Contraction stage and formation of the distance sex bivalent in Oedionychina (Coleoptera, Alticidae)

At the beginning of PM I in Omophoita cyanipennis F abr. of Oedionychina (Coleoptera, Alticidae), a notable reserve of unoriented gel is left at the site of the contraction clump, together with the sex chromosomes. Although the sex chromosomes might be involved in the gel from the beginning of the PM I movements, the initially low viscosity of the gel allows departure and movement of X and Y towards the opposite poles, reorientation when necessary, and other minor but relatively rapid movements. Polar connections and PM I movements of the sex chromosome centromeres are slightly retarded as compared with the autosomal centromeres. However, before the sex chromosomes have time to reach the vicinity of the poles, their centromeres attain the metaphasic condition, and the poleward pull weakens. The gelic link is then sufficient to maintain the distance bivalent. It seems also possible that the gel body becomes more rigid by metaphase. The lateral location of the sex spindle is initially determined by the lateral contraction site of the chromosomes which in turn is due to the one‐sided accumulation of the clearzone material just prior to the breakdown of the nuclear membrane. Formation of the equatorial plate at the height of the contraction clump saves the distance sex bivalent from longer congression movements. Additional factors keeping the sex spindle in its lateral position are retarded initiation of PM I movement of the sex chromosomes, lack of centripetal component of the pulling force at their centromeres, and the gel body in which they are involved.