II.27 Mapping chromosome 6.
R. T. Ramage and Michele Paluska. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, USA.
Balanced tertiary trisomics can be used to locate the positions of translocation break-points and genes in chromosomes (Barley Newsletter 10: 44-49). In a BTT, the extra chromosome is a translocated chromosome and carries the locus of the marker gene. The break-positions of a translocated chromosome can be recognized more easily in a BTT than in a diploid as the extra chromosome can be compared with the two pairs of normal chromosomes from which the translocated chromosome derives its segments. As the dominant allele of the marker gene must be carried on the translocated chromosome of the BTT, its locus can be assigned to either the centromere or the translocated segment of the extra chromosome. We are using BTT lines to map both translocation break-points and genes. This communication reports progress in mapping the male sterile gene msg,,bk and break-points of translocations involving chromosome 6.
Twenty-three translocations involving chromosome 6 were crossed onto male sterile ms,,bk plants. One head from each F2 plant was harvested and planted in F3 rows to provide linkage information. Semisterile (heterozygous) plants in the F3 rows were harvested in bulk. The 300 lightest seed in about 2 pounds of each of the bulks were separated with an aspirator. The 300-seed lots were thin-planted (12 to 18 inches apart) in the field. Plants that appeared to be trisomic (actually, any plant that did not look like a diploid) were harvested and sown in plant rows. Some of these rows segregated in a typical BTT ratio, e.g., about 3 male sterile diploids to one male fertile trisomic. Trisomic plants from the apparent BTT rows were harvested. Root-tips of trisomics found in this seed were examined cytologically and the extra chromosome compared morphologically with the two pairs of normal chromosomes from which it derived its segments.
F3 linkage data are presented in Table 1. Recombination values ranged from .01 to .24. As the number of lines classified was small, standard errors of recombination values are large. From this data, it was concluded that the gene msg,,bk is located about 6 or 7 recombination units from the centromere of chromosome 6. This agrees with linkage data concerning msg,,bk and the orange lemma gene, o (BGN 4: 11-15).
The number of "trisomic" plants selected and the number of BTT lines found among their progenies are also presented in Table 1. At least one BTT line was obtained from every cross except that with T2-6h. The percentage of selected "trisomics" that were BTT's ranged from 7 to 28 demonstrating the ease of obtaining BTT lines.
Root-tip cells of BTT plants from the T6-7a cross contain an extra chromosome that has a satellite on each arm (Figure l). This is expected as the msg,,bk locus is located on the short arm of chromosome 6. Based on this observation and appropriate linkage data, it is concluded that msg,,bk is carried on the short arm of chromosome 6 about 6 or 7 recombination units from the centromere.
Figure 1. Root-tip chromosomes from BTT lines.
Other examples of extra chromosomes in BTT lines are shown in Figure l. In BTT's derived from Tl-6f, T3-6j and T4-6g, the extra chromosomes have satellites of chromosome 6 and long arms that are longer than a normal chromosome 6. These translocations should be broken in the long arm of chromosome 6. In the BTT derived from T2-6e, the extra chromosome has a short arm that is longer than a normal chromosome 6. The extra chromosome should be T26e with the break in chromosome 6 being between msg,,bk and the centromere. The extra chromosome derived from T5-6a is indistinguishable from a normal chromosome 6. Therefore, the break-point of this chromosome can not be determined from this observation.
