Question

How do cells initially become different from each other in C. elegans?

Answer 1

Initially the cells in C. elegans are different from each other. The worm is typical of most animals in the early specification of the cells that will eventually give rise to the germ cells (eggs or sperm). The worm's germ line is produced by a strict series of asymmetric cell divisions of the fertilized egg. The asymmetry originates with a cue from the egg's environment: the sperm entry point defines the future posterior pole of the elongated egg. The proteins in the egg then interact with one another and organize themselves in relation to this point so as to create a more elaborate and extreme asymmetry in the interior of the cell. The proteins involved are mainly translated from the accumulated mRNA products of the genes of the mother. Because this RNAis made before the egg is laid, it is only the mother's genotype that dictates what happens in the first steps of development. Genes acting in this way are called maternal-effect genes.

A subset of maternal-effect genes are specifically required to organize the asymmetric pattern of the nematode egg. In the nematode egg, the Par proteins (the products of the par genes) serve to bring a set of ribonucleoprotein particles called P granules to the posterior pole, so that the posterior daughter cell inherits P granules and the anterior daughter cell does not. Throughout the next few cell divisions, the Par proteins operate in a similar way, orienting the mitotic spindle and segregating the P granules to one daughter cell at each mitosis, until, at the 16-cell stage, there is just one cell that contains the P granules.