Abstract
A sample of 62 Hemerobius marginatus males was studied, and ten individuals were found to carry one or two B-chromosomes, which were isopycnotic in meiosis. At pachytene, univalent B-chromosomes or a B-chromosome bivalent showed association with the heteropycnotic sex chromosome pair. No such association was found at diakinesis. At metaphase I, syntelically orientated univalent B-chromosomes showed preferential segregation from each other. The presence of univalent B-chromosomes-either one or two-was found to increase sex chromosome nondisjunction significantly. Among sex chromosome nondisjunctionals, a B-chromosome was found to segregate preferentially from sex chromosomes, indicating preferential segregation of nonhomologous chromosomes. Both preferential segregation of univalent B-chromosomes from each other and a B-chromosome from sex chromosomes are explained by the model according to which pachytene association of chromosomes is able to determine segregation even in those cases where chromosomes are physically unlinked at meiotic prophase.
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