Twenty‐two bivalents were observed in X117 oat (Avena sativa L.), a isogenic line resistant to crown rust (Puccinia coronata Cda. avenae Fraser and Led.), and with 42 + 2 fragment chromosomes. The homologous fragment chromosomes are either telocentric or acrocentric. Transmission through the egg, calculated from the backcross data, was 18.2%. A dominant gene, Pc‐15, conferring resistance to races 264 and 290 of crown rust of oats is located on each of the fragments.

F2 data of field‐grown plants of a cross between translocation 172‐11‐3 (purple flower, W1) and type collection line T93A (white flower, w1) of soybean [Glycine max (L.) Merr.] revealed a significant deviation from the expected ratio of 3 purple: 1 white. To determine the cause(s) of the abnormal ratio, the chromosome number and constitution of the F2 progenies of another cross involving 172‐11‐3 and T161 (white flower) were determined. Translocation 172‐11‐3 has cytologically recognizable interchange chromosomes, and the short interchange chromosome was present as a univalent in 46% of the pollen mother cells (PMCs) in interchange heterozygotes. Sixty‐five percent of the F2 progeny were disomics of the N/N, N/T, and T/T types, 18% were trisomics, 16% were 40‐chromosome plants with a duplication‐deficiency gamete, and 1% were 42‐chromosome plants. Assuming that the univalent was lost in the cytoplasm 50% of the time, the observed number of N/N and N/T plants fit the expected, but the number of T/T did not. A recombination frequency of 1.9% between w1 and the breakpoint was calculated. The F2 data of duplex trisomic hybrids indicated that the w1 locus was on the satellite chromosome. The breakpoint on the long arm of the satellite chromosome is distal to the w1 locus. Our data indicated no linkage between the breakpoint and the genes y10 (chlorophyll deficiency) and t (pubescence color).

Avena magna is a new tetraploid species morphologically similar to the hexaploid A. sterilis, having a high concentration of protein, large caryopses, and outstanding resistance to crown rust. One genome in A. magna appears homologous to the As genome present in hexaploid, tetraploid, and one group of diploid species. Avena magna is a possible ancestor of cultivated oats.

Diploid Avena strigosa Schreb. and A. wiestii Steud., tetraploid A. abyssinica Hochst., and hexaploid A. sativa L. and A. sterilis L. were cross‐compatible with A. magna Murphy and Tcrrell All hybrids except the pentaploids were sterile. Partial homology of the A genome of the diploid species with one genome of A, magna was indicated by the chromosome associations observed in the triploid hybrids A. magna ✕ A. wiestii and A. strigosa ✕ A. magna. In the tetraploid hybrid, A. abyssinica ✕ A. magna, chromosome associations suggest that the two species have one or both genomes partially common. In the pentaploid hybrids, the frequencies and kinds of chromosome associations observed suggest that A. magna may have two genomes partially in common with two genomes of the hexaploid species, a common genome and the remaining genomes partially common, or two genomes partially in common with the three genomes of the hexaploids.

Synopsis Crown rust resistance of the diploid variety, C.D. 3820, has been transferred to 43 and 42 chromosome hexaploid types by using a derived tetraploid, C.I. 7232. Resistance of a second diploid variety, C.I. 3815, and a tetraploid variety, C.I. 7233, has been transferred to the second and third backcross generations.