Altered mitotic domains reveal fate map changes in Drosophila embryos mutant for zygotic dorsoventral patterning genes.

The spatial and temporal pattern of mitoses during the fourteenth nuclear cycle in a Drosophila embryo reflects differences in cell identities. We have analysed the domains of mitotic division in zygotic mutants that exhibit defects in larval cuticular pattern along the dorsoventral axis. This is a powerful means of fate mapping mutant embryos, as the altered position of mitotic domains in the dorsoventral pattern mutants correlate with their late cuticular phenotypes. In the mutants twist and snail, which fail to differentiate the ventrally derived mesoderm, mitoses specific to the mesoderm are absent. The lateral mesectodermal domain shows a partial ventral shift in twist mutants but a proportion of ventral cells do not behave characteristically, suggesting that twist has a positive role in the establishment of the mesoderm. In contrast, snail is required to repress mesectodermal fates in cells of the presumptive mesoderm. In the absence of both genes, the mesodermal and the mesectodermal anlage are deleted. Mutations at five loci delete specific pattern elements in the dorsal half of the embryo and cause partial ventralization. Mutations in the genes zerknüllt and shrew affect cell division only in the dorsalmost cells corresponding to the amnioserosa, while the genes tolloid, screw and decapentaplegic (dpp) affect divisions in both the prospective amnioserosa and the dorsal epidermis. We demonstrate that in each of these mutants dorsally placed mitotic domains are absent and this effect is correlated with an expansion and dorsal shift in the position of more ventral domains. The loss of activity in each of the five genes results in qualitatively similar alterations in the mitotic pattern; mutations with stronger ventralizing phenotypes affect increasingly greater subsets of the dorsal cells. Double mutant analysis indicates that these genes act in a concerted manner to specify dorsal fates. The correlation between phenotypic strength and the progressive loss of dorsal pattern elements in the ventralized mutants, suggests that one of these gene products, perhaps dpp, may provide positional information in a graded manner.