Cytogenetics of Stem‐Rust Resistance in an Agrotricum Line1

Low transmission of resistance to stem rust and to adult‐plant leaf rust in crosses between the Agrotricum line NE66536 and Triticum aestivum L. em. Thell. prompted this cytogenetic study. Monosomic analysis showed that stem‐rust resistance was carried on an Agropyron chromosome which had substituted for chromosome 6D of wheat but could also substitute for 6A. Pollen with the Agropyron chromosome instead of 6D was slightly less competitive than pollen with a complete set of wheat chromosomes. Transmission of stem‐rust resistance in crosses to ‘Chinese Spring’ was a good fit to expected segregation ratios for plants having two independent univalents in place of a bivalent. Transmission of resistance in crosses to ‘Omaha’ was considerably lower than expected. It is proposed that differential alien chromosome instability in different genetic backgrounds leads to variability in the rate of univalent misdivision and somatic loss, resulting in variable transmission rates.