GrainGenes
A Database for Triticeae and Avena
Ramage*and Michele Paulska, pp. 38-40
II.24 Use of balanced tertiary trisomics to map translocation break-points.
R. T. Ramage*and Michele Paulska, Agronomy Department, University of Arizona, Tucson, Arizona 85721, U.S.A. (* Research Geneticist, Plant Science Division, ARS, USDA).
From a breeding program designed to establish tertiary trisomics balanced for both a seedling lethal and a male sterile gene, we have obtained information concerning the location of translocation breakpoints. Four balanced tertiary trisomics (BTT's) have been established that use a male sterile gene whose locus is on the centromere segment of the extra chromosome very close to the centromere. These BTT's are: 51a ms1, 51b ms1, 51c ms1 and 21a ms2. All of the extra chromosomes have a translocated segment of chromosome 1. The gene for albino seedling, ac2, is located on chromosome 1. Its locus is on the centromere segment of the extra chromosome in the BTT 17c ac2 and is on the translocated segment of the extra chromosome in the BTT 61a ac2. Trisomic plants from the male sterile BTT's were used as the female parent in crosses with the albino seedling BTT's. The F1 trisomics should have had a diploid complement heterozygous for both ms and ac2. If the ac2 locus is not carried on the translocated segment of the extra chromosome, the F2 should segregate 3 green : 1 albino seedling. Trisomic F2 plants should produce F3 progenies in a ratio of 1 "all green" : 2 "segregating 3 green : 1 albino". If the ac2 locus is carried on the translocated segment of the extra chromosome, the percentage of albino seedlings in the F2 would be dependent upon the frequency of crossing over between the gene and the centromere and upon the frequency of transmission of the extra chromosome. As ac2 is probably close, genetically, to the centromere, the F2 ratio should be dependent primarily upon the frequency of transmission of the extra chromosome. The frequency of albino seedlings in the F2 should be between 12.5% (with 50% transmission) and 25% (with no transmission of the extra chromosome). Trisomic F2 plants should produce F3 progenies in a ratio of 1 "all green" : 2 "segregating less than 25% albinos: " 1 "BTT for ac2". In this study, F3 progenies were classified as "all green", "segregating less than 50% albinos", or "segregating more than 50% albinos".
From crosses of BTT 51a ms1, 38 trisomic F1's produced F2 progeny containing from 13 to 32 percent albino seedlings. The total F2 progeny consisted of 4436 green and 1478 albino seedlings. F3 progenies were grown from 475 trisomic F2 plants: 162 were "all green" and 313 "segregated less than 50% albinos". These ratios are not significantly different from the 3:1 and 1:2 ratios expected if the ac2 locus is not carried on the extra chromosome. If the ac2 locus is not on the translocated segment of chromosome T51a, it should be on the centromere segment of chromosome T15a.
From crosses of BTT 51b ms1, 23 trisomic F1's produced F2 progeny containing from 10 to 29 percent albino seedlings. The total F2 progeny consisted for 2522 green and 388 albino seedlings. F3 progenies were grown from 297 trisomic F2 plants: 71 were "all green", 145 "segregated less than 50% albinos" and 81 "segregated more than 50% albinos". The 81 F3 progenies classified as BTT's contained an average of 24.7% green and 75.3% albino plants. These ratios fit expectations if the ac2 locus is carried on the translocated segment of chromosome T51b. The ms1 and ac2 loci would be linked in the homozygous translocation T1-5b.
From crosses of BTT 51c ms1, 18 trisomic F1's produced F2 progeny containing from 12 to 28 percent albino seedlings. The total F2 progeny consisted of 1330 green and 309 albino seedlings. F3 progenies were grown from 240 trisomic F2 plants: 58 were "all green", 132 "segregated less than 50% albinos" and 50 "segregated more than 50% albinos". The 50 F3 progenies classified as BTT's contained an average of 27.2% green and 72.8% albino seedlings. The ratios fit expectations if the ac2 locus is carried on the translocated segment of chromosome T51c. The ms1 and ac2 loci would be linked in the homozygous translocation T1-5c.
From crosses of BTT 21a ms2, 23 trisomic F1's produced F2 progeny containing from 22 to 57 percent albino seedlings. The total F2 progeny consisted of 1675 green and 1315 albino seedlings. F3 progenies were grown from 514 trisomic F2 plants: 17 were "all green" and the other 497 segregated from 8% to 78% albino seedlings. Twenty-eight of the F3 progenies contained less than 31% albinos, 423 contained from 31% to 60% albinos, and 46 contained more than 60% albinos. These data do not conform to any of the expectations. Some mechanism such as gametophyte factors or preferential chromosome pairing is needed to explain the observed ratios. We are continuing the investigation of this material.