II.26 Translocation linkage tests - T2-7a x male sterile genes.
R. T. Ramage and R. F. Eslick. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721 and Department of Plant and Soil Science, Montana State University, Bozeman, Montana 59715, USA.
Plants homozygous for the translocation T2-7a are light green seedling lethals. Chromosome T27a is composed of the long arm of chromosome 2 and a short arm consisting of parts of the short arms of chromsomes 2 and 7 including the satellite of chromosome 7. Chromosome T72a 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 (BGN 3: 47-49). Such a translocation is very useful in assigning genes to chromosomes. 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. When T2-7a is used in linkage tests with male sterile genes, the F2 should consist of 1 normal fertile : 2 semisteriles : 1 male sterile if the gene and translocation break-point are inherited independently and should consist of 2 semisteriles : 1 male sterile if the gene and break-point are absolutely linked. If the gene and translocation break-point are linked, but not absolutely, normal fertile plants will be found in the F2 as a result of crossing over between the gene and break-point. Thus, unlocated male sterile genes can be associated with the break-point of T2-7a with a minimum of work. After a male sterile gene has been associated with the break-point of T2-7a, other translocations can be used to assign it to either chromosome 2 or 7.
Forty male sterile genes that have not been assigned to chromosomes were crossed as female with the balanced primary trisomic 7 interchange homozygote T2-7a. Also, msg2, which is on chromosome 2 very closely linked with the break-point of T2-7a, and msg16, which is on chromosome 7 less closely linked with the break-point, were crossed to provide check populations. The F2's were classified for normal fertile : semisterile : male sterile at harvest time (Table 1). In 3 of the populations, msg,,o, msg,,bf, and msg,,bh, male sterility was difficult to distinguish from semisterility at harvest, so, the populations were classified as normal fertiles and "steriles". None of the 3 genes are closely linked to the T2-7a break-point as normal fertiles were observed in numbers close to those expected with independence of the gene and T2-7a break-point. Thirty-one of the F2 populations contained approximately 1 normal fertile : 2 semisteriles : 1 male sterile. It was concluded that these 31 genes are not closely linked with the break-point of T2-7a.
Table 1. Classifications for normal fertility : semisterility : male sterility of F2 populations of male sterile genes X T2-7a crosses.
Populations involving msg,,f and msg,,be contained no normal fertiles indicating absolute linkage between them and the T2-7a break-point. Populations involving msg2, msg9, msg16, msg18, msg,,bq and msg,,cw contained fewer normals than expected if the genes were inherited independently of the T2-7a break-point.
F3 rows from the 8 F2 populations that indicated linkage between the gene and T2-7a break-point were grown and classified for segregation for male sterility. Due to the lethality of the homozygous translocations, only semisterile F2 plants were used to produce F3 rows for calculating linkage intensities. F3 data are presented in Table 2. Some of the populations were also segregating for other characters known to be located on chromosome 2 or 7. F3 linkage data for these characters are also presented in Table 2.
Table 2. Semisterile F3 linkage data for gene to translocation breakpoint regions from populations derived from crosses of male sterile genes with T2-7a.
The genes msg2, msg,,f and msg,,be are very closely linked and the genes msg9, msg16 and msg,,cw are linked with the break-point of T2-7a. All should be located on either chromosome 2 or 7. The genes msg18 and msg,,bq show loose linkage with the T2-7a break-point. They may or may not be located on chromosome 2 or 7; if they are, they are not close to either centromere.
The gene msg7 is inherited independently of the T2-7a break-point. The population was also segregating for two- vs. six-row (V vs. v). Linkage data from the F3 of this population indicate that the V locus is about 24 recombination units from the break-point. This is in agreement with previous data. Data from the other populations that were segregating for two-row gave recombination values ranging from .20 to .50. These values and their standard errors are so large that not much confidence can be placed in them.
The msg9 population was also segregating for rough vs. smooth awns (R vs. r). F3 data indicate a recombination value of .30 for the R to break-point region. The msg,,f population was also segregating for long vs. short rachilla hairs (S vs. s). The F3 data indicate a recombination value of .33 for the S to break-point region. Again, these recombination values and their standard errors are so large that not much confidence can be placed in them.
The msg2 population was also segregating for elongated outer glume (E vs. e). The F3 data indicate a recombination value of .03 for the e to break-point region and a value of .04 for the e to msg2 region. As the recombination value for the msg2 to break-point region is .01, the most likely map of chromosome 2 would be: msg2 - centromere - T2-7a - e. However, this speculation is based on only 5 F3 rows which could have been the result of outcrossing rather than crossing over in the F1 generation.
BGN 5 toc
BGN Main Index