BINARY SPECIFICATION OF THE EMBRYONIC LINEAGE IN CAENORHABDITIS-ELEGANS

In Caenorhabditis elegans, the early embryo contains five somatic foun der cells (known as AB, MS, E, C and D) which give rise to very differ ent lineages, Two simply produce twenty intestinal (E) or muscle (D) c ells each, whereas the remainder produce a total of 518 cells which co llectively contribute in a complex pattern to a variety of tissues(1). A central problem in embryonic development is to understand how the d evelopmental potential of blastomeres is restricted to permit the term inal expression of such complex differentiation patterns, Here we iden tify a gene, lit-1, that appears to play a central role in controlling the asymmetry of cell division during embryogenesis in C. elegans. Mu tants in lit-1 suggest that its product controls up to six consecutive binary snitches which cause one of the two equivalent cells produced at each cleavage to assume a posterior fate. Most blastomere identitie s in C, elegans may therefore stem from a process of stepwise binary d iversification.