The multi sex combs gene of Drosophila melanogaster is required for proliferation of the germline.

We present a genetic analysis showing that the Drosophila melanogaster gene multi sex combs (mxc; Santamaria and Randsholt 1995) is needed for proliferation of the germline. Fertility is the feature most easily affected by weak hypomorphic mutations of this very pleiotropic locus. Pole cell formation and early steps of gonadogenesis conform to the wild-type in embryos devoid of zygotic mxc product. mxc mutant gonad phenotypes and homozygous mxc germline clones suggest a role for mxc in control of germ cell proliferation during the larval stages. mxc requirement is germ cell autonomous and specific in females, whilst in males mxc product is also needed in somatic cells of the gonads. Although mxc can be classified among the Polycomb group (Pc-G) of genes, negative trans-regulators of the ANT-C and BX-C gene complexes, germline requirement for mxc appears independent of a need for other Pc-C gene products, and mxc gonad phenotypes are different from those induced by mutations in BX-C genes. We discuss the possible functions of the mxc product which helps to maintain homeotic genes repressed and prevents premature larval haemocyte differentiation and neoplasic overgrowth, but promotes growth and differentiation of male and female gonads.