Abstract
V. Ambros and R. Horvitz have identified a set of genes that may control the timing of developmental events in the nematode Caenorhabdit~ e/e- gans 1. After hatching, C. e/egans proceeds to the adult stage via four juvenile stages, each followed by a molt. Each juvenile stage is characterized by a distinct pattern of cell divi- sion and differentiation 2 called an S1, $2, $3 or $4 lineage pattern 1. Ambros and Horvitz have found that the S1-$4 lineage patterns are modular; that is, a lineage pattern (including hy- podermal, neuronal, muscle, and intestinal divisions and dif- ferentiations) characteristic of one stage can be expressed at a different stage. Each behaves like a complex lineage cassette that can be inserted into one or more of the available temporal slots. Ambros and Horvitz have isolated mutations (which they call heterochronic mutations) that replace lineage patterns characteristic of one stage with patterns characteristic of a different stage. By analogy with homeotic mutations of
Published Version
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