GrainGenes

II.10 Gene order near the centromere of chromosome 2.

P. T. Gymer. Rothwell Plant Breeders, Rothwell, Lincoln, United Kingdom.

Some evidence suggests that the allocation of genes to chromosome arms by telotrisomic analysis may not always be reliable.

The accepted gene order on chromosome 2 is T2-7d - e - msg2, with the centromere between e and msg2. According to telotrisomic analysis, e is on the short arm, and msg2 on the long arm (Tsuchiya and Singh, 1973). The linkage is very close, so that the balanced tertiary trisomic, BTT 2-7d msg2, is very stable. The BTT has E-msg2 on the two normal chromosomes, and e-Msg2 on the 2-7d (extra) chromosome (Fig. 1).

Occasional crossovers have, however, been identified in 18-17, the most widely used stock of BTT 2-7d msg2. In the first generation, recombinant individuals are normal and fully fertile, but being heterozygous for e and msg2, will segregate in subsequent generations; they are presumed to be the result of crossing-over between e and the translocation break-point (Fig. 2). The presence of the e gene confirms that recombination, and not out-crossing, has occurred.

At Rothwell, one apparently recombinant individual was not only homozygous for e but also for msg2. Pollination by 18-17 (trisomic) gave diploid male-sterile F1 with seven bivalents in meiosis. Pollination by a diploid homozygous ee plant gave ee F1 with a quadrivalent in meiosis. These results indicate that the plant had two chromosomes 2, and did not contain the 2-7d translocation.

There are three possible explanations for this recombinant plant; in each case, the crossover chromosome would first have been transmitted to a trisomic plant, which in turn would have produced the double homozygote. The three theories are:

1. That the accepted gene order is correct, and this was a double crossover (Fig. 3). In view of the very tight linkage in this region, this seems rather improbable.

2. That the T2-7d break-point is in the long arm of chromosome 2, not in the short arm as previously thought (Fig. 4). However, the arm opposite the satellite on the 2-7d chromosome would then be the short arm of chromosome 2, which is generally accepted to be slightly shorter than the long arm of chromosome 6. In fact, in the normal BTT 2-7d, the unsatellited arm is distinctly longer than this, conforming to the long arm of chromosome 2, and only a little shorter than the long arm of chromosome 7 (Gymer, 1974).

That the correct ordering on the chromosome is T2-7d - msg2 - e (Fig. 5). This conflicts with the results of telotrisomic analysis (Tsuchiya and Singh, 1973). However, the telotrisomic evidence consists simply of the fact that, in the F2 of the cross Telo 2L x ee, there were many homozygous ee trisomics. Now this only means that the e locus is absent from the Telo 2L chromosome; this could easily be because of interstitial loss, which has indeed been observed on this arm before (Gymer, 1974). The cross, Telo 2S x ee has not yet been reported, but on this theory would be expected to give some homozygous ee trisomics. Even if it did not, this theory is not ruled out, since Telo 2S may include a small proximal part of the long arm, including the e locus. This recalls a suggestion already made concerning two of the fragment versions of the 2-7d chromosome (Gymer, 1974).

The msg2 locus may be on the long arm, proximal to the e locus, rather than as in Figure 5. This would agree with telotrisomic analysis (Tsuchiya and Singh, 1973), in which the F2 of Telo 2L x msg2msg2 gave no male-sterile trisomics; indicating that msg2 is on the long arm. However, it is again possible that Telo 2L may include a small proximal part of the short arm, perhaps including the msg2 locus.

Recombinant plants which are fertile ee homozygotes would, on this theory, be formed by crossing-over between Msg2 and the break-point.

The possibility that telotrisomic lines may actually contain deletions, or may not be strictly telosomic, must call for caution in the interpretation of the results of telotrisomic analysis.

Fig. 1. BTT 2-7d msg2
Fig. 2. The production of e e plants by crossing-over in BTT 2-7d
Fig. 3. Theory 1
Fig. 4. Theory 2
Fig. 5. Theory 3

References:
Gymer, R. T., 1974. BGN 4: 28-29.
Tsuchiya, T. and Singh, R. J., 1973. BGN 3: 75-78.

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