Maternal-to-zygotic transition: soma versus germline

See research article http://genomebiology.com/2012/13/2/R11 webciteIn many species, the earliest stages of embryonic development occur in the absence of transcription and depend on maternally loaded mRNAs and proteins. During the maternal-to-zygotic transition (MZT), the control of development switches from the maternal genome to the zygotic genome. This is achieved by the combination of two processes: the massive degradation of maternal mRNAs and the activation of the zygotic genome (Figure 1). Over the past few years, genome-wide analyses of the MZT have been performed in whole embryos from several different species, including Drosophila [1-3].Germ cells are specified and segregate from somatic lineages during early embryogenesis. In both Drosophila and Caenorhabditis elegans, the formation of germ cells depends on specific maternal components that accumulate in a specialized cytoplasm within the egg, called the germ plasm. An important function of the germ plasm is to protect germ cell progenitors from somatic differentiation. This is achieved in part by repressing transcription until the germ cell fate is established [4]. In Drosophila, the germ plasm (also called pole plasm) is localized at the posterior pole of the embryo. There, the germ cell progenitors, the primordial germ cells (PGCs), are the first cells to form. Just as in the somatic part of the embryo, MZT must occur in the PGCs to allow the switch from maternal to zygotic gene expression in these cells. Until recently, the low numbers of PGCs present in embryos have precluded large-scale analyses of their MZT; but in a new study published in this issue of Genome Biology, Siddiqui et al. [5] used flow cytometric sorting of GFP-labeled PGCs, proteomics and microarray-based gene-expression profiling to define the MZT in Drosophila PGCs.In a first step, they identified proteins and mRNAs that were specific to or that were enriched in somatic cells or in PGCs. They found that different proteins and mRNAs w