Insect sex chromosome evolution: conservation, turnover, and mechanisms of dosage compensation

Abstract

Sex chromosomes have evolved many times throughout the tree of life, and understanding what has shaped their unusual morphological, sequence, and regulatory features has been a long-standing goal. Most early insights into insect sex chromosome biology came from a few model species, such as the fruit fly Drosophila melanogaster, which limited broad-scale evolutionary inferences. More recently, extensive comparative genomics studies have uncovered several unexpected patterns, which we highlight in this review. First, we describe the conservation of the ancestral X chromosome over 450 million years but also its recurrent turnover (i.e. its reversal to an autosome when a new X chromosome arose) in at least one order. We then summarize classical and more recent findings on how insects modulate the expression of X-linked genes following the degradation of the Y chromosome and how the diverse mechanisms of dosage compensation identified may elucidate important principles of sex chromosome regulatory evolution.