A translocation between a B chromosome and the short arm of chromosome 2 bearing the B-Peru allele has been recovered and designated as TB-2Sa. When homozygous for an r seed-color allele the hyperploid progeny are easily identified by their deeply colored scutella and colorless aleurones. Because no wt seedlings were observed (0/29) among the hypoploid progeny of a cross of gl2 wt, r-g x hyperploid TB-2Sa r-g, the breakpoint of the translocation probably lies between the B and wt loci. This translocation should prove useful in some studies of the distal half of 2S because the only other B-2S translocation available to date is an unmarked compound translocation involving 3L.
Thirty-seven B-Peru r-g multi-B-chromosome plants, whose tassels were just beginning to emerge from the whorls, were treated with 500 to 594 r of gamma irradiation given at a rate of 0.95 to 1.13 r/sec. Irradiation was accomplished by hanging the potted plants upside-down next to a retractable in-air 60Co source at the University of Missouri Research Reactor. Pollinations were made from the irradiated plants onto various r-g testers 5 to 10 days after irradiation. The plant that gave rise to TB-2Sa received 500 r at a rate of 0.95 r/sec and was used as a pollen source six days after irradiation.
Among the 53,770 treated F1 progeny 234 TB-2S candidates were selected on the basis of their colorless aleurones and colored scutella. The selected F1's were planted in flats and screened for germination (205 germinated) and viability. Of the 143 individuals transplanted into the field, 64 were tested by crossing onto a gl2 wt, r-g tester. Only eight of those tested segregated for more than one non-corresponding (colorless aleurone/colored scutellum) kernel each and are considered to be likely candidates for TB-2S lines marked with B-Peru.
Further testing has only been performed on one of the candidates, TB-2Sa, which gave 2 kernels with colored scutella out of 45 with colorless aleurones from the F1 x tester cross. Both of these kernels were planted and crossed again onto gl2 wt, r-g. Only one of these crosses resulted in segregation of non-corresponding kernels. Out of the 304 kernels from that one cross (179 colorless and 125 colored aleurones) there were 37 hyperploid and 29 hypoploid (colored aleurone/colorless scutellum) kernels.
The 37 hyperploids are being reserved for stock increases and further mapping of the breakpoint. Sandbench testing of the 29 presumptive hypoploids resulted in 13 gl2 and no wt seedlings. Thirty of the colored/colored euploid kernels were scored in the sandbench and 5 were gl2 and none were wt. The presence of a few gl2 seedlings in the fully colored euploid class is thought to be due to crossing-over between B -Peru on the translocated chromosome and gl2 on the normal homolog. The distance between B and gl2 is 19 map units.
The initial idea for this experiment was provided by Dave Hoisington, the stocks were provided by Jack Beckett and Ed Coe, and the gamma-rays were provided by Vicki Spate and all of the other fine folks at MURR.
Craig Echt
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