Different mechanisms may control specific subsets of genes. Polyploidy generated by whole-genome duplication theoretically causes no chromosome imbalance 59 and can even be advantageous, perhaps by increasing variation in gene expression due to rapid genetic and epigenetic changes Banaszynski, L.
Additional sex differences result from biallelic X expression during specific stages of female embryogenesis and from specific sex chromosome expression in germ cells. In aneuploid genomes, gene expression is altered not just in the aneuploid region but also globally, suggesting enhanced gene expression instability unseen in euploid genomes, which may partly explain overlapping features of different chromosomal disorders 47 The MSL complex is not sufficient to achieve a precise doubling of gene expression, and other components are also involved, including some repressive factors 51
To understand this relationship and its implications for a potential role in gene silencing, we review DNA methylation patterns from birds through to mice. Polyploidy is tolerated in many organisms despite massive changes in dosage throughout the genome.
Summary DNA methylation is an important epigenetic mark that has long been recognized for its silencing role in X chromosome inactivation in eutherian mammals. Antequera F. In contrast, uniformly high DNA methylation at promoters as on the inactive X in eutherians correlates with reduced gene expression—but so does uniformly low DNA methylation.
The mouse X chromosome is enriched for sex-biased genes not subject to selection by meiotic sex chromosome inactivation. This meiotic sex chromosome inactivation MSCI is triggered by the largely unpaired nature of the X and Y compartment, which attracts a specific epigenetic machinery involving the histone H2AX
Smchd1-dependent and - independent pathways determine developmental dynamics of CpG island methylation on the inactive X chromosome. Independently evolved dosage compensation systems in reptiles and fish use many of the same epigenetic silencing mechanisms non-coding RNA, DNA methylation, and modified and variant histones as do compensation systems in mammals.
Al Nadaf S. CpG position was determined relative to annotated TSSs in each species.