Dosage compensation of sex chromosomes in animals.

The origin and evolution of sex chromosomes have long been a focus of research in biology. Two of the most studied systems are the XY system and the ZW system. Due to Y/W degeneration, heterozygous (XY/ZW) sex-linked genes are absent and their dosage is reduced when compared to autosomal genes and homozygous (XX/ZZ) sex-linked genes, creating an issue of dosage imbalance between sex chromosomes and autosomes. Multiple evolutionary models have been proposed to explain the evolutionary mechanism of dosage compensation that might resolve such dosage imbalance. In this review, we summarize the findings related to the dosage effect of sex chromosomes from a variety of perspectives, including transcriptomes, proteomes, haploid cells, and single cells. In addition, a summary of the dosage effect of sex chromosomes in major phylogenetic branches of multiple species is provided. Finally, we approved an overview of the related theoretical models and future research directions at the end of this paper.