TB-10L19 has a break in the proximal part of the distal heterochromatic region of the B chromosome. Its break on chromosome 10 is very close to the centromere on the long arm (Lin, Genetics 92:931-945, 1979). Since the 10-B chromosome carried the major portion of the distal heterochromatic region of the B chromosome, the exact break position could not be determined due to presence of the heterochromatic mass at its end. Additional observations to be reported here indicate the break position being right at the 10 centromere. A plant carrying the 10 10-B B-10 B-10 chromosome constitution was studied at pachytene stage. Two B-10 chromosomes were seen to pair most of the time. One of the two B-10s paired occasionally with 10 and 10-B chromosomes to become a trivalent with complete pairing in two arms but an incomplete pairing in the other arm as would be expected. There were several exceptional cells where 10, 10-B and B-10 were associated in a T-configuration. Pairing in three arms was perfect. Two of the three arms were from chromosome 10, and the third arm is the distal heterochromatic region of the B chromosome. The centromere of chromosome 10 was located at a place where the three arms met. What is unusual about these cells is that the B-10 chromosome did not carry the expected B structures, i.e., the centromeric knob, the proximal euchromatic region and the proximal portion of the distal heterochromatic region. The B-10 chromosome of these cells, however, carried the same B part as that borne on the 10-B chromosome, i.e., the distal heterochromatic region. In other words, this B-10 chromosome did not have the centromere of either chromosome 10 or the B chromosome. The fact that it is acentric and present at the pachytene stage, suggests a premeiotic origin.
The breakpoint of TB-10L30 was also identified by virtue of its abnormal structure (Lin, MGNL 52:114-116, 1978). This translocation has a break close to the junction between the proximal euchromatic region and the distal heterochromatic region on the B long arm. The 10-B chromosome did not carry the expected B structure, i.e., the distal heterochromatic region. Instead, it carried the proximal B parts, i.e., the same B parts carried by the B-10 chromosome. The pachytene picture of a heterozygous plant had a T-configuration with complete pairing in three arms, two of which are from chromosome 10 and the other from the proximal portions of the B chromosome. The break position on the long arm of chromosome 10 can thus be measured directly. The average of three separate measurements (0.14, 0.12 and 0.13) is 0.13.
TB-7Lb has a break on the euchromatic region of the long arm of the B chromosome (Lin, MGNL 59:44-45, 1985). Its breakpoint on the long arm of chromosome 7 was measured only once with tassels of a 7 7-B B-7 B-7 plant. Chromosome 7 in this particular cell paired with the 7-B chromosome. B parts of the 7-B chromosome (including part of the proximal euchromatic region followed by the entire distal heterochromatic region) did not fold back and thus allowed an accurate measurement of the breakpoint, which is at the 0.34 position.
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