Activation and repression activities of ash2 in Drosophila wing imaginal discs.

Polycomb (PcG) and trithorax (trxG) group genes are chromatin regulators involved in the maintenance of developmental decisions. Although their function as transcriptional regulators of homeotic genes has been well documented, little is known about their effect on other target genes or their role in other developmental processes. In this study, we have used the patterning of veins and interveins in the wing as a model with which to understand the function of the trxG gene ash2 (absent, small or homeotic discs 2). We show that ash2 is required to sustain the activation of the intervein-promoting genes net and blistered (bs) and to repress rhomboid (rho), a component of the EGF receptor (Egfr) pathway. Moreover, loss-of-function phenotypes of the Egfr pathway are suppressed by ash2 mutants, while gain-of-function phenotypes are enhanced. Our results also show that ash2 acts as a repressor of the vein L2-organising gene knirps (kni), whose expression is upregulated throughout the whole wing imaginal disc in ash2 mutants and mitotic clones. Furthermore, ash2-mediated inhibition of kni is independent of spalt-major and spalt-related. Together, these experiments indicate that ash2 plays a role in two processes during wing development: (1) maintaining intervein cell fate, either by activation of intervein genes or inhibition of vein differentiation genes; and (2) keeping kni in an off state in tissues beyond the L2 vein. We propose that the Ash2 complex provides a molecular framework for a mechanism required to maintain cellular identities in the wing development.