Chromosome pairing problems in interploidy transfer of leaf rust resistance in oats

Summary

Advanced generation lines of the cross, C.I. 7232, a derived tetraploid (2n=28) x Clarion, a hexaploid (2n=42) oat, originally made to transfer leaf rust (C.O.-Puccinia coronata Cda. f. sp. avenae Fraser & Led.) resistance genes from C.I. 7232 into Clarion, were unstable for resistance and possessed dark lemmas. F₁₁-F₁₃ progeny lines were classified into two phenotypic classes−(1) plants with dark lemmas and resistant to rust (D-R) averaged 60% and (2) remaining plants with light lemmas and suceptibility to rust (L-S). Resistant F₁₂-F₁₃ plants and their F₁ hybrids resulting from crosses with Clarion had 20″ + 2′ (20 bivalents and two univalents), while susceptible plants had 21″. This indicated that a chromosome of C.I. 7232 (alien) which carried the gene(s) for reaf rust resistance (R) and another gene for dark lemma color (D) had replaced a hexaploid oat chromosome in the resistant plants. The alien chromosome was poorly transmitted through the pollen (0.16) indicating an advantage of euploid (n=21′) over alien substitution (n=20′+1′A) male gametophytes. Nonrecovery of recombinant types (L-R and D-S), and meiotic observations in resistant and susceptible plants indicated poor pairing affinity of the alien chromosome with the chromosome of hexaploid oat, resulting in whole chromosome substitution. Use of irradiation is suggested for leaf rust resistance transfer in this material.