SEX-CHROMATIN IN LEPIDOPTERA

Like mammals, Lepidoptera possess female-specific sex chromatin. In a compilation of new and published data, 81% of the 238 Investigated Lep idoptera species display one or more heterochromatin bodies in female somatic interphase cells, but not in male cells. In contrast with the similar phenomenon in mammals, this sex-specific heterochromatin does not function as a dosage compensation mechanism. Most Lepidoptera have a WZ/ZZ sex chromosome mechanism, and tile sex chromatin is derived f rom the univalent W sex chromosome. Sex chromatin is regarded as an in dicator of an advanced stage of W chromosome evolution. In species wit h a Z/ZZ sex chromosome mechanism, loss of the W chromosome is accompa nied by loss of the female-specific heterochromatin. Since sex chromat in can be discerned easily in interphase nuclei, and especially so in the highly polyploid somatic cells, it is a useful marker for diagnosi ng chromosomal sex of embryos and larvae, and of identifying sex chrom osome aberrations in mutagenesis screens. All species with sex chromat in belong to the Ditrysia, the main clade of Lepidoptera that contains more than 98% of all extant species. Sex chromatin has not been repor ted for clades that branched off earlier. The nonditrysian clades shar e this character with Trichoptera, a sister group of the Lepidoptera. We propose that Lepidoptera originally had a Z/ZZ sex chromosome mecha nism like Trichoptera; the WZ/ZZ sex chromosome mechanism evolved late r in the ditrysian branch of Lepidoptera. Secondary losses of the W ch romosome account for the sporadically occurring Z/ZZ sex chromosome sy stems in ditrysian families. The lepidopteran sex chromatin, therefore , appears to mirror the full evolutionary life cycle of a univalent se x chromosome from its birth through heterochromatinization to sporadic loss.