| A major deletion of part of chromosome 5A of Triticum
aestivum T.E. MILLER and S.M. READER Plant Breeding Institute, Cambridge, England Many aneuploid types have been established in bread wheat, Triticum aestivum (2n= 6x=42). In the variety Chinese Spring nullisomics, monosomics, trisomics, tetrasomics, telocentrics, isochromosomes and nullisomic-tetrasomic compensating stocks exist (SEARS 1954; SEARS & SEARS 1978). However, few lines containing chromosomes with deletions of part of an arm have been established or reported despite their potential use for genetic analysis (SEARS 1977; GILL & KIMBER 1974; GIORGI 1978,1981). A line containing one complete chromosome 5A and one 5A with a major deletion of part of the long arm has been established in Chinese Spring. The effects of the deletion were first observed in a disomic addition plant (2n=6x=44) of chromosome 5R of Secale cereale cv. Imperial to Chinese Spring. The plant had a hairy neck produced by the Hp gene on 5R but also exhibited the speltoid phenotype typical of the reduced dosage of the Q gene on 5A. Although not examined cytologically this plant presumably contained a single deletion chromosome as its progeny contained plants with one, two or no very short unequal armed chromdsomes which could be recognised in mitotic metaphase preparations (Fig.1a). Moreover, only those plants with the distinctive chromosome had the speltoid phenotype. A disomic 5R addition containing a pair of the deletiop chromosomes (20"+deletion"+ 1"5R) was crossed with Chinese Spring euploid and with Chinese Spring double ditelocentric 5A. The euploid cross produced plants that were monosomic 5R but contained a single deletion chromosome (20"+1/deletion"+1'5R). Self pollination of such a hybrid produced a plant lacking 5R but retaining a deletion chromosome (20"+1/deletion", Fig. 1c), which still exhibited the speltoid phenotype (Fig. 1b). The double ditelocentric cross gave plants that contained a 5R chromosome, a deletion chromosome and a long and a short telocentric 5A chromosome. At first metaphase of meiosis the 5R and the 5AL chromosomes remained unpaired, whereas the deletion chromosome and the 5AS chromosome formed a bivalent (20"+st/deletion"+1t'+1'5R, Fig. 1d). The Q gene is carried on 5AL (SEARS 1952) and the deletion chromosome produces a speltoid phenotype indicating the loss of part of 5AL. This is supported by the morphology of the deletion chromosome and confirmed by the double ditelocentric cross. Further evidence is provided by gel electrophoresis of Beta-amylase isozymes. Chromosome 5A carries a gene Beta-Amy-A2 at 22 map units from the centromere (GALE, AINSWORTH & BAIRD 1982). Gel electrophoresis of the 5R addition containing a pair of the deletion chromosomes showed that the Beta-Amy-A2 gene was missing. The deletion point must therefore be less than 22 units from the centromere. |
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