| Homoeologous relationships between two Agropyron
intermedium chromosomes and wheat T. T. THE and E. P. BAKER Department of Agricultural Botany, Faculty of Agriculture, University of Sydney, Sydney, Australia Certain chromosomes of Agropyron elongatum and Ag. intermedium have been found to compensate in genetic activity for wheat chromosomes. An Ag. elongatum chromosome conditioning stem rust resistance substituted effectively for homoeologous group 6 wheat chromosomes (KNOTT 1964, JOHNSON 1966, ANDERSON and DRISCOLL 1967). Two chromosomes from the same species each conferring leaf rust resistance were found to substitute for wheat chromosomes-one for 3D (BAKSHI and SCHLEHUBER 1958) and the other for 7D (SHARMA and KNOTT 1966, QUINN and DRISCOLL 1967) and 7A (NANDA 1968). An Ag. elongatum chromosome conditioning blue endosperm substituted for a specific wheat chromosome (KNOTT 1958). Weique is a commercial wheat cultivar in which a pair of wheat chromosomes is replaced by an Ag. intermedium pair conditioning stem rust resistance (WIENHUES 1965). A second chromosome conditioning rust resistance also substitutes for a specific wheat chromosome (WIENHUES 1966). Seed of two disomic Ag. intermedium addition lines to Vilmorin 27 wheat were kindly supplied by Dr. Y. CAUDERON, Institut National de la Recherche Agronomique, Versailles, France. TAF1 possesses resistance to leaf rust and TAF2 resistance to stem rust due to the alien chromosomes. During maintenance of these lines root-tip chromosome counts revealed that certain resistant lines in both accessions possessed 42 chromosomes and had presumably arisen through alien chromosome substitution. Resistance of TAF1 to Australian leaf rust strains is of the adult plant type and is difficult to recognise in the Vilmorin background since the latter possesses a measure of this type of resistance. However, when 42 chromosomes plants in each accession were crossed with T. aestivum, hybrid plants invariably exhibited 20II+2I at meiosis thus verifying their alien chromosome substitution constitution. Meiotic configurations observed are described according to the system suggested by KIMBER and SEARS (1968). A TAF2 substitution line was crossed with Chinese Spring stocks monosomic, monotelosomic or monoisosomic for members of the D genome. Except for crosses involving 7D, hybrids with 41 chromosomes consistently exhibited 1IV+17II+3I and those with 42 chromosomes 1IV+18II+2I in the case of monosomic crosses and 1IV+17II+t1II (or i1II)+2I where monotelosomics (or monoisosomics) were implicated. The 41 chromosome hybrids in crosses involving monoiso-7D consistently showed 1IV+18II+1I whilst those with 42 chromosomes exhibited 1IV+18II+1I+i, thus indicating that the Ag. intermedium chromosome in TAF2 had spontaneously substituted for wheat chromosome 7D. The quadrivalent observed in all crosses indicated a translocation difference in Vilmorin 27 relative to Chinese Spring involving chromosomes in the A or B genomes. A 42-chromosome plant in the "critical" cross involving monoiso-7D proved to be exceptional, exhibiting a modal pairing of 1IV+1III+17II+1I. One explanation is that the trivalent arose through non-disjunction of a wheat chromosome either during mega- or microsporogenesis, or in an early somatic division. |
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