COEVOLUTION OF SEX CHROMOSOMES AND

ABSTRACT Sex chromosome evolution is accompanied by significant divergence in morphology and gene content and results in most genes of one of the sex chromosomes being present in two dosages in one sex and in one dos-age in the other. To eliminate the difference in the expression levels of these genes between sexes and to restore equal expression levels of the genes between sex chromosomes and autosomes, mechanisms of dosage compensa-tion have appeared. Studies of three classical objects, Drosophila melanogaster, Caenorhabditis elegans, and mam-mals, have shown that dosage compensation of X-linked genes can be achieved through completely different chromosome-wide mechanisms. New data on sex chromosome gene expression demonstrating that many sex chromosome genes can be expressed at different levels in males and females were recently obtained from birds and butterflies. In this review, dosage compensation mechanisms in D. melanogaster, C. elegans, and mammals are considered and the data on sex chromosome gene expression in birds and butterflies, and their influence on our view of dosage compensation, are discussed