| Telocentric mapping of the squarehead (valgare)
gene Q on chromosome 5A of hexaploid wheat M.V. PRABHAKARA RAO Biology and Agriculture Division, Bhabha Atomic Research Centre Trombay, Bombay-400 085, India The long arm of chromosome 5A of common wheat carries the gene Q which is responsible for the suppression of speltoid effect and for squarehead spike characteristic of the variety Chinese Spring (SEARS 1954). In the present study, an attempt was made to map the genetic distance between the Q locus and centromere by using the telocentric method (SEARS 1966). Materials and Methods Chinese Spring wheat ditelosomic for chromosome arm 5AL was crossed to the substitution line 'spelta-5A'. Foundation seed of these two lines was kindly supplied by Dr. E.R. Sears, University of Missouri, Columbia, Mo., U.S.A. Spikes of the F1 plants (2n=41+t) were speltoid and nonsquarehead. In the variety Chinese Spring, Q behaves as a partially dominant gene for the suppression of speltoid effect and as far as the squarehead effect is concerned, the gene is hemizygous ineffective and fully recessive. The F1 plants were selfed as well as test-crossed as male parents to Chinese Spring. Results and Discussion The test-cross progeny consisted of 44 plants of which 26 were of vulgare-type and 18 of spelta-type. This phenotypic segregation conforms to a 1:1 ratio (X2=1.46; p=0.20-0.30) indicating independence of the Q locus and the chromosome 5A centromere. The chromosome constitution and phenotype of the test cross plants are summarized in Table 1. The 16 vulgare-type plants with 42 normal chromosomes and the 2 speltoids with 2n=41+t were scored as crossovers. The remaining 26 plants were non-crossovers. Out of the 32 plants with 2n=42, the 16 vulgare-type plants were crossovers and the 16 spelta-type plants were non-crossovers. However, among the 12 monotelodisomic plants (2n=41+t), only the 2 spelta-types were crossovers. The reduced frequency of crossover telocentrics cannot be explained with the present data. Male transmission of telocentric-5AL in competition with complete 5A was 12/44 or 27.3%. The F2 progeny of the selfed monotelodisomic F1 consisted of 227 plants of which 166 were of spelta-type (nonsquarehead) and 61 were of vublgare-type (squaerhead). This segregation was a very good fit for a 3:1 ratio (X2=0.424; p=0.50-0.70). A random sample of 80 F2 plants were used for meiotic studies to determine their chromosome constitution. The chromosome numbers and phenotypes of these F2 plants are given in Table 2. Within each of the three chromosomal classes and over all 80 plants, the distribution of spelta-type: vulgare-type conformed with 3:1 ratio. The data of both test cross and F2 progenies indicated that the Q locus is genetically independent of the chromosome 5A centromere. Hence it can be concluded that the gene Q is located 50 or more crossover units from the centromere, i.e. it has a distal location on the long arm of chromosome 5A. Literature Cited SEARS, E.R. 1954. The aneuploids of common wheat. Mo. Agr. Exp. Sta. Res. Bul., 572: 59. SEARS, E.R. 1966. Chromosome mapping with the aid of telocentrics. Proc. 2nd Int. Wheat Genetics Symp., (Lund, 1963), Hereditas Suppl. Vol. 2, 370-381. |