Elimination of synaptonemal complex irregularities in a Lolium hybrid

The diploid hybrid ryegrass, Lolium temulentum × L. perenne (2n = 14), contains two sets of chromosomes which are structurally and genetically dissimilar. Whole-mount surface spreading of synaptonemal complexes from 35 pollen mother cell nuclei shows that chromosome pairing during meiotic prophase is highly irregular, with homoeologues and heterologues pairing together to form multivalents. These are eliminated, however, during meiotic prophase in favour of homoeologous bivalents. In addition, the length differential between homoeologous chromosomes is accommodated in a coordinated fashion and is absorbed into the structure of the synaptonemal complexes. The hybrid thus displays a remarkable capacity to eliminate synaptonemal complex irregularities and to produce homoeologous bivalents, which are functionally and morphologically almost indistinguishable from their homologous counterparts in the parents.