Telocentric mapping of a second gene for grass-clump dwarfism

R.A. MCINTOSH and E.P. BAKER

Department of Agricultural Botany, The University of Sydney, Australia

MCMILLAN (l937) explained grass-clump dwarfism on the bssis of interaction between four genes, two of which were completely linked in repulsion. HERMSEN (1963) proposed a three gene system which could adequately explain MCMILLAN's results. He designated the genes D1, D2 and D3. The results of HURD and MCGINNIS (1958), and HERMSEN (1963) showed that chromosomes 2D, 2B and 4B, respectively, carry these genes. MCINTOSH and BAKER (1968) indicated that D1 is located 3.2 cross-over units to the right of the 2D centromere. This report describes a Preliminary study involving telocentric mapping of D2 and indicates the locus order for centromere, D2 and Sr9b on 2B.

Kenya W744 (number refers to the Sydney University Wheat Accession Register), d1 D2 d3, produces F₁ dwarfs in crosses with Gabo, D1 d2 D3. In a preliminary experiment a (Chinese Spring mono-2B x Kenya³ ) monosomic plant was pollinated with Gabo. Somatic counts on ten F₁ plants showed that nine possessed 41 and one, 42 chromosomes. The phenotype of the nine monosomic plants was normal and that of the disomic plant was dwarf.

The Kenya parent was then used to pdllinate a Chinesse Spring stock (d1 d2 d3) ditelosomic for the right arm of chromosome 2B. These F₁s were used as pollen parents in test-crosses to Gabo. Observations were made on the test-cross seedlings to deterimine their chromosome constitution by root tip counts, the presence of the gene Sr9b conditioning stem rust resistance known to be on chromosome 2B and for dwarf versus normal habit. Kenya carries Sr15 in addition to Sr9b and Gabo possesses Sr11. Stem rust strain 34-2,4,5 (culture 64231) was used to detect Sr9b. This strain is virulent on plants with Sr11 and glasshouse temperatures at the time of study were sufficiently high to prevent the operation of Sr15.

On the basis of test-cross results, which are presented below (Table 1), the most likely gene order is shown in Figure 1.

Recombination estimates from the data are:

Centromere	-	Sr9b	18.2%	( 6/33)
Centromere	-	D2	48.5 %	(16/33)
Sr9b	-	D2	42.2%	(14/33)

SEARS and LOEGERING (1968) estimated recombination between the centromere and Sr9a to be 10.6% which at P=0.05 does not differ from the value of 18.2 + or - 6.7% obtained in the current investigations. It appears that D2 is located on the right arm considerably distal to Sr9b. As SEARS and LOEGERING showed that Sr16 was distal to, and independent of Sr9a, linkage between Sr16 and D2 is probable.

MCINTOSH and BAKER (1968) located D2 on chromosome 2D close to the centromere. Since D2 appears to be independent of the 2B centromere it appears unlikely that these genes represent mutations at "homoeologous" loci.

References

HERMSEN, J.G. TH. 1963. Euphytica 12 : 126-9.

HURD, E.A. and R.C. MCGINNIS 1958. Can. J. Plant Sci. 38 : 506.

MCMILLAN, J.R.A. 1937. C.S.I.R. Bull. No. 104, Melbourne, Australia.

MCINTOSH, R.A. and E.P. BAKER 1968. Proc. Third International Wheat Genetics Symposium. Australian Academy of Science, Canberra. 305-9.

SEARS, E.R. and W.Q. LOEGERING 1968. Crop Sci. 8 : 371-3.

(Received August 1, 1969)