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

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Abstract
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.