Physical distribution of homoeologous recombination in individual chromosomes of Festuca pratensis in Lolium multiflorum.

Crossing over-based recombination is a powerful tool for generating new allelic combinations during sexual reproduction. It usually occurs between homologous chromosomes. However, under some conditions, homoeologues may also be capable of crossing over. Whether homologous and homoeologous crossovers are equivalent and governed by the same rules has never been established. Here we report on chromosome distribution of homoeologous crossovers in a unique system of Festuca x Lolium hybrids. Unlike in most other hybrids, in these intergeneric hybrids, homoeologous chromosomes are capable of pairing and crossing over with frequencies approaching that of homologues. At the same time, genome divergence makes cytological detection of chromosome recombination feasible. We analyzed the distribution of homoeologous recombination along individual chromosomes in a complete set of intergeneric single chromosome substitutions from F. pratensis into tetraploid L. multiflorum. Homoeologous recombination sites were not evenly distributed along the chromosomes, being concentrated in intercalary regions of the arms and reduced in proximal and distal regions. Several recombination hotspots and cold spots were found along individual chromosomes and the recombination was not affected by the presence of a secondary constriction. Our results indicate that despite the uneven distribution of homoeologous recombination, introgression of any part of the F. pratensis genome into L. multiflorum is feasible.