Transmission of a deficiency for nearly the entire nucleolus organizer region

Last year we reported the transmission of a deficiency thought to include the entire NOR (MNL 57:131-132). The deficiency was initially detected by the presence of polymitotic (po) plants among the progeny of a cross between heterozygous T6-9(067-6) Females and heterozygous +/po males. Pachytene analysis of the po plants indicated heterozygosity for a Dp-Df chromosome. Based on the lack of apparent NOR-heterochromatin and the previously reported breakpoint of 6S.39, we believed the break was proximal to the NOR and that a Dp-Df chromosome deficient for the entire NOR had been transmitted. However, later quartet analysis of Dp-Df heterozygotes that were not hemizygous for po (-/+ instead of -/po) revealed a nucleolus in every microspore. We expected 50% to have a diffuse nucleolus if the plants were heterozygous for a complete NOR-deficiency. This observation indicated (1) that the break must be in the NOR and interestingly (2) that even a small portion of the NOR is sufficient to form a microspore nucleolus. Pachytene analysis of the homozygous interchange revealed two chromosomes associated with the nucleolus. The break appears to be in the proximal 1-2% of the NOR-heterochromatin. Thus, these tests indicate the transmission of a deficiency for 98-99% of the NOR and not the entire NOR.

R. L. Phillips, A. S. Wang and W. P. Bullock

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