Kinetic organization of metaphase I bivalents in spermatogenesis of Lepidoptera and Trichoptera species with small chromosome numbers

Abstract

The fine structure of bivalents in metaphase I spermatocytes of two Lepidoptera species, Orgyia thyellina Butler (n = 11) and O. antiqua (L.) (n = 14) (Lymantriidae), and a Trichoptera species, Limnephilus decipiens (Kolenaty) (n = 10) (Limnephilidae) were studied using a series of ultrathin sections and transmission electron microscopy. The bulk of species in both orders possess a haploid chromosome number of about 30. Thus, the experimental species have relatively small chromosome numbers. This study showed that metaphase I bivalents in both Lepidoptera species are polykinetic; attachment of kinetochore microtubules is found scattered throughout the entire poleward chromosomal surface. The microtubules were inserted in material of medium electron density. A pair of distinct kinetochore plates, consisting of material of about the same electron density as the chromatin, was detected at each poleward chromosomal surface in metaphase I bivalents of the caddis-fly, L. decipiens. The observations suggest that DNA elements responsible for the organization of the kinetochores are dispersed throughout the chromosomes in the two Lepidoptera species, whereas they are narrowly clustered in the Trichoptera. Thus, karyotype evolution in the closely related Lepidoptera and the Trichoptera involved widely differing mechanisms.