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Spontaneous and Induced Interspecific Gene Transfer For Crown Rust Resistance in Avena¹

Crown rust resistance from Avena tetraploid CI 7232 was transferred to Avena sativa L. utilizing irradiation with thermal neutrons. The relationship between an alien chromosome from CI 7232 and its homologue in hexaploids was also studied.

CI 7232, derived from a cross between A. abyssinica Hochst. (2n=28) and A. strigosa Schreb. (2n=14), possesses resistance (Pc-15) to crown rust (Puccinia coronata Cda. f. sp. avenae Fraser and Led.) from A. strigosa. Earlier, CI 7232 was crossed with the susceptible hexaploid ‘Clarion’, and dark kernel color (D) and resistance (R) from the tetraploid parent were linked in nearly all F₈ F₁₀ lines. Only one of 1,355 F₇ plants was found to have light kernel color (L) and resistance. We previously reported that all D-R and L-R plants were unstable for rust resistance, and that this instability was caused by the failure of a substituted alien chromosome (1'_A) to transmitted through the pollen.

In this study, seeds from two L-R plants with 20'' 1'+1'_A were irradiated with thermal neutrons (Total dose: Flux x time of 8.8 x 10¹² / N^th / cm²). Irradiated seeds were space planted in a field nursery, inoculated with crown rust in early tillering stages, and classified prior to heading. Resistant plants were then crossed as pollen parents to Clarion as a screen for functional male gametes carrying resistance. Two of the resulting I-F_l's were resistant, and ratios in their respective I-F₂ and I-Ff₃ populations confirmed monofactorial inheritance. Thus, Pc-15 had been transferred from the alien chromosome to one in the hexaploid complement which had normal male and female transmission. Metaphase I and tetrad observations in two homozygous and two heterozygous I-F₃ plants revealed completely normal meiosis.

In the original, nonirradiated, monosomic alien substitution plants (20''+l'+l'_A), L-R or D-R alien chromosomes were loosely associated with their hexaploid mate in about 85% of the cells, but recombination between the two marker loci occurred only rarely. The L-R recombinant chromosomes evidently still carry a large, non. homologous segment of the alien chromosome (including Pc.15) which completely blocks its pollen transmission.

Key Words: Chromosome substitution • Polyploids • Avena strigosa L. • Avena sativa L. • Derived-tetraploid • Meiosis • Meiotic index • Irradiation • Thermal neutrons

² Former graduate research assistant and professor, respectively.

Received for publication January 22, 1977.

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