II.4 Balanced tertiary trisomic 27c msg2.
C. E. Fastnaught and R. T. Ramage. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, USA.
The translocation T2-7c, in the variety Bonus, was crossed onto a male sterile, msg2, plant in a Coast type background. Trisomics balanced for the male sterile gene were obtained from the F3 of this cross. The trisomics also segregated for the two-row locus (V vs. v) in a ratio expected if the two-row locus was carried on the extra chromosome. Balanced tertiary trisomic plants resembled very closely plants that were primary trisomics for chromosome 2.
The two-row locus is carried on the long arm of chromosome 2 about 25 recombination units from the centromere. The msg2 locus is carried on the long arm of chromosome 2 very close to the centromere. Also, according to Tsuchiya (BGN 1: 73) "--it is almost impossible to distinguish morphologically the primary trisomic for complete 2 from the telotrisomic for 2L". Therefore, the BTT 27c msg2 should have almost the entire long arm of chromosome 2 included in its extra chromosome. The translocation T2-7c is broken in the long arm of chromosome 7. The extra chromosome in BTT 27c msg2 should consist of the satellite, short arm, centromere and part of the long arm of chromosome 7 plus almost the entire long arm of chromosome 2.
In root-tip cells of BTT plants, 3 chromosomes with satellites of chromosome 7 were observed (Figure 1). Two of these should be normal chromosomes 7 and the other should be the extra chromosome. The extra chromosome, the one on the left in Figure 1, can be recognized by its longer long arm.
In this BTT, the long arm of chromosome 2 determines it's morphological characteristics rather than the satellite, short arm and part of the long arm of chromosome 7.
Figure 1. Chromosome T27c and two normal chromosomes 7 from the BTT 27c msg2
