Misdivision of an Agropyron elongatum chromosome

T.T.THE and E.P.BAKER

Department of Agricultural Botany, Faculty of Agriculture, University of Sydney, Sydney, N.S.W. 2006, Australia

Agropyron elongatum (Host) BEAUV. provides a source of resistance to both wheat stem rust (Puccinia graminis tritici ERIKS. and HENN.) and wheat leaf rust (P. recondita ROB. ex DESM.). One means of making use of these resistances has been via the amphidiploid P.P.W. 327 (2n=56) which has the three genomes of Chinese Spring wheat together with one genome (7 chromosome pairs) from polyploid A. elongatum (2n=70). This amphiploid was produced by L. H. SHEBESKI in Canada. It is difficult to determine whether Agropyron has contributed the adult plant type leaf rust resistance exhibited by the amphiploid since Chinese Spring also possesses resistance of this type. Disomic addition lines possessing seedling resistance to Australian stem rust strains have been produced at this Institution by backcrossing P.W. 327 with susceptible cultivars as recurrent parents. Besides stem rust resistance these addition lines possess resistance to leaf rust carried by the Agropyron chromosome pair.

KNOTT (1961) suggested that the gene(s) conditioning stem and leaf rust resistances were carried on the same chromosome in single Agropyron chromosome addition lines to wheat, the added chromosome pair being derived from P.W. 327. The same Agropyron chromosome was apparently involved in both instances since in both KNOTT'S and our studies it substitutes for group 6 wheat chromosomes. KNOTT found that some families segregated for resistance to stem rust but not leaf rust and vice versa and proposed that the gene(s) for stem and leaf rust resistances were on opposite arms of the same Agropyron chromosome. He suggested that chromosome misdivision producing telocentric chromosomes, evidence for which he did not obtain cytologically, could result in the separation of the two resistances.

In the present studies the Agropyron univalent was found to undergo spontaneous misdivision permitting isolation of the two derived telocentric stocks. These have been maintained as ditelosomic addition lines in a Gabo background. One telosome possesses resistance to stem rust, whereas the other carries adult plant leaf rust resistance (Fig. 1 a and b), thus verifying KNOTT'S suggestion.

One of the features exhibited by seedlings possessing either the monosomic or telocentric chromosome conditioning stem rust resistance is frequent somatic loss (approx. 1 in 450 seedlings) resulting in chimaerism for infection type which is always observed initially on the primary seedling leaf (Fig. 1 c). The extent of sectoring in the primary leaf varies markedly. Frequently the line of demarcation is at the midrib; in other instances smaller or larger sectors are resistant, indicating presumably that loss may occur at different stages in ontogeny. Somatic loss of the telocentric conditioning leaf rust resistance probably occurs but is less readily detected since such resistance is only operative at the adult plant stage when screening of large populations is technically more difficult.

Literature cited

KNOTT, D.R. 1961. The inheritance of rust resistance. VI. The transfer of stem rust resistance from Agropyron elongatum to common wheat. Can. J. Plant Sci. 41:109-123.

(Received Nov. 28, 197l)