Interspecific gene transfer in lolium
Nearly two thousand diploid hybrid plants originating from crosses between triploids (with two sets of Italian ryegrass chromosomes and one set of perennial ryegrass chromosomes) and diploid Italian ryegrass were clonally replicated and examined for their expression of five qualitative and five quantitative traits in a glasshouse. The qualitative trait were: presence of awns, red leaf base and three isozyme variants; phosphogluco isomerase (PGI), glutamate oxaloacetate transminase (GOT) and superoxide dismutase (SOD). The quantitative traits were; leaf width, leaf length, per cent stem, regrowth score and days to ear emergence. The purpose of using triploids during backcrossing was to reduce the proportion of genes originating from the donor parent. Assuming normal mendelian inheritance, the progeny would have 1/6 or 17% of the genes of perennial ryegrass origin compared to 1/4 or 25% in two generations of ordinary backcrossing: only a small advantage in using triploids. But loss of chromosomes of the donor species due to preferential pairing at meiosis in the triploid would greatly increase that advantage. The occurrence of such chromosome loss was best revealed by the frequency of hybrid plants with qualitative traits. The frequency of plants with the perennial ryegrass allele at the PGI/2 isozyme locus and with red base was much lower than expected but the frequency of plants with the perennial ryegrass allele at the GOT locus was exactly as expected. Since these three loci were unlinked, the results suggest considerable loss of some chromosomes but not others. There was a clear linkage between leaf length and red base, PGI and GOT suggesting that genes for leaf length were distributed widely over the genome. Genetic correlation between quantitative traits suggested linkage between early flowering and stemmy regrowth, both traits of Italian ryegrass. Potentially useful hybrid clones which showed only a single perennial ryegrass trait were identified. Some had no awns, some with red base, some had entirely leafy regrowth and some were late flowering. However, because some of the quantitative traits had high coefficients of variation and because the traits examined only represent a small proportion of the ryegrass genome, further work is required to determine the value of these clones for breeding.