CHARACTERIZATION OF A REGION OF THE X-CHROMOSOME OF DROSOPHILA INCLUDING MULTI SEX COMBS (MXC), A POLYCOMB GROUP GENE WHICH ALSO FUNCTIONS AS A TUMOR-SUPPRESSOR

Genetic analysis of the 8D3;8D8-9 segment of the Drosophila melanogast er X chromosome has assigned seven complementation groups to this regi on, three of which are new. A Polycomb group (Pc-G) gene, multi sex co mbs (mxc), is characterized and mutant alleles are described. Besides common homeotic transformations characteristic of Pc-G mutants that mi mic the ectopic gain of function of BX-C and ANT-C genes, mxc mutants show other phenotypes: they zygotically mimic, in males and females, t he characteristic lack of germ line seen in progeny of some maternal e ffect mutants of the so-called posterior group (the grandchildless phe notype). Loss of normal mxc function can promote uncontrolled malignan t growth which indicates a possible relationship between Pc-G genes an d tumour suppressor genes. We propose that gain-of-function of genes n ormally repressed by the wild-type mxc product could, in mxc mutants, give rise to an incoherent signal which would be devoid of meaning in normal development. Such a signal could divert somatic and germ line d evelopmental pathways, provoke the loss of cell affinities, but allow or promote growth.