The nonsignificance of distance pairing for the regular segregation of the sex chromosomes in Hemerobius marginatus male. (Hemerobidae, Neuroptera)

Abstract

Pachytene nuclei in Hemerobius marginatus male showed five autosomal bivalents and loosely paired X and Y chromosomes. In part of the cells, the paired condition was retained until late prometaphase. In these cells, the segregation of sex chromosomes was determined by the bivalent condition. In another part of the cells, sex chromosomes appeared as spatially well separated univalents at diakinesis, followed by their congression separately to the equator at the beginning of prometaphase, after which they were pulled near the poles by spindle fibers during the stabilization process of metaphase structure. On the basis of the behaviour of univalent sex chromosoms during prometaphase, it was concluded that no pairing occurring late at prometaphase was involved in the determination of their segregation. During metaphase I, the X and Y chromosomes did not reorientate, but showed stabilized positions near opposite poles, i.e., distance pairing. Nondisjunction of sex chromosomes occurred in only 0.6% of metaphase I cells. The determination of the segregation of univalent sex chromosomes was explained by the model according to which nonrandom desynapsis movement of chromosomes is responsible for their initial orientation to opposite spindle poles at the beginning of prometaphase.