| A new interpretation of the mechanism regulating chromosome
pairing in Triticum Mahesh D. UPADHYA Department of Horticulture, University of Hawaii, Honolulu, Hawaii, U.S.A. The cytological studies of 27-chromosome hybrids between Secale cereale and monosomics of T. aestivum variety Chinese Spring, deficient in turn for all the twenty-one chromosomes, had revealed that the absence of chromosome 5B leads to a reduction and the absence of 6A leads to an increase in condensation of chromosomes at the first meiotic division. These observations thus suggested the presence of genes on chromosomes 5B and 6A which control the condensation cycle of meiotic chromosomes. This control of condensation process might have certain bearing on the control of homeologous pairing among the wheat chromosomes has been suggested. The details are being published elsewhere (Biol. Zentral. in press). However, the recent reports of FELDMAN (Proc. Natl. Acad. Sci. U.S. 55, 1966) and FELDMAN et al. (Proc. Natl. Acad. Sci. U.S. 56, 1966) have opened a new approach to the understanding of the mechanism controlling chromosome pairing in polyploid wheats. The data on the control of chromosome condensation by 5B and 6A, when viewed in the light of these reports on the premeiotic associations of homologous chromosomes, lend themselves to an interesting new interpretation of the control of diploid-like meiotic behavior of polyploid wheats. FELDMAN (Proc. Natl. Acad. Sci. U.S. 55, 1966) has shown that with the normal two doses of 5B, homologous chromosomes are associated before meiosis begins to give rise to regular bivalent formation in hexaploid wheat. And it has also been shown by FELDMAN et al. (Proc. Natl. Acad. Sci. U.S. 56, 1966) from their studies of the somatic associations of the homologues in the dividing root tip cells of T. aestivum, that the homologues tended to lie near each other. Therefore, it would seem that pairing of the homologous chromosomes during meiosis is predetermined by their associations at the last premeiotic mitosis. MOENS (Chromosome, Berl. 15, 1964) had observed that in the microsporocytes of tomato (Lycopersicon esculentum), the earliest visible chromosomes are fully paired bivalents, and thus has suggested that synapsis may therefore have occurred during or prior to premeiotic interphase. He had further indicated that the observations of MCCLINTOCK (Amer. J. Bot. 32, 1945) and SINGLETON (Amer. J. Bot. 40, 1953) in Neurospora crassa could well be applicable to higher plants. That is after the last premeiotic mitosis the condensed chromosomes pair immediately and undergo decondensation, and that the despiralization of the paired chromosomes facilitates point-by-point pairing. Also, the pairing of condensed chromosomes is, to some extent, supported by the absence of interlocking of bivalents. SMITH (Canad. J. Res Sec. D. 20, 1942) had also concluded from his studies that the meiotic pairing consumated at pachytene is initiated at the latest by the telophase of the last premeiotic division, and that at the anaphase of the last premeiotic division homologues become associated in pairs and reappear in the following prophase relationally coiled. He had also indicated that telophase pairing considered in relation to the time, phase and degree of relational coiling have an obvious bearing on such diverse phenomenon as "asynapsis" and somatic reduction. These conslusions give an indication that premeiotic association serves as an essential step in regulating meiotic pairing. |
| --> Next |