Genome Silencing and Elimination: Insights from a “Selfish” B Chromosome

Abstract

B chromosomes are non-essential components of numerous plant and animal genomes. Because many of these “extra” chromosomes enhance their own transmission in ways that are detrimental to the rest of the genome, they can be thought of as genome parasites. An extreme example is a paternally inherited B chromosome known as paternal sex ratio (PSR), which is found in natural populations of the jewel wasp Nasonia vitripennis. In order to ensure its own propagation, PSR severely biases the wasp sex ratio by converting diploid female-destined embryos into transmitting haploid males. This action occurs at the expense of the other paternally inherited chromosomes, which fail to resolve during the first round of division and are thus eliminated. Recent work has revealed that paternal genome elimination by PSR occurs through the disruption of a number of specific histone post-translational modifications, suggesting a central role for chromatin regulation in this phenomenon. In this review, we describe these recent advances in the light of older ones and in the context of what is currently understood about the molecular mechanisms of targeted genome silencing and elimination in other systems.

Controlling Apomixis: Shared Features and Distinct Characteristics of Gene Regulation

“…Similarly, mechanisms of depletion of the paternal genome after occasional fertilization might be reinforced by mechanisms related to the heterochromatic B-like chromosomes in Boechera, as previously demonstrated for jewel wasps [157]. In the jewel wasp Nasonia vitripennis paternally inherited B-chromosomes promote their own transmission at the expense of other paternal chromosomes which are eliminated to form a haploid embryo [157]. Future studies will be needed to uncover, if similar pathways are active in plants.…”

A chromosome‐level genome assembly provides new insights into paternal genome elimination in the cotton mealybug Phenacoccus solenopsis

“…To further uncover the epigenetic factors that possibly contributed to the heterochromatinization of paternal chromosomes in male cotton mealybugs, we chose several histone modifications relevant to heterochromatin formation for further analysis (Aldrich & Ferree, 2017; Bongiorni et al., 2007; Bongiorni et al, 2009; Hodson et al., 2017). Western blot experiments showed that H3K9me3 and H3K27me3 are substantially accumulated in male chromatin compared to female, whereas H4K20me1 and H4K20me3 were not detected in either sex (Figure 4c, Figure S7).…”

“…According to Western blot experiments, the heterochromatinization status in males might be maintained by H3K9me3 and H3K27me3, similar to other species such as Planococcus citri Risso (Bongiorni et al., 2007), Liposcelis sp. (Hodson et al., 2017) and Nasonia vitripennis Walker (Aldrich & Ferree, 2017; Aldrich, Leibholz, Cheema, Ausio, & Ferree, 2017). The signals of H4K20me1 and H4K20me3 modifications could not be detected.…”

How Next-Generation Sequencing Has Aided Our Understanding of the Sequence Composition and Origin of B Chromosomes

“…The most drastic impact of B chromosomes documented in the literature relates to the jewel wasp ( Nasonia vitripennis ), in which the B chromosomes (also referred to as the “paternal sex ratio” (PSR) chromosomes) are transmitted exclusively via the sperm, and act to eliminate one set of A chromosomes during the zygote’s first mitosis [ 17 , 18 ]. As a result, a female zygote is converted into a male embryo (see review by Aldrich and Ferree [ 19 ]). The PSR-induced elimination of the paternal A chromosomes is regulated by post-translational modifications to the histones associated with the sperm’s chromatin [ 20 ].…”