II.30. Genetics and cytology of the translocation T2-7a.
R. T. Ramage, Michele Paluska and G. A. Wiebe. Agricultural Research Service, U. S. Department of Agriculture, Agronomy and Plant Genetics Department, University of Arizona, Tucson, Arizona 85721, U.S.A.
The translocation T2-7a was X-ray induced in the cultivar Mars C.I. 7015 (1,2). Plants homozygous for the translocation are light green seedling lethals. All plants heterozygous for the translocation that have been progeny tested have segregated for the seedling lethal. The translocation stock is maintained by selecting heterozygous (semisterile) plants from the selfed progeny of heterozygous plants.
A translocation whose break-point is absolutely linked with a seedling lethal has advantages as a linkage tester. The typical translocation gives an F2 ratio of 1 standard normal : 2 heterozygous translocations : 1 homozygous translocation. Standard normals and homozygous translocations both have normal fertility and can not be separated phenotypically. In the F2 of a translocation such as T2-7a, the homozygous translocations are seedling lethals so all F2 plants with normal fertility are standard normals. Also, all F9 plants heterozygous for the translocation will segregate for seedling lethatity in F3.
T2-7a has been used in linkage tests with gene markers on both chromosomes 2 and 7. For markers on chromosome 2, no recombination between ms2 and the break-point and about 22% between V and the break-point have been observed. These values indicate that the break-point is either very close to the centromere or in the short arm of chromosome 2. For markers on chromosome 7, no recombination between ms19 and the break-point, about 5% between ddt and the break-point, about 8% between ms16 and the breakpoint and about 50% between R and the break-point have been observed. These values indicate that T2-7d is broken in the short arm of chromosome 7.
Observations of mitotic prophases in root-tip squashes confirms the positions of the break-points as predicted by genetic data. Chromosome T2-7a is composed of the long arm of chromosome 2 and a short arm consisting of parts of the short arms of chromosomes 2 and 7 including the satellite of chromosome 7. Chromosome T7-2a is composed of the long arm of chromosome 7 and a short arm consisting of parts of the short arms of chromosomes 7 and 2. Both chromosomes are easily recognized in root-tip squashes. Chromosome T2-7a, which has the satellite of chromosome 7, can be distinguished from chromosome 7 by its longer short arm and shorter long arm. Chromosome T7-2a, which has the long arm of chromosome 7, can be distinguished from chromosome 2 by its long long arm and from chromosome 7 by its non-satellited short arm.
Three balanced tertiary trisomic plants have been selected from the F2 of linkage tests involving ms16 and T2-7a. The F2 plants were selected as trisomics because they exhibited a phenotype resembling plants trisomic for chromosome 2. They were identified as balanced tertiary trisomics based on their F3 progenies. The F3 progenies contained male sterile diploids, male fertile trisomics and male fertile diploids in ratios of 15 : 8 : 2, 12 : 1 : 2, and 20 : 11 : 5. The male fertile diploids could have been the result of either crossing over in the BTT complex or of outcrossing in the field. Progenies were grown from the 20 trisomic plants. They contained a total of 80 male sterile diploids, 29 male fertile trisomics and 32 male fertile diploids. Again, the male fertile diploids could have been the result of either outcrossing in the field or of crossing over in the BTT complex. Progenies produced from trisomic plants grown in the greenhouse (where crossing over is minimal) also contain male fertile diploids as well as trisomics that are not balanced for ms16. This indicates that crossing over between ms16 and the break-point of the extra chromosome is occurring and accounts for at least some of the male fertile diploids found in the selfed progeny of the BTT.
Root-tip squashes of balanced tertiary trisomic plants were examined and found to contain a normal diploid complement plus an extra T2-7a chromosome. As the ms16 locus is carried on chromosome 7, it must be distal to the break-point of the translocation T2-7a. If the rather large number of male fertile diploids found in the selfed progeny of BTT plants is indicative of the amount of crossing over occurring in the BTT complex, there must be considerable pairing between the satellited ends of chromosomes 7 and T2-7a.
Four balanced primary trisomic interchange homozygotes were selected from the F2 of linkage tests involving T2-7a. The F2 plants were selected as trisomics because their phenotypes resembled that of plants trisomic for chromosome 7. They were identified as balanced trisomics by their F3 progenies. The F3 progenies contained trisomic plants and seedling lethals in ratios of 3 : 7, 2 : 5, 14 : 32 and 7 : 16. Progenies were grown from the 26 trisomic plants. They contained a total of 51 trisomic and 103 seedling lethal plants.
Root-tip squashes of balanced trisomic plants were examined and found to contain a diploid complement homozygous for T2-7a and an extra normal chromosome 7. They are described as balanced primary trisomic interchange homozygotes that are homozygous for T2-7a, have an extra chromosome 7 and are balanced for the seedling lethal that is associated with the break-point of T2-7a.
The normal chromosome 7 carries genetic material that is dominant to the seedling lethal associated with the break-point of T2-7a. It is not known if seedling lethality is the result of a recessive gene absolutely linked with the break-point or the result of a deficiency associated with the break-point. Also, it is not known if a normal chromosome 2 carries genetic material that is dominant to the seedling lethal. This appears unlikely because a number of F2 plants trisomic for chromosomes 2 and T2-7a have been selected and progeny tested. None were balanced for the seedling lethal. The seedling lethal is probably associated with the break-point of chromosome T2-7a only.
Balanced primary trisomic interchange homozygotes greatly facilitate the maintanance and use of translocations such as T2-7a. The only viable progeny of a selfed balanced trisomic are also balanced trisomics. The translocation stock can be maintained by harvesting surviving plants in the selfed progeny of the balanced primary trisomic interchange homozygote. The only functional pollen produced by the balanced trisomic carries the translocated chromosomes. When it is used as a male parent in crosses all crossed seed would be heterozygous for the translocation.
References:
Burnham, C. R., F. H. White and R. W. Livers. 1954. Chromosomal interchanges in barley. Cytologia 19:191-202.
Ramage, R. T., C. R. Burnham and A. Hagberg. 1961. A summary of translocation studies in barley. Crop Science 1:277-279.