General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

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    314 General cytogenetics §7.2 chromosomes. The centromeres seem to play a decisive role. Af¬ finity has been studied most thoroughly in mice, but there are reports (mostly not confirmed) of affinity in other species, in¬ cluding plants. The attraction between non-homologous chromo- meres from the same parent seems to be restricted to cases where the genomes are of distinctly different origin: heterozygotes formed by hybridizing two different inbred strains or an inbred strain with a wild strain. Segregations of genes within inbred strains do not show affinity. Again a different system is distributive pairing (Grell, 1964): non-homologous chromosomes of similar size sometimes have a tendency to associate. Although no crossing-over takes place, there is metaphase coorientation, leading to separation at anaphase: the two chromosomes are combined in one gamete less frequently than expected with random segregation. The phenomenon appears to be relatively common in Drosophila, but seems to be scarce in other organisms. A special case of correlated coorientation without apparent physical connection was first described by Payne (1916) and by White ( 1954); in meiosis of mole cricket males (Gryllotalpa) there is a single metacentric X-chromosome in addition to 10 normal bivalents and one heteromorphic bivalent consisting of a large and a small chromosome. The large chromosome of the heteromorphic bivalent and the single X-chromosome always pass to the same pole and are included together in the same daughter nucleus. Artifi¬ cial re-orientation of the heteromorphic bivalent (Camenzind and Nicklas, 1968) resulted in subsequent re-orientation of the X. The reversed did not occur: when the X was re-oriented, the hetero¬ morphic bivalent did not follow, but the X returned to its original pole, to which the large chromosome of the bivalent went. This correlated orientation insures proper segregation, but the mecha¬ nism is not understood.