Cytogenetics of a moth species with a low chromosome number, Orgyia thyellina

While most Lepidoptera have numerous small chromosomes, Orgyia thyellina has few (n = 11) and relatively big chromosomes. We exploited this situation in a light microscopic study of chromosome morphology in mitosis and in various stages of male and female meiosis. - There is no indication of a primary constriction in mitotic chromosomes. Meiosis is chiasmatic in males and achiasmatic in females. Bivalents in the late female pachytene stage have a chromomere pattern of an unusually high degree of resolution. It is especially suited to test structural homology of synapsed chromosomes in bivalents. The chromomere pattern is homologous in the synapsed autosomes but partly differential in the equally synapsed sex chromosomes. The female pachytene affords a good basis for cytogenetic investigations in Lepidoptera.