Abstract
Relatively modest levels of genetic gain have been achieved in conventional ryegrass breeding when compared to cereal crops such as maize, current estimates indicating an annual improvement of 0.25–0.6% in dry matter production. This property is partially due to an inability to effectively exploit heterosis through the formation of F1 hybrids. Controlled crossing of ryegrass lines from geographically distant origins has demonstrated the occurrence of heterosis, which can result in increases of dry matter production in the order of 25%. Although capture of hybrid vigor offers obvious advantages for ryegrass cultivar production, to date there have been no effective and commercially suitable methods for obtaining high proportions of F1 hybrid seed. Continued advances in fine-scale genetic and physical mapping of the gametophytic self-incompatibility (SI) loci (S and Z) of ryegrasses are likely in the near future to permit the identification of closely linked genetic markers that define locus-specific haplotypes, allowing prediction of allelic variants and hence compatibility between different plant genotypes. Given the availability of such information, a strategy for efficient generation of ryegrass cultivars with a high proportion of F1 hybrid individuals has been simulated, which is suitable for commercial implementation. Through development of two parental pools with restricted diversity at the SI loci, relative crossing compatibility between pools is increased. Based on simulation of various levels of SI allele diversity restriction, the most effective scheme will generate 83.33% F1 hybrids. Results from the study, including the impact of varying flowering time, are discussed along with a proposed breeding design for commercial application.
Highlights
Limited gains have been made in ryegrass breeding over the past 80 years, estimates varying from 0.25 to 0.6% and 1.18% annual genetic improvement in dry matter production for perennial and Italian ryegrass, respectively (Woodfield, 1999)
Proportions of F1 Hybrid Production The two designs based on selective restriction of SI allele diversity both displayed higher potential for F1 hybrid production than those which represented population hybrid-based schema
Breeding scheme Se1 obtained the highest level of simulated hybrid seed production (83.33%), compared to 76.36% for scheme Se2, while none of the non-selective schemes (P1, P2, P3, and P4) exceeded 57.14% (Table 2)
Summary
Limited gains have been made in ryegrass breeding over the past 80 years, estimates varying from 0.25 to 0.6% and 1.18% annual genetic improvement in dry matter production for perennial and Italian ryegrass, respectively (Woodfield, 1999). One of the largest constraints on genetic gain in ryegrass breeding has been an inability to effectively exploit heterosis through generation of F1 hybrids. Contemporary ryegrass cultivars are commonly bred from a limited number of elite parents (4–12) that are polycrossed to form the first synthetic generation (Syn). The progeny undergo further polycrossing in order to obtain sufficient seed for commercial sale. Capture of heterosis would offer clear advantages for ryegrass breeding, no effective methods for obtaining high proportions of F1 seed on a commercial scale have to date been made available
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