Supernumerary chromosomes in Alopecurus pratensis L.

The interspecific hybridization between A. pratensis (4x) and A. pratensis ssp. alpestris (4x) gave fertile F 1 plants in high frequency. One of the two A. pratensis genomes (A p B p ) is suggested to be homologous with one of the two genomes (A a B a ) in A. p. ssp. alpestris. The other genomes in the two species are suggested to be at least partially homologous. Supernumerary chromosomes are found to reduce homoeologous chromosome pairing, giving 7.6% fewer meiotic irregularities in the F 1 's with 2n>28 compared with the F 1 's with 2n=28. In a field experiment, F 1 outyielded parent A. pratensis by 69% and had 20.9% less outwintering over two years. In a diallel cross among 8 F 1 plants, the 2n=28 F 1 's, compared with the 2n>28 F 1 's, had higher g.c.a. and lower s.c.a. for chromosome number and seed production, lower g.c.a. and higher s.c.a. for plant height and no significant differences for fodder yield. Two breeding procedures for stabilizing the heterotic effect are proposed and discussed. One deals with selection among F 2 and following generations, and the other is concerned with colchicine‐induced chromosome doubling of F 1 to get an 8x amphidiploid. Genome balance and recombination effects in the first procedure and use of supernumerary chromosomes and optimum ploidy level in the second procedure are discussed as main problems for developing a new stable synthetic variety.