II. 33. Location of a light green dwarf mutant on chromosome 2.
R. T. Ramage and R. A. Ronstadt-Smith. USDA-ARS, Plant Sciences Department, University of Arizona, Tucson, Arizona, U.S.A. "R"
A number of mutants were induced in a two-row, backeross derived, form of the variety "Glacier". The mutants were backcrossed to normal six-row Glacier. The F2's of the backcrosses were examined for linkage between the mutants and the row number locus. A light green dwarf mutant was found to be linked with the row-number locus with .15 + .051 recombination between them (BGN 11:37-38).
The row number locus is located in the long arm of chromosome 2, about 26 recombination units from the centromere. The light green dwarf locus could be between the centromere and the row number locus or distal to the row number locus. We use the male sterile gene, msg2, as a marker for the centromere of chromosome 2 and are still not sure in which arm it is located.
To determine the location of the light green dwarf locus, male fertile, six-row, dwarf plants were crossed onto male sterile, two-row, normal plants and male fertile, two-row, dwarf plants were crossed onto male sterile, six-row, normal plants. In both kinds of crosses, the male sterile locus and the light green dwarf locus were in repulsion. In one kind of cross, the row number locus and the dwarf locus were in coupling with the row number and male sterility loci being in repulsion. In the other kind of cross, the row number locus and the dwarf locus were in repulsion with the row number and male sterility loci being in coupling. The two kinds of crosses are referred to as "coupling" and "repulsion" crosses in this report.
The F1 generation was grown in the summer at Bozeman, Montana and the F2 generation was grown in the winter at Tucson, Arizona. A total of 495 plants from the coupling cross and 445 plants from the repulsion cross were classified for the three kinds of characters. In both kinds of crosses, two-row-six-row heterozygotes were easily and accurately identified.
Chi-square values for fit to expected ratios were calculated and they indicated that in both kinds of crosses, all three characters segregated as expected. The chi-square values for a fit to a 3 normal:l light green dwarf ratio were 1.488 in the coupling cross, 0.001 in the repulsion cross and 0.817 in the total 940 plants. The chi-square values for a fit to a 3 male fertile:l male sterile ratio were 1.132 and 0.468 in the two kinds of crosses and was 0.091 in the total population. The chi-square values for a fit to a 1 two-row:2 heterozygote:l six-row ratio were 2.895 and 0.009 in the two kinds of crosses and was 1.821 in the total 940 plants.
Chi-squares values for independent inheritance were calculated. In both kinds of crosses, light green dwarf and male sterility segregated independently as indicated by the chi-square values of 0.005 and 0.096 in the coupling and repulsion crosses and of 0.090 in the total 940 plants. Row number and dwarf were associated in their inheritance as indicated by chi-square values of 101.268 in the coupling cross and 136.098 in the repulsion cross. Row number and male sterility were also associated in their inheritance as indicated by chi-square values of 6.897 and 44.011 in the two kinds of crosses. The associated inheritance of row number and dwarf and of row number and male sterility and the independent inheritance dwarf and male sterility indicates a gene order of : male sterility - row number - light green dwarf.
Recombination values between the three characters were calculated by the maximum likelihood method. Light green dwarf and male sterility were in repulsion and exhibited independent inheritance in both crosses. Recombination values between them were .50 + .034, .51 + .035 and .51 + .024 in the two kinds of crosses and the combined populations. Light green dwarf and row number showed .19 + .020, .24 + .022 and .22 + .015 recombination in the coupling, repulsion and combined crosses. Row number and male sterility showed .32 + .026, .43 + .027 and .39 + .019 recombination in the coupling, repulsion and combined crosses. Based on recombination values from combined data, the map for these three genes in the long arm of chromosome 2 is: msg2--39--V--22 dwarf.
The recombination values found in this study are larger than we expected based on previous experience; particularly those in the repulsion crosses. We are aware of large variations in recombination values in this region of chromosome 2. The recombination value of 26 units that we assume for the msg2 to V region is based on the combined data from crosses of one female to 6 different males. Recombination values in the 6 crosses ranged from 16 to 36. Another factor that might be involved in the inflated values observed in this study is the environment in which the recombination occurred. The Fl's were grown in the summer at Bozeman, Montana whereas our previous experiences have been with Fl's grown in the winter in Arizona.
The light green dwarf locus is distal to the row number locus in the long arm of chromosome 2. It may be useful in connecting the liguleless, li, and triple awned lemma, tr, loci to the msg2-V linkage map. Crosses have been made to assess these linkage relationships.