SEX-CHROMOSOME DIFFERENTIATION IN SOME SPECIES OF LEPIDOPTERA (INSECTA)

Sex chromosome morphology of eight Lepidoptera species was studied, ex ploiting predominantly the pachytene stage when chromosomes display a remarkable chromomere pattern. Six species had a WZ/ZZ sex chromosome system, one species a W(1)W(Z)Z/ ZZ system and one species was of the Z/ZZ type. Much like XY chromosomes in groups with male heterogamety, the lepidopteran sex chromosomes showed various degrees of structural differentiation. Differences between Z and W chromomere patterns range d from undetectable to obviously non-homologous. A common property of the W chromosomes (the W-1 in the W(1)W(2)Z/ZZ system) was the possess ion of a block of heterochromatin. The heterochromatin block comprised a small or a large segment of the W or even the entire W, depending o n the species. Segments with apparent structural homology are evolutio narily young parts of the sex chromosomes recently fused autosomes tha t have not had sufficient time for differentiation. The 'primitive' le pidopteran species Micropterix calthella had a Z/ZZ sex chromosome sys tem. This supports the hypothesis that the lepidopteran W chromosome c ame into being at the base of the 'advanced' Lepidoptera; it was presu mably an autosome whose homologue fused to the original Z chromosome.