Chromosome association at first meiotic metaphase in tetraploid hybrids between Lolium perenne and L. multiflorum was compared with that in autotetraploid L. perenne. The hybrids were found to have significantly higher levels of bivalent frequency, and lower levels of multivalent and chiasma frequency. A significant increase in multivalent frequency with increasing chiasma formation was found in both groups, but the increase was much less in the hybrids. These differences in chromosome associations between the two groups must therefore reflect differences in chiasma distribution and it is suggested that the results indicate a significant degree of preferential bivalent pairing in the hybrids.

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INDEX WORDSFestuca arundinacea, tall fescue, seed weight, coleoptile tiller, seedling vigour, selection criterion. SUMMARYTwo characters, previously shown to influence the rate of seedling growth in tall fescue, were studied in detail in three distinct populations as part of a programme to improve seedling vigour in the species. The effect of seed weight was found to be confined to the very early stages of growth and was apparent in seedling dry matter yield but not in tiller number. The development of a tiller in the axil of the coleoptile, which varied significantly between populations, had a far greater and more permanent effect both in terms of dry matter yield and tiller number. The increase in tiller number attributable to the coleoptile tiller and its derivatives increased geometrically in step with the total tiller number, so that the intitial difference of 28~ in the first harvest remained practically unchanged in the subsequent three harvests. Similarly, the percentage difference in dry matter yield between plants with and without a coleoptile tiller remained more or less constant at about 22~ from the second harvest onwards.It is concluded that the development of a coleoptile tiller may prove to be a useful selection criterion in the breeding of this species.

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Polyploidy has featured strongly in plant evolution as a means of conserving favoured hybrid combinations during sexual reproduction. Combining different genomes is likely to extend adaptation rather than increase yield per se . Subsequent stabilization by polyploidy depends on the degree of differentiation of the genomes and on the breeding system, but success is mediated by low chromosome numbers while reduced fertilities may be offset where the crop is harvested primarily for vegetative parts and/or is perennial. In old established grain crops like the cereals, raw synthetic polyploids are not likely to offer immediate advantages but may be subsequently improved by arduous selection in an extended gene pool (e.g. triticale). The relatively undeveloped herbage grasses offer unique opportunities. Flexibility in grass swards is presently sought through unstable mixtures of races and species. It is the breeders’ aim to combine genetically the complementary features of these in stable varieties. Agronomically useful hybrids between diploid Lolium species have been stabilized at the tetraploid level through tetrasomic inheritance reinforced by a degree of preferential pairing. This preferential pairing may be genetically enhanced, thus raising the possibility of producing a new agriculturally useful amphiploid ryegrass species. Prospects for developing useful amphiploid hybrids between less closely related Lolium / Festuca species is considered and related to more limited objectives of transferring desirable genes or gene complexes.