A novel structure of the B-10 chromosome of TB-10L6 --Cheng, Y-M, Lin, B-Y The B chromosome consists of two unequal arms. The long arm comprises, proximal to distal, a centric knob, a proximal euchromatic region, four distal heterochromatic regions (1, 2, 3, and 4) and a distal euchromatic tip (Lin, Genetics 92:931-945, 1979). The B breakpoint of TB-10L6 is located in the proximal euchromatic region (Lin, Genetics 90: 613-627, 1978). Accordingly, the 10-B chromosome contains the distal portion of the proximal euchromatic region, all four heterochromatic regions and the distal euchromatic tip; the B-10 chromosome carries the centric knob and the proximal portion of the proximal euchromatic region. The predicted pachytene structure of heterozygous TB-10L6 is a trivalent with a complete pairing in two arms and no pairing in the third, because the B part of 10-B and B-10 are not homologous.

Such pachytene structure was not observed in this study. Instead, of the 18 cells with a clear standard structure of 10-B and B-10 mentioned above, a T-configuration with two arms completely paired and the third arm partly paired was detected (Figure 1). The distal portions of the proximal euchromatic region of B-10 and heterochromatic region 3 of 10óB pair together (I, Figure 1). This pairing brings the portion of 10L adjacent to the breakpoint and heterochromatic region 2 in close association (II, Figure 1), leaving the unpaired portion of 10L to form a single-strand loop (III, Figure 1). The pairing between two different B parts were documented previously by Longley (Longley, Am. J. Bot. 43:18-22, 1956). He observed in heterozygous TB-9La a pairing between the proximal euchromatic region and the distal euchromatic tip of the B long arm in a reversed fashion, which he termed "dyscentric pairing". An exchange in the paired region resulted in the formation of a small chromosome carrying the centromeric knob and two tiny euchromatic arms. No such "dyscentric pairing" was observed in this study.

Seven other cells possessed an unexpected heterozygous structure. In those cells, the B-containing arm of the T-configuration had a standard 10-B and an anomalous B-10 which had a complete pairing at the end (I, Figures 2 and 3). The paired portions, identical in both chromosomes, included heterochromatic regions 3, 4, and the distal euchromatic tip, leaving heterochromatic regions 1 and 2 unpaired (II, Figure 3). In other words, the B-10 of these cells did not carry the expected B-structure; i.e., the centric knob and the proximal euchromatic region. Instead, its B-portion covers only heterochromatic regions 3 and 4 but not the centric knob and the proximal euchromatic region.. Such a B-10 structure is reminiscent of an early finding by Lin (Lin, MNL 60:54, 1986). He observed an acentric B-10 chromosome in the hyperploid of TB-10L19 and suggested a premeiotic origin. The current observation seems to be consistent with his supposition. In a premeiotic division, the proximal euchromatic region and heterochromatic region 3 may pair (I, Figure 1), and an exchange in this region would result in an acentric B-10 that carried heterochromatic regions 3 and 4 (I, Figure 3) but not regions 1 and 2 (II, Figure 3). Further work is needed to substantiate this supposition.

Figure 1. Chromosome pairing of a heterozygous TB-10L6 with an excepted B-10 structure. Arrow, centromere 10; *, the centric knob; I, pairing between heterochromatic region 3 and the proximal euchromatic region; II, association of heterochromatic region 2 with part of 10L; III, unpaired portion of 10L.

Figure 2. Chromosome pairing of a heterozygous TB-10L6 with an unexpected B-10 structure. Only the distal portion of the B-containing arm is focused. The distal portions of 10-B and B-10 have an identical structure, comprising heterochromatic regions 3, 4, and the distal euchromatic tip. Arrow, centromere 10; I, complete pairing of the heterochromatic regions 3 and 4 plus the distal euchromatic tip of 10-B and B-10.

Figure 3. Chromosome pairing of a heterozygous TB-10L6 with an unexpected B-10 structure. All three arms are focused. The distal portions of 10-B and B-10 have an identical structure, consisting of heterochromatic regions 3, 4, and the distal euchromatic tip. Arrow, centromere 10; I, complete pairing of heterochromatic regions 3 and 4 plus the distal euchromatic tip of 10-B and B-10; II, unpaired heterochromatic regions 1 and 2 of 10-B.
 
 


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