Absence of dosage compensation for a sex-linked enzyme in butterflies ( Heliconius ) 4

6-phosphogluconate dehydrogenase (6PGD) is sex-linked in Heliconius butterflies. Within each of two species tested, the specific activity of 6PGD in males (the homogametic sex) is approximately twice that in females. This confirms that sex-linked genes in lepidoptera, as in birds, are not dosage-compensated. This absence of dosage compensation may be the basis for the frequent female-limitation of mimicry, and explains the peculiarity that the loci involved are never sex-linked, whereas male-limited sexual characters can be both sex-linked and autosomal.