Localization and segregation of lineage-specific cleavage potential in embryos of Caenorhabditis elegans

Early embryogenesis of the nematode Caenorhabditis elegans is characterized by the continuous visibility of a germline and the stepwise separation of all somatic cells from it. Germline and somatic cells exhibit different cleavage patterns. Typical for the germline is a series of stemcell-like, unequal cleavages generating blastomeres, which differ in size, cell cycle periods, and fate. Typical for members of somatic cell lineages during early development are their equal and synchronous cleavages generating cells of similar appearance. Using a laser microbeam various experiments have been carried out to investigate the conditions that lead to the two different types of cleavage. Development of partial embryos demonstrates that the potential for germline-like cleavage is localized in the posterior region of the fertilized egg prior to both the formation of pronuclei and the posterior aggregation of germline-specific granules. Experimental alteration of the cleavage plane can result in a switch from unequal to equal cleavage, with an apparent correlation between the orientation of the mitotic spindle and the type of cleavage. Nuclear transfer experiments indicate that nuclei and centrioles are not involved in the decision as to which type of cleavage will be executed. Cytoplasmic transfer from soma-like to germline-like cleaving cells and vice versa does not alter the cleavage type in the recipient cell. But if separation of germline from soma is delayed after the removal of a centrosome, germline-like cleavage may be completely suppressed, all cells thereafter dividing soma-like.