Dosage compensation and sex-chromatin in non-mammals

Abstract

1. Inactivation of one X chromosome in somatic cells of female mammals is a form of dosage compensation of sex-linked genes, but the mechanism is entirely different from that operating inDrosophila. The latter is designated as dosage compensationsensu strictu.2. There is no dosage compensation of barred,sex-linked dilutionorslow-featheringin domestic fowls, ofalmondorfadedin pigeons, or ofcinnamonin canaries. Among Lepidoptera the same is true ofsex-linked melanisminLymantria monachaand of a locus controlling haemolymph colour inChoritoneuraspp. There is no positive evidence that dosage compensation occurs outside Drosophila and mammals.3. Sex-chromatin in female birds (heterogametic) has been reported by several authors; the genetical evidence is against the possibility that this represents (as in mammals) an inactivated X chromosome. Sex-chromatin in the heterogametic sex also occurs in some (not all) Lepidoptera and Heteroptera; in Heteroptera it usually represents a heteropyknotic Y chromosome.4. Some complications in Muller's theory of dosage compensationsensu strictuare discussed. Not all ‘compensatory modifiers’ are necessarily sex-linked.5. The problem of dosage compensation in species with impaternate males is discussed;fusedinHabrobraconis not compensated.