[
1987]
Work in our laboratory over the past several years has focused on the nature of early determinative decisions in embryos of the free-living nematode Caenorhabditis elegans. Two of these decisions regard determination of sex and determination of the level of X-chromosome expression. C. elegans has two sexes, self-fertilizing hermaphrodites and males. Hermaphrodites normally have two X chromosomes, and males have only one (there is no Y chromosome). Genetic and molecular evidence suggest that C. elegans compensates for this difference in X dosage, not by X inactivation as in mammals, but rather by global regulation of the X chromosome as in Drosophila; that is, X-linked genes are expressed at a higher level per chromosome in 1X than 2X animals, so that levels of X expression are similar in the two sexes. Also as in Drosophila, the primary signal that dictates both sex determination and level of X expression in C. elegans is the ration of the number of X chromosomes to the number of sets of autosomes (X/A ratio) rather than the absolute number of X chromosomes.|
[
1982]
Caenorhabditis elegans is a free-living, nonparasitic nematode. It is a self-fertilizing hermaphrodite. Males arise spontaneously by nondisjunction of X-chromosomes. Of all eukaryotic organisms C. elegans has probably been most extensively studied at the cellular level. Within 12 hours the fertilized egg develops into a young larva with 558 nuclei (560 in the male). During postembryonic development the animal proceeds through four larval stages increasing its number of nuclei to 959 (1,031 in the male) plus some 2,000 germ cells (about 1,000 in the male). The cell lineages from fertilization to adulthood have been completely analyzed in living embryos and animals. This and its well-established genetics (more than 300 genes have been mapped on the six linkage groups) make it a suitable model organism to study problems of gene action and development. Various techniques have been used to interfere with normal development (including laser-induced cell ablations) and to analyze development on the subcellular level (including recombinant DNA technology). The characteristic features of rigidly determined development, the low cell number, and the knowledge of cellular events should make it possible to identify molecular action in situ and relate it to the structure and