Do male and female heterogamety really differ in expression regulation? Lack of global dosage balance in pygopodid geckos

Differentiation of sex chromosomes is thought to have evolved with cessation of recombination and subsequent loss of genes from the degenerated partner (Y and W) of sex chromosomes, which in turn leads to imbalance of gene dosage between sexes. Based on work with traditional model species, theory suggests that gene copy disequilibrium leads to the evolution of mechanisms to counter this imbalance. Dosage compensation, or at least achieving dosage balance in expression of sex-linked genes between sexes, has largely been documented in lineages with male heterogamety (XX/XY sex determination), while ZZ/ZW systems are assumed to be usually associated with the lack of chromosome-wide gene dose regulatory mechanisms.

性染色体（sex chromosomes）分化被认为是随着重组终止（cessation of recombination）以及性染色体退化配偶体（Y与W染色体）发生基因丢失而逐步演化而来的，该过程进而引发了两性间的基因剂量（gene dosage）失衡。基于传统模式物种的相关研究，现有理论指出基因拷贝失衡（gene copy disequilibrium）会推动抵消该失衡的调控机制演化。目前，剂量补偿（dosage compensation）——至少是实现两性间性连锁基因（sex-linked genes）表达的剂量平衡——已在雄性异配性别（male heterogamety，XX/XY性别决定系统）的演化支中得到广泛证实；而ZZ/ZW性别决定系统通常被认为与缺乏全染色体水平的基因剂量调控机制（chromosome-wide gene dose regulatory mechanisms）相关联。