Cytology of F1 hybrids and chromosome number of F2 and BC1 progeny of the cross Bromus riparius x B. inermis

Summary Hybrids between B. inermis Leyss (2n=8x=56) and B. riparius Rehm. (2n=10x=70) were easily made. The F₁ hybrids had a fertility of 20%–50% under open pollination and backcrossing to B. inermis. Chromosome pairing in B. riparius was predominantly as bivalents (29.04–33.85 per cell for plant means). Bivalents also predominated in the F₁ hybrid (2n=9x=63) and there was a high level of pairing with no reduction in chiasma frequency. It was impossible to estimate the frequency of auto-versus allosyndetic pairing. Chromosome pairing in a hybrid between B. arvensis (2n=2x=14) and B. riparius confirmed that the B. riparius complement is capable of complete autosyndetic pairing. Chromosome numbers in the F₂ progeny ranged from 2n=56 to 72 but they were skewed towards 2n=63 to 70. Backcrosses ranged from 2n=56 to 63, as expected, with the distribution skewed towards 2n=56. Selection towards the 2n=56 level would be difficult in the F₂. Empirical observation suggested that cytoplasm had a major influence on morphology in the backcrosses. Additional studies are required to determine the best breeding scheme to introgress germ plasm between B. inermis and B. riparius.