Fig. 4.

Maternal regulation of chromosomal imprinting in coccids. a Heterochromatinization of the paternal chromosome set in coccids. The sperm (nucleus in blue) fertilizes the egg (nucleus in red). The zygote contains 10 chromosomes (2n = 10), which is the typical number found in coccid species. At around the 7th cleavage division in male embryos, a wave of heterochromatinization begins at one of the poles (shown in male embryo beginning at the right-hand pole). Heterochromatinization leads to the formation of a chromocenter (blue) that consists of the aggregation of the paternal chromosome set, which is shown magnified in the inset above the male embryo. The chromocenter is enriched in two heterochromatin-specific histone modifications H3K9me3 (yellow) and H4K20me3 (purple) as well as the non-histone chromosomal protein HP1 protein (black). In the rest of the male embryo individual, paternal (blue) and maternal (red) chromosomes can be observed and there is no chromocenter. This is the case throughout the female embryo given on the left. The “dots” of H3K9me3 (yellow), H4K20me3 (purple), and HP1 protein (black) simply represent enrichment—their distributions overlap on the heterochromatic set in embryos (Bongiorni et al. 2007; Cowell et al. 2002; Kourmouli et al. 2004). b The development of parthenogenetic males and females embryos in Pulvinaria hydrangeae. Meiosis gives rise to a haploid maternal nucleus containing 5 chromosomes; chromosomes at all stages are given in red as they are entirely maternal in origin. Polar bodies (PBs) I and II, once extruded, take no part in the events that follow. The maternal nucleus undergoes a haploid mitosis and the products fuse to give a diploid zygote substitute, 95% of which give rise to females where both sets are euchromatic. Five percent of the embryos produce males where one set of chromosomes becomes heterochromatic. Since the males are derived without any paternal contribution, the imprinting leading to subsequent heterochromatinization is therefore under maternal control and determined by conditioning of the maternal ooplasm. c A model for the maternal regulation of imprinting in coccids. In the top row, the maternal nucleus (in red) lies in an oocyte cytoplasm conditioned to produce females after fertilization (sperm nucleus is given in blue). Both chromosome sets remain euchromatic at all stages of development shown. In the bottom row, oocytes conditioned to produce males the egg are spatially differentiated with a region (shaded in blue) that contains factors or determinants laid down by the mother that can “imprint” chromosomes in the paternal pro-nucleus for later heterochromatinization. Heterochromatinzation is depicted as the step-wise aggregation of the paternal chromosomes (blue) into a chromocenter. The maternal chromosomes are colored red