| Mapping of the compactum gene C on chromosome
2D of wheat M. V. PRABHAKARA RAO Biology and Agriculture Division, Bhabha Atomic Research Centre. Trombay, Bombay-85, India The compactum gene C was located on chromosome 2D by UNRAU (1950). MCINTOSH and BAKER (1968) failed to recover any crossover involving C when the right arm telocentric 2D of Chinese Spring was combined with 2D of compactum variety, Poso, suggesting that the gene is probably located on the left arm of 2D. In the present study, the substitution line, Poso-2D (CC), was crossed with Chinese Spring (cc) ditelo-2D (left) monotelo-2D (right). F1 plants without the right arm telocentric (2n=42) (20II+1 hetII) were selected. These F1 plants (Cc) were selfed as well as used as male parents in testcrosses to Chinese Spring. Out of 222 F2 plants, 215 were of compactum-type and 7 of aestivum-type. Three of the aestivum-type plants were studied cytologically and two of them showed 20II+1 hetII (crossovers) at meiosis and the third plant had 20II+tII (non-crossover). Since the two plants with an entire 2D showed the aestivum phenotype, the C gene is located on the left arm of 2D. The testcross progeny consisted of 177 plants of which 35 were of aestivum-type and the remaining 142 of compactum-type. Cytological analysis of the aestivum-type plants revealed that 31 of them had the paternal telocentric (20II + 1 hetII) and were therefore non-crossovers, while four plants had the entire paternal 2D (21II) and were crossovers. Thus, a crossover value of 2.26% between C and centromere was obtained. The data indicates that the compactum gene is located on the left (beta) arm of 2D, close to the centromere. A few compactum plants with a crossover telocentric are expected in the population besides the 31 aestivum-type plants. However, the crossing over of C with the centromere being very low, the number of telocentrics carrying C is likely to be small and these would further be subject to competition with the entire 2D. Hence, if the occurrence of compactum plants with a crossover telocentric is ignored, the male transmission of telocentric 2D (left) in competition with the entire 2D is 31/177 i.e., 17.5%. Literature cited UNRAU, J. 1950. The use of monosomes and nullisomes in cytogenetic studies of common wheat. Sci. Agr., 30, 66-89. MCINTOSH, R. A. and E. P. BAKER 1968. A linkage map for chromosome 2D. Proc. 3rd Intern. Wheat Genet. Symp., pp. 343-347. (Received May 15, 1972) |