Fig. 6.

A maternal-to-zygotic-transition gene block. a) In the ovary, oocytes express maternal 5S rRNA (green vertical bars on the chromosome sketch) and spliceosome genes (U1, U6; blue and mauve) in Region-2 but not znf genes (yellow) in Region-2 or the adjacent mir430 genes (white) (Locati, Pagano, Ensink et al. 2017; Locati, Pagano, Girard et al. 2017; Locati et al. 2018; Pagano, Dekker et al. 2020; Pagano, Locati et al. 2020). The 5S ribosomal and the spliceosomal maternal transcripts help make the machinery to translate proteins essential for embryogenesis from maternal mRNAs before expression of the zygotic genome. Maternal 45S genes are in Region-4 (not indicated in the sketch) (Ortega-Recalde et al. 2019). b) Reciprocally, early embryos at the beginning of the maternal-to-zygotic transition do not express maternal 5S rRNA or maternal spliceosome (U1, U6) genes in Region-2, but do express Region-2 znf genes and the nearby mir430 genes. Znf proteins may help to stabilize the mobility of transposable elements (White et al. 2017; Hadzhiev et al. 2019; Wells et al. 2023) and the mir430 mature products promote clearance of maternal messenger RNAs (Giraldez et al. 2005; Takacs and Giraldez 2016; Giraldez et al. 2006; Lund et al. 2009). R1, R2, R3, and R4 represent Regions-1 to -4.