Modifiers of muscle and heart cell fate specification identified by gain-offunction screen in Drosophila

The homeobox genes ladybird in Drosophila and their vertebrate counterparts Lbx1 genes display restricted expression patterns in a subset of muscle precursors and are both implicated in diversification of muscle cell fates. In order to gain new insights into mechanisms controlling conserved aspects of cell fate specification, we have performed a gain-of-function (GOF) screen for modifiers of the mesodermal expression of ladybird genes using a collection of EP element carrying Drosophila lines. Amongst the identified genes, several have been previously implicated in cell fate specification processes, thus validating the strategy of our screen. Observed GOF phenotypes have led us to identification of an important number of candidate genes, whose myogenic and/or cardiogenic functions remain to be investigated. Amongst them, the EP insertions close to rhomboid, yan and rac2 suggest new roles for these genes in diversification of muscle and/or heart cell lineages. The analysis of loss and GOF of rhomboid and yan reveals their new roles in specification of ladybird-expressing precursors of adult muscles (LaPs) and ladybird/tininan-positive pericardial cells. Observed phenotypes strongly suggest that rhomboid and yan act at the level of progenitor and founder cells and contribute to the diversification of mesodermal fates. Our analysis of rac2 phenotypes clearly demonstrates that the altered mesodermal level of Rho-GTPase Rac2 can influence specification of a number of cardiac and muscular cell types including those expressing ladybird. Finding that in rac2 mutants ladybird and even skipped-positive muscle founders are overproduced, indicate a new early function for this gene during segregation of muscle progenitors and/or specification of founder cells. Intriguingly, rhomboid, yan and rac2 act as conserved components of Receptor Tyrosine Kinases (RTKs) signalling pathways, suggesting that RTK signalling constitutes a part of a conserved regulatory network governing diversification of muscle and heart cell types. (C) 2003 Elsevier Ireland Ltd. All rights reserved.