Lateral element lengths and nuclear disposition in Lolium

The 50 per cent difference in nuclear DNA amount between Lolium perenne and L. temulentum translates into a 50 per cent difference in mean total lengths per cell of lateral elements at pachytene, indicating that the same proportion of the genome is incorporated into synaptonemal complexes in these two species. Quantitative comparisons were not able to show that the accommodation of the length differential between homoeologous lateral elements in the diploid F1 hybrid is accomplished either by an equal stretching of the shorter chromosomes or a contraction of the longer. Ph-like genes in one genotype of L. perenne appear to induce irregular associations of A and B chromosomes, whereas in their absence synapsis of A chromosomes is normal and the B chromosome usually segregates to the periphery of the nucleus during meiotic prophase. The differences in chromosome behaviour at this stage have remarkably little effect on bivalent and chiasma frequencies at metaphase I, and on the fertility of the organism.