GENETIC INTERACTIONS AND DOSAGE EFFECTS OF POLYCOMB GROUP GENES IN MICE

In Drosophila and mouse, Polycomb group genes are involved in the main tenance of homeotic gene expression patterns throughout development. H ere we report the skeletal phenotypes of compound mutants for two Poly comb group genes bmi1 and M33. We show that mice deficient for both bm i1 and M33 present stronger homeotic transformations of the axial skel eton as compared to each single Polycomb group mutant, indicating stro ng dosage interactions between those two genes. These skeletal transfo rmations are accompanied with an enhanced shift of the anterior limit of expression of several Hox genes in the semitic mesoderm. Our result s demonstrate that in mice the Polycomb group genes act in synergy to control the nested expression pattern of some Hox genes in semitic mes odermal tissues during development.