ABSENCE OF SEX-CHROMATIN CORRESPONDS WITH A SEX-CHROMOSOME UNIVALENT IN FEMALES OF TRICHOPTERA

Five Trichoptera species, representing four different families of thre e suborders, have been examined for sex chromatin status in relation t o their sex chromosome system. These were Hydropsyche sp., Polycentrop us flavomaculatus (Pictet), Rhyacophila sp., Anabolia furcata Brauer a nd Limnephilus decipiens (Kolenaty;). None of the species displayed se x-specific heterochromatin in highly polyploid nuclei of the Malpighia n tubule cells. Such sex chromatin is a characteristic trait of the he terogametic female sex in the sister order Lepidoptera; it is derived from the heterologous sex chromosome W. Hence, the absence of sex chro matin in somatic nuclei of Trichoptera females indicated the lack of a W chromosome in their karyotype. Correspondingly, diploid chromosome sets of the females consisted of an odd chromosome number, two sets of autosomes and one sex chromosome Z. Thus, the Z/ZZ chromosome mechani sm of sex determination has been confirmed. In pachytene and postpachy tene oocytes, the Z chromosome having no pairing partner formed a univ alent. In Hydropsyche sp., the Z-univalent was distinct as a compact, positively heteropycnotic element. Whereas, in two other caddis-flies, P. flavomaculatus and L. decipiens, it formed a negatively heteropycn otic thread. In postpachytene nuclei of nurse cells of A. furcata, two sister chromatids of the Z chromosome separated as a result of chromo some degeneration and formed a negatively heteropycnotic pseudobivalen t. The species-specific differences in pycnosis may reflect a transcri ptional activity/inactivity of the Z chromosome during meiotic prophas e. The absence of sex chromatin and the sex chromosome system in Trich optera are characters in common with the ''primitive'' Lepidoptera. Th is supports a hypothesis that the common ancestor of both orders had a Z/ZZ sex chromosome mechanism.