a group known as imaginal cells survives, dividing and maturing to create pupal and adult tissues. Each of these developmental steps are linked to a particular genetic program deployed in response to a single stage-specifying gene. For instance, the activation of the Br-C gene triggers the transition from larva to pupa, while E93 initiates the transformation of the pupa into an adult. However, which stage-specifying gene controls larval identity remains unclear. Recent studies suggest that in fruit flies, a gene known as chinmo could be playing this role. In response, Chafino et al. explored how chinmo shapes the development of fruit fly larvae. The experiments showed that chinmo is activated in the juvenile stage, and that it is required for the larvae to grow properly and for larval and imaginal tissues to form. Conversely, it must be switched off for the insect to become a pupa and then an adult. Further work suggested that the role of chinmo as a larval specifier could have emerged early in insect evolution. Moreover, Chafino et al. revealed that chinmo could repress Br-C, an important characteristic since stage-specifying genes usually switch on sequentially by regulating each other. A closer look suggested that, in imaginal cells, chinmo promotes development by inhibiting Br-C; in larval cells, however, chinmo not only has a Brc-repressing role but it is also necessary for larval cells to grow. Additional experiments exploring the role of the stage-specifying genes in tumor formation showed that chinmo promotes cells proliferation while Br-C and E93 had tumor-suppressing properties. Overall, the work by Chafino et al. sheds new light on the genetic control of insect development, while also potentially providing a new perspective on how genes related to chinmo and Br-C contribute to the emergence of human cancers.