Gene dosage and the evolution of gene expression

In many diploid species, sex determination is linked to a pair of sex chromosomes that evolved from a pair of autosomes. In theseorganisms, thedegenerationof thesex-limitedYorWchromosomecausesa reduction ingenedose in theheterogametic sex forX-orZ-linked genes. Variations in gene dose are detrimental for large chromosomal regions when they span dosage-sensitive genes, andmany organisms were thought to evolve complete mechanisms of dosage compensation to mitigate this. However, the recentrealization that a wide variety of organisms lack complete mechanisms of sex chromosome dosage compensation has presented aperplexing question: How do organisms with incomplete dosage compensation avoid deleterious effects of gene dose differencesbetween the sexes? Here we use expression data from the chicken (Gallus gallus) to show that ohnologs, duplicated genes known tobe dosage-sensitive, are preferentially dosage-compensated on the chicken Z chromosome. Our results indicate that even in theabsence of a complete and chromosome wide dosage compensation mechanism, dosage-sensitive genes are effectively dosagecompensated on the Z chromosome.