Characterization and practical use of self-compatibility in outcrossing grass species

ISSN:1095-8290ISSN:0305-736

Genetic dissection of self-compatibility in perennial ryegrass (Lolium perenne L.) and its exploitation for breeding

In plants, reproductive strategies determine the degree of genetic variation of a species and influence the procedures to develop new cultivars. Self-compatibility (SC), defined as the ability for a plant to self-fertilize, is a reproductive trait evolved through the loss of ancestral self-incompatibility (SI) during the evolution of many plant species. Studying the molecular mechanisms underlying the transition from SI to SC and its consequences for plant adaptation and speciation has engaged the scientific community for decades. For some crops, SC is a tool to efficiently produce homozygous plant material by self-pollination, which breeders have used to generate parental lines for hybrid breeding schemes. This makes SC not only a trait of fundamental importance for plant biology but also of great agricultural interest to produce high yielding grass hybrids. This thesis has investigated genetic causes of SC in perennial ryegrass (Lolium perenne L.), one of the world’s most important forage crops. Perennial ryegrass is characterized by a SI system that prevents inbreeding and makes it a strictly allogamous species. Naturally occurring sources of SC in perennial ryegrass have been documented, however their regulation is just starting to be understood at the genetic level. Moreover, a general utilization of SC for breeding is lacking. The goal of this thesis was to advance the current understanding of the genetic architecture of SC in perennial ryegrass by identifying genomic regions and molecular determinants responsible for SC. The research described in chapters 2-5 and here summarized, provide both biological as well as practical insights for the application of SC in forage grass breeding. After a general introduction, chapter 2 reviews the state of the art about the SI system in grasses and describes in detail known causes of SC. Strategies for the discovery of novel SC sources are discussed, together with a review of the possibilities that SC presents for forage grass breeding. Specifically, the importance of using SC to purge deleterious alleles and to fix genetic variation in heterotic groups in order to implement hybrid breeding strategies is addressed. Moreover, the perspectives of using SC as a tool to develop immortalized mapping populations for the genetic dissection of agronomic traits are discussed. Chapter 3 describes a fine mapping experiment aimed at narrowing down a genomic region harboring a SC causal gene in a perennial ryegrass population. Previous classical genetic studies on this population mapped a SC locus on linkage group (LG) 5. However, the region identified was still considerably large, making it difficult to suggest candidate genes. With a combination of quantitative trait locus (QTL) mapping and analysis of segregation distortion based on a genome-wide linkage map, we confirmed the presence of the QTL on LG 5. In addition, we further fine mapped the locus to a 0.26 cM region, considerably improving the mapping resolution achieved in previous studies. Lastly, projecting the interval onto a genome assembly, we identified potential candidate genes. Chapter 4 reports the development and genetic characterization of a perennial ryegrass population segregating for SC. A segregating population was developed starting from germplasm provided by the seed company Deutsche Saatvederelung AG (DSV) and consisted of S1 and S2 lines originating from two founder self-compatible genotypes. To investigate structure and genetic diversity within the population, SNP markers were developed from a genotyping-by-sequencing (GBS) library. Combining 69,002 SNPs with SC phenotypic variation in a genome-wide association study, we identified SNPs with a significant association. Significant SNPs mapped on chromosome 2 and 7 and indicated the presence of three candidate QTL, revealing novel sources of SC and providing a fundamental resource for SC diagnostic markers in marker-assisted selection programs. Lastly, a pilot study described in chapter 5 investigates the occurrence of a temperature-induced SC in perennial ryegrass, a phenomenon called pseudo-self-compatibility (PSC). A set of 16 self-incompatible genotypes selected from the DSV perennial ryegrass germplasm was incubated at 30 °C during flowering and pollen tube growth was assessed using in vitro self-pollinations. The PSC response was found to be highly genotype-dependent, revealing the presence of variation for this trait. This study paves the way to further genetic mapping studies aimed at determining the genetic control of PSC. In conclusion, chapter 6 discusses the findings showed in the previous chapters in a broader context. Also, future perspectives are addressed, describing possible further experimental designs to complement the results presented in this thesis

Identification of Candidate Genes for Self-Compatibility in Perennial Ryegrass (Lolium perenne L.)

Self-incompatibility (SI) is a genetic mechanism preventing self-pollination in ~40% of plant species. Two multiallelic loci, called S and Z, control the gametophytic SI system of the grass family (Poaceae), which contains all major forage grasses. Loci independent from S and Z have been reported to disrupt SI and lead to self-compatibility (SC). A locus causing SC in perennial ryegrass (Lolium perenne L.) was previously mapped on linkage group (LG) 5 in an F2 population segregating for SC. Using a subset of the same population (n = 68), we first performed low-resolution quantitative trait locus (QTL) mapping to exclude the presence of additional, previously undetected contributors to SC. The previously reported QTL on LG 5 explained 38.4% of the phenotypic variation, and no significant contribution from other genomic regions was found. This was verified by the presence of significantly distorted markers in the region overlapping with the QTL. Second, we fine mapped the QTL to 0.26 centimorgan (cM) using additional 2,056 plants and 23 novel sequence-based markers. Using Italian ryegrass (Lolium multiflorum Lam.) genome assembly as a reference, the markers flanking SC were estimated to span a ~3 Mb region encoding for 57 predicted genes. Among these, seven genes were proposed as relevant candidate genes based on their annotation and function described in previous studies. Our study is a step forward to identify SC genes in forage grasses and provides diagnostic markers for marker-assisted introgression of SC into elite germplasm. © Copyright © 2021 Cropano, Manzanares, Yates, Copetti, Do Canto, Lübberstedt, Koch and Studer.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Characterization and practical use of self-compatibility in outcrossing grass species

• Background Self-incompatibility (SI) systems prevent self-fertilization in several species of Poaceae, many of which are economically important forage, bioenergy and turf grasses. Self-incompatibility ensures cross-pollination and genetic diversity but restricts the ability to fix useful genetic variation. In most inbred crops it is possible to develop high-performing homozygous parental lines by self-pollination, which then enables the creation of F1 hybrid varieties with higher performance, a phenomenon known as heterosis. The inability to fully exploit heterosis in outcrossing grasses is partially responsible for lower levels of improvement in breeding programmes compared with inbred crops. However, SI can be overcome in forage grasses to create self-compatible populations. This is generating interest in understanding the genetical basis of self-compatibility (SC), its significance for reproductive strategies and its exploitation for crop improvement, especially in the context of F1 hybrid breeding. • Scope We review the literature on SI and SC in outcrossing grass species. We review the currently available genomic tools and approaches used to discover and characterize novel SC sources. We discuss opportunities barely explored for outcrossing grasses that SC facilitates. Specifically, we discuss strategies for wide SC introgression in the context of the Lolium–Festuca complex and the use of SC to develop immortalized mapping populations for the dissection of a wide range of agronomically important traits. The germplasm available is a valuable practical resource and will aid understanding the basis of inbreeding depression and hybrid vigour in key temperate forage grass species. • Conclusions A better understanding of the genetic control of additional SC loci offers new insight into SI systems, their evolutionary origins and their reproductive significance. Heterozygous outcrossing grass species that can be readily selfed facilitate studies of heterosis. Moreover, SC introduction into a range of grass species will enable heterosis to be exploited in innovative ways in genetic improvement programmes.This article is published as Cropano, Claudio, Iain Place, Chloé Manzanares, Javier Do Canto, Thomas Lübberstedt, Bruno Studer, and Daniel Thorogood. "Characterisation and practical use of self-compatibility in outcrossing grass species." Annals of Botany 127 (2021): 841–852. doi:10.1093/aob/mcab043.</p

The rs7903146 Variant in the TCF7L2 Gene Increases the Risk of Prediabetes/Type 2 Diabetes in Obese Adolescents by Impairing \u3b2-Cell Function and Hepatic Insulin Sensitivity

In this study, we aimed to explore the mechanism by which TCF7L2 rs7903146 risk allele confers susceptibility to impaired glucose tolerance (IGT) or type 2 diabetes (T2D) in obese adolescents