Footprints of Selection Derived From Temporal Heterozygosity Patterns in a Barley Nested Association Mapping Population

Nowadays, genetic diversity more than ever represents a key driver of adaptation to climate challenges like drought, heat, and salinity. Therefore, there is a need to replenish the limited elite gene pools with favorable exotic alleles from the wild progenitors of our crops. Nested association mapping (NAM) populations represent one step toward exotic allele evaluation and enrichment of the elite gene pool. We investigated an adaptive selection strategy in the wild barley NAM population HEB-25 based on temporal genomic data by studying the fate of 214,979 SNP loci initially heterozygous in individual BC1S3 lines after five cycles of selfing and field propagation. We identified several loci exposed to adaptive selection in HEB-25. In total, 48.7% (104,725 SNPs) of initially heterozygous SNP calls in HEB-25 were fixed in BC1S3:8 generation, either toward the wild allele (19.9%) or the cultivated allele (28.8%). Most fixed SNP loci turned out to represent gene loci involved in domestication and flowering time as well as plant height, for example, btr1/btr2, thresh-1, Ppd-H1, and sdw1. Interestingly, also unknown loci were found where the exotic allele was fixed, hinting at potentially useful exotic alleles for plant breeding.

Genic introgression from an invasive exotic fungal forest pathogen increases the establishment potential of a sibling native pathogen

Significant hybridization between the invasive North American fungal plant pathogen Heterobasidion irregulare and its Eurasian sister species H. annosum is ongoing in Italy. Whole genomes of nine natural hybrids were sequenced, assembled and compared with those of three genotypes each of the two parental species. Genetic relationships among hybrids and their level of admixture were determined. A multi-approach pipeline was used to assign introgressed genomic blocks to each of the two species. Alleles that introgressed from H. irregulare to H. annosum were associated with pathways putatively related to saprobic processes, while alleles that introgressed from the native to the invasive species were mainly linked to gene regulation. There was no overlap of allele categories introgressed in the two directions. Phenotypic experiments documented a fitness increase in H. annosum genotypes characterized by introgression of alleles from the invasive species, supporting the hypothesis that hybridization results in putatively adaptive introgression. Conversely, introgression from the native into the exotic species appeared to be driven by selection on genes favoring genome stability. Since the introgression of specific alleles from the exotic H. irregulare into the native H. annosum increased the invasiveness of the latter species, we propose that two invasions may be co-occurring: the first one by genotypes of the exotic species, and the second one by alleles belonging to the exotic species. Given that H. irregulare represents a threat to European forests, monitoring programs need to track not only exotic genotypes in native forest stands, but also exotic alleles introgressed in native genotypes.

Organic farming - Deep genotyping reveals specific selection footprints in barley populations

Organic farming has the potential to tackle the imminent task of sustainable food production, if the yields could be raised. Here, the benefits of additional exotic alleles, the necessity of increased genetic heterogeneity in organically farmed systems and the buffering capacities by the pronounced plasticity of root traits are demonstrated. Two barley populations, naturally adapted for more than two decades to organic and conventional farming systems, were compared by a novel strategy of whole genome resequencing of pooled samples. Substantial allele frequency deviations between the farming systems were uncovered (for various agronomically relevant chromosomal regions) by testing multiple generations. In contrast to the organic adapted population, an early equilibrium in the conventional population was observed, accompanied reduced genetic diversity. Differences between the populations were revealed in root morphology, developmental processes and abiotic stress responses. These findings indicate that wild genetic resources play a critical role in the development of organically adapted varieties and distinct variations in ecosystems demand different genetic compositions.

You Had Me at “MAGIC”!: Four Barley MAGIC Populations Reveal Novel Resistance QTL for Powdery Mildew

Blumeria graminis f. sp. hordei (Bgh), the causal agent of barley powdery mildew (PM), is one of the most important barley leaf diseases and is prevalent in most barley growing regions. Infection decreases grain quality and yields on average by 30%. Multi-parent advanced generation inter-cross (MAGIC) populations combine the advantages of bi-parental and association panels and offer the opportunity to incorporate exotic alleles into adapted material. Here, four barley MAGIC populations consisting of six to eight founders were tested for PM resistance in field trials in Denmark. Principle component and STRUCTURE analysis showed the populations were unstructured and genome-wide linkage disequilibrium (LD) decay varied between 14 and 38 Mbp. Genome-wide association studies (GWAS) identified 11 regions associated with PM resistance located on chromosomes 1H, 2H, 3H, 4H, 5H and 7H, of which three regions are putatively novel resistance quantitative trait locus/loci (QTL). For all regions high-confidence candidate genes were identified that are predicted to be involved in pathogen defense. Haplotype analysis of the significant SNPs revealed new allele combinations not present in the founders and associated with high resistance levels.

Study of the genetic structure of a Brassica napus canola population derived from six interspecific crosses of B. napus × B. oleracea

Broadening the genetic base of the C genome of Brassica napus canola is needed for continued improvement of this crop. For this, we developed few hundred canola lines from B. napus × B. oleracea interspecific crosses involving a B. napus canola line and six B. oleracea accessions belonging to four varieties, viz. vars. alboglabra, botrytis, capitata and italica, and following two breeding methods (F2- and BC1 (F1 × B. napus)-derived lines). The objective of this study was to understand the genetic structure of this population regarding the alleles introgressed from B. oleracea by using SSR markers, and to investigate the inheritance of B. oleracea alleles in these re-constituted canola lines. Marker analysis showed that the four B. oleracea varieties were genetically quite distinct. Several canola lines derived from these six crosses tended to group together with their B. oleracea parent demonstrating that the wide diversity of the B. oleracea gene pool can be exploited for broadening the genetic base of the C genome of B. napus canola. Loss of several B. oleracea alleles occurred during the development of these inbred lines. While comparing the two breeding methods for introgression of B. oleracea alleles, significantly greater loss of alleles occurred in the F2-derived population as compared to the BC1-derived population. Thus, the knowledge from this study can be used for efficient introgression of exotic alleles from B. oleracea into B. napus for broadening the genetic base of this crop.

Origin Specific Genomic Selection: A Simple Process To Optimize the Favorable Contribution of Parents to Progeny

Modern crop breeding is in constant demand for new genetic diversity as part of the arms race with genetic gain. The elite gene pool has limited genetic variation and breeders are trying to introduce novelty from unadapted germplasm, landraces and wild relatives. For polygenic traits, currently available approaches to introgression are not ideal, as there is a demonstrable bias against exotic alleles during selection. Here, we propose a partitioned form of genomic selection, called Origin Specific Genomic Selection (OSGS), where we identify and target selection on favorable exotic alleles. Briefly, within a population derived from a bi-parental cross, we isolate alleles originating from the elite and exotic parents, which then allows us to separate out the predicted marker effects based on the allele origins. We validated the usefulness of OSGS using two nested association mapping (NAM) datasets: barley NAM (elite-exotic) and maize NAM (elite-elite), as well as by computer simulation. Our results suggest that OSGS works well in its goal to increase the contribution of favorable exotic alleles in bi-parental crosses, and it is possible to extend the approach to broader multi-parental populations.

Origin Specific Genomic Selection: a simple process to optimize the favourable contribution of parents to progeny

Modern crop breeding is in constant demand for new genetic diversity as part of the arms race with genetic gain. The elite gene pool has limited genetic variation and breeders are trying to introduce novelty from unadapted germplasm, landraces and wild relatives. For polygenic traits, currently available approaches to introgression are not ideal, as there is a demonstrable bias against exotic alleles during selection. Here, we propose a partitioned form of genomic selection, called Origin Specific Genomic Selection (OSGS), where we identify and target selection on favourable exotic alleles. Briefly, within a population derived from a bi-parental cross, we isolate alleles originating from the elite and exotic parents, which then allows us to separate out the predicted marker effects based on the allele origins. We validated the usefulness of OSGS using two nested association mapping (NAM) datasets: barley NAM (elite-exotic) and maize NAM (elite-elite), as well as by computer simulation. Our results suggest that OSGS works well in bi-parental crosses, and it is possible to extend the approach to broader multi-parental populations.

Genetic diversity and population structure in a rice drought stress panel

A drought stress panel composed of diverse accessions selected from upland, aerobic, rainfed lowland and irrigated lowland environments, was assembled to serve as germplasm for aerobic adaptation breeding. Aerobic rice requires significant levels of tolerance to drought stress due to intermittent water deficit and high soil impedance caused by aerobic conditions. Genomic information may be utilized to investigate the nature of the panel to guide varietal improvement. Using 153 simple sequence repeat and 384 single nucleotide polymorphism markers, the aim of the study was to compare the allelic properties of the two marker types, infer population structure of the panel, and estimate kinship among the accessions. There was a general agreement between the results derived from the two marker types. Marker alleles were found to occur at low frequencies, as the panel was composed mostly of improved accessions with some landraces. The panel clustered into japonica (JA), aus (AU), upland-adapted indica (UL) and lowland-adapted indica (LL) subpopulations. The AU and JA subpopulations were more divergent from the rest of the subpopulations than were the LL and UL subpopulations. Average marker-based kinship for related accessions was less than 0.20, indicating a low degree of genetic relatedness in the panel. Within the LL and UL subpopulations, the low levels of kinship imply that there is still much genetic gain to be expected from utilizing the accessions in breeding. Thus, an understanding of the genetic variation in the panel suggests focusing on improving the mean in the short term, and tapping into the exotic alleles from the AU and JA subpopulations when genetic gain declines.