How does an early embryonic cell become committed to a particular path of development and differentiation? One of the current areas of investigation in invertebrate development concerns the respective roles of maternal and embryonic gene activities in setting up first general, then specific, patterning in the early stages of embryogenesis. In Drosophila, the principles are becoming clear: localizcd maternally expressed gene products are used initially to define body axes and broad regional zones in which various combinations of genes are zygolically expressed to further define the emerging pattern of segmentation [reviewed in ref.
- In the nematode C. elegans, although the embryonic cell lineage is well understood(2) and there is ample evidence of a high degree of maternal control of cell individual genes that control the processes of asymmetric early cell division and lineage-specific initiation of zygotic gene expression have not until now bcen identified.
Two recent papers, onc by C. C. Mello, B. A. Draper, M. Krause, H. Weintraub, and J. R. Priess@), and an earlier paper by B. Bowerman, B. A. Eaton, and J. R. Priessi9) report the isolation of maternal-effect mutations in three genes, pie-I, mex-I. and skn-I, which change the developmental fatc of particular blastomeres in the 8-cell embryo. The genetic and phenotypic analysis of these mutants, and the cloning of the skn-1 gene, lead to a consistent model for how the fate of a single blastomere is determined, and the major genes involved, as well as providing insight -and defining further questions -regarding the mechanisms for specifying individual fate in each of the major cell lineages.