A new genetic locus for self-compatibility in the outcrossing grass species perennial ryegrass (Lolium perenne)

Abstract

Background: Self-incompatibility (SI) is a physiological mechanism that many flowering plants employ to prevent self-fertilization and maintain heterozygosity. In the grass family this is known to be controlled by a two locus (S-Z) system; however, the SI system is intrinsically leaky. Modifier genes of both the S and Z loci and a further locus, T, are known to override SI leading to self-fertilization and self-seed production. This has implications for the ecological and evolutionary success as well as the commercial breeding of grasses. Here we report a study where the genetic control of self-compatibility (SC) was determined from the results of self-pollinating an F2 population of perennial ryegrass from two independently derived inbred lines produced by single-seed descent.

Methods: In vitro self-pollinations of 73 fertile plants were analysed. A genetic association analysis was made with a panel of 1863 single-nucleotide polymorphism (SNP) markers, generated through genotype-by-sequencing methodology. Markers were placed on a recombination map of seven linkage groups (LGs) created using Joinmap v.5. The seed set on self- and open-pollinated inflorescences was determined on 143 plants, including the 73 plants analysed for self-pollination response.

Key results: Self-pollinations revealed a bimodal distribution of percentage SC with peaks at 50 and 100 %. A single quantitative trait locus (QTL) was identified with peak association for marker 6S14665z17875_11873 that mapped to LG 6. Peak position was associated with maximum marker segregation distortion. The self-compatible plants were equally fecund after self- and open pollination.

Conclusions: This is the first report in the Poaceae family of an SC locus located on LG 6. This new SC QTL discovery, as well as indicating the complex nature of the pollen-stigma recognition process and its evolutionary significance, provides an additional source of SC for breeding perennial ryegrass.

Keywords: Genetic linkage map; Poaceae; genotyping by sequencing; heterosis; inbreeding; quantitative trait locus; reproductive assurance; self-incompatibility; single-nucleotide polymorphism.

Fig. 1.

In vitro pollination of (A) genotype 212 showing a 100 % self-compatible pollen–stigma reaction and (B) genotype 362 showing a 50 % self-compatible pollen–stigma reaction.

Fig. 2.

Frequency distribution of estimated percentage SC in the F₂ population.

Fig. 3.

Genome-wide SC QTL analysis. Marker LOD scores are represented as filled colour circles based on analysis of the complete population, and χ² test probabilities of marker segregations for agreement with a 1:2:1 ratio are represented as black crosses across seven LGs. The genome-wide permutation test 95 % significance LOD threshold is 3.7.

Fig. 4.

Genome-wide SC MQM analysis made after selecting marker 6S14665z17875_11873 as a cofactor. The genome wide permutation test 95 % significance LOD threshold is 3.8.

Fig. 5.

Correlation between mean numbers of seeds set per spikelet for plants self-pollinated and open-pollinated (R = 0.61).