Germ cell specification

Abstract

The germline of Caenorhabditis elegans derives from a single founder cell, the germline blastomere P(4). P(4) is the product of four asymmetric cleavages that divide the zygote into distinct somatic and germline (P) lineages. P(4) inherits a specialized cytoplasm ("germ plasm") containing maternally encoded proteins and RNAs. The germ plasm has been hypothesized to specify germ cell fate, but the mechanisms involved remain unclear. Three processes stand out: (1) inhibition of mRNA transcription to prevent activation of somatic development, (2) translational regulation of the nanos homolog nos-2 and of other germ plasm mRNAs, and (3) establishment of a unique, partially repressive chromatin. Together, these processes ensure that the daughters of P(4), the primordial germ cells Z2 and Z3, gastrulate inside the embryo, associate with the somatic gonad, initiate the germline transcriptional program, and proliferate during larval development to generate ∼2,000 germ cells by adulthood.

Fig. 2.1

Embryonic origin of the germline. Abbreviated embryonic lineage from the 1-cell stage to the ~88-cell stage and embryo schematics corresponding to each stage shown in the lineage tree. Germ plasm is denoted in purple, germ granules are darker purple dots. High levels of MEX-5/6 inherited by somatic blastomeres are denoted in blue. Red nuclei are not competent for mRNA transcription