Genetic studies on self-fertility in perennial ryegrass (Lolium perenne L.) with implications for hybrid breeding in allogamous grasses

Abstract

Perennial grasses have diverse uses and are relevant from the agronomic and economic point of view, with main uses as forage, turf and bioenergy. In the grass family polyploidy is prevalent and both autopolyploids and allopolyploids are present. Also, within grasses there are a range of breeding systems, but hermaphrodite flower is the most frequent floral condition. Cross-pollination in species with hermaphrodite flowers is imposed by a gametophytic genetic self-incompatibility (SI). SI is controlled by two multiallelic and independent loci, S and Z. The incompatibility phenotype of the pollen grain is determined by its haploid genome. A pollen grain is incompatible when the same S and Z alleles carried by pollen are present in the pistil. This SI mechanism keeps its functionality at higher ploidy levels. Understanding the mechanisms involved in the breakdown of SI are crucial for implementing novel breeding practices. The aim of this work is to increase the knowledge of SF in outcrossing grasses for the purpose of inbred line development and making hybrid breeding possible. Mutations at S, Z and at a third locus are known to cause self-fertility (SF). In perennial ryegrass (Lolium perenne), a locus conferring SF is located in linkage group 5. Using segregation and linkage analysis, the SF locus region was reduced to 1.6 cM. This locus explained 94% of the observed variability. By aligning the flanking marker sequences to the Brachypodium distachium reference genome, it was found that it corresponds to an 807 Kbp region in B. distachium. This locus was studied at the tetraploid level and it was found that SF remained functional, the SF locus genotype was the main determinant of pollen compatibility explaining 54% of the variation, and there is incomplete dominance between alleles at this locus in the diploid pollen grain. The prospects of migrating the SF locus from perennial ryegrass to other related self-incompatible were discussed. Based on the available information on hybridization between Lolium and Festuca species, different types of crosses were propose according to the particular species involved. The results and observations presented here contribute to a better understanding of the trait at both diploid and tetraploid levels and are promising as SF may readily be incorporated into breeding programs.