In 1972, Phillips and Wang (MGCNL 46:123) listed 20 interchanges with a break in the nucleolus organizer region (NOR). The nucleolar association of two bivalents at diakinesis in homozygous interchange stocks was the criterion employed to determine that the break was in the NOR. The three interchanges T4-6(7037), T6-9(4778), and T6-10(5253) were thought to have a break proximal to the NOR in 6S because only one bivalent associated with the nucleolus in the homozygote and the satellite was of normal size. In the process of cytogenetically mapping
polymitotic (po) by hemizygous F1 tests in which the deficiency derived from heterozygous interchanges (see article by Phillips, Patterson, and Buescher, this issue), the above three interchanges all transmitted a deficiency-duplication complement that uncovered po. If the breaks are in 6S proximal to the NOR, the expected deficiency would include the entire NOR. Believing this to be unlikely, pachytene analysis was performed on the three interchange homozygotes. All three were found to have a break in the NOR heterochromatin.
The NOR break in T4-6(7037) was determined to be about 90% of the distance from the proximal to distal end of the heterochromatin (Het.90). Only a small heterochromatic segment (10%) was associated with the nucleolus. The bivalent possessing the large proximal segment (90%) was nearly always not associated with the nucleolus and usually formed no nucleolus. This 64 bivalent formed a small nucleolus comprising approximately 0.2% of the total nucleolar volume in less than 1% of the pachytene cells. Thus this NOR segment, consisting of nearly the entire NOR heterochromatin, was largely inactive in nucleolar formation at pachytene in the homozygote.
The T6-10(5253) interchange was found to have identical properties to T4-6(7037) in terms of nucleolar capacity. However, the break was determined to be at .3 in the NOR heterochromatin (Het.3). Thus the NOR heterochromatin can be separated at these two points (.3 and .9) with essentially no nucleolar capacity associated with the different proximal portions. These observations are consistent with the functional map developed for the NOR and presented by Phillips at the International Symposium on Genetics and Breeding, September, 1975.
The break in T6-9(4778) was extremely close to the distal end of the NOR heterochromatin (Het.95). This interchange differed from the other two in that both bivalents with NOR segments associated with a single nucleolus in at least 53% of the cells. The earlier cytology at diakinesis indicating only one bivalent associated with the nucleolus apparently was in error, perhaps representing a sampling problem. In 19% of the 136 pachytene cells analyzed, the 69 bivalent possessing nearly the entire NOR heterochromatin had associated a small nucleolus representing 1.7% of the total nucleolar volume. Apparently the NOR breakpoint in this interchange is positioned such that some nucleolar capacity is associated with the proximal heterochromatic segment.
In summary, these homozygous interchanges separate the NOR-heterochromatin into two parts with the proximal segment possessing a very low nucleolar formation capacity. The finding that the breaks are in the NOR explains why these interchanges uncovered po in appropriate crosses. Deficiencies for various parts of the NOR have been shown to be ovule transmissible (see Phillips, Patterson and Buescher, this News Letter).
R. L. Phillips and P. J. Buescher
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