A target enrichment method for gathering phylogenetic information from hundreds of loci: An example from the Compositae.

UnlabelledPremise of the studyThe Compositae (Asteraceae) are a large and diverse family of plants, and the most comprehensive phylogeny to date is a meta-tree based on 10 chloroplast loci that has several major unresolved nodes. We describe the development of an approach that enables the rapid sequencing of large numbers of orthologous nuclear loci to facilitate efficient phylogenomic analyses. •Methods and resultsWe designed a set of sequence capture probes that target conserved orthologous sequences in the Compositae. We also developed a bioinformatic and phylogenetic workflow for processing and analyzing the resulting data. Application of our approach to 15 species from across the Compositae resulted in the production of phylogenetically informative sequence data from 763 loci and the successful reconstruction of known phylogenetic relationships across the family. •ConclusionsThese methods should be of great use to members of the broader Compositae community, and the general approach should also be of use to researchers studying other families

GENETIC MAPPING OF GENE EXPRESSION LEVELS: EXPRESSION LEVEL POLYMORPHISM ANALYSIS FOR DISSECTING REGULATORY NETWORKS OF PLANT DISEASE RESISTANCE

The genetic basis of inherited traits has been studied through di erent approaches in many areas of science. Examples include quantitative trait locus (QTL) analysis and mutant analysis in genetics, genome sequencing and gene expression analysis in genomics. Each of these approaches is used for the investigation of complex traits, such as disease resistance, but also provides knowledge on components of complex biological systems. We introduce a novel functional genomics approach that integrates two areas, genetics and genomics, by applying QTL analysis to quantitative di erences in the mRNA abundance of trait-related genes. This approach allows comprehensive dissection of regulatory networks for complex traits at a systems biology level. We also address statistical issues, and suggest guidelines for future experiments in this new framework

Wheat rusts never sleep but neither do sequencers: will pathogenomics transform the way plant diseases are managed?

Field pathogenomics adds highly informative data to surveillance surveys by enabling rapid evaluation of pathogen variability, population structure and host genotype

Isolation of Putative Disease Resistance Gene Clones from Chickpea and Pigeonpea

Nucleotide binding sites conferring disease resistance were cloned from chickpea (Cicer arietinum) and pigeon pea (Cajanus cajan). The majority of amplified resistance gene candidates showed high homology to N (tobacco) and L6 (flax) resistance genes

Plant disease resistance genes encode members of an ancient and diverse protein family within the nucleotide-binding superfamily

The nucleotide binding site (NBS) is a characteristic domain of many plant resistance gene products. An increasing number of NBS-encoding sequences are being identified through gene cloning, PCR amplification with degenerate primers, and genome sequencing projects. The NBS domain was analyzed from 14 known plant resistance genes and more than 400 homologs, representing 26 genera of monocotyledonous, dicotyle-donous and one coniferous species. Two distinct groups of diverse sequences were identified, indicating divergence during evolution and an ancient origin for these sequences. One group was comprised of sequences encoding an N-terminal domain with Toll/Interleukin-1 receptor homology (TIR), including the known resistance genes, N, M, L6, RPP1 and RPP5. Surprisingly, this group was entirely absent from monocot species in searches of both random genomic sequences and large collections of ESTs. A second group contained monocot and dicot sequences, including the known resistance genes, RPS2, RPM1, I2, Mi, Dm3, Pi-B, Xa1, RPP8, RPS5 and Prf. Amino acid signatures in the conserved motifs comprising the NBS domain clearly distinguished these two groups. The Arabidopsis genome is estimated to contain approximately 200 genes that encode related NBS motifs; TIR sequences were more abundant and outnumber non-TIR sequences threefold. The Arabidopsis NBS sequences currently in the databases are located in approximately 21 genomic clusters and 14 isolated loci. NBS-encoding sequences may be more prevalent in rice. The wide distribution of these sequences in the plant kingdom and their prevalence in the Arabidopsis and rice genomes indicate that they are ancient, diverse and common in plants. Sequence inferences suggest that these genes encode a novel class of nucleotide-binding protein

Isolation and fine mapping of Rps6: An intermediate host resistance gene in barley to wheat stripe rust

A plant may be considered a nonhost of a pathogen if all known genotypes of a plant species are resistant to all known isolates of a pathogen species. However, if a small number of genotypes are susceptible to some known isolates of a pathogen species this plant maybe considered an intermediate host. Barley (Hordeum vulgare) is an intermediate host for Puccinia striiformis f. sp. tritici (Pst), the causal agent of wheat stripe rust. We wanted to understand the genetic architecture underlying resistance to Pst and to determine whether any overlap exists with resistance to the host pathogen, Puccinia striiformis f. sp. hordei (Psh). We mapped Pst resistance to chromosome 7H and show that host and intermediate host resistance is genetically uncoupled. Therefore, we designate this resistance locus Rps6. We used phenotypic and genotypic selection on F2:3 families to isolate Rps6 and fine mapped the locus to a 0.1 cM region. Anchoring of the Rps6 locus to the barley physical map placed the region on two adjacent fingerprinted contigs. Efforts are now underway to sequence the minimal tiling path and to delimit the physical region harbouring Rps6. This will facilitate additional marker development and permit identification of candidate genes in the region

Crop rotation and genetic resistance reduce risk of damage from Fusarium wilt in lettuce

Fusarium wilt of lettuce, caused by the soilborne fungus Fusarium oxysporum f. sp. lactucae, affects all major lettuce production areas in California and Arizona. In trials at UC Davis, we found that lettuce cultivars differ significantly in susceptibility to the disease, with some leaf and romaine types highly resistant under all test conditions. For more susceptible cultivars, disease severity is strongly influenced by inoculum levels and ambient temperature. Management of Fusarium wilt requires an integrated approach that includes crop rotation to reduce soil inoculum levels and the use of resistant cultivars during the warmest planting windows

Multiple alleles for resistance and susceptibility modulate the defense response in the interaction of tetraploid potato (Solanum tuberosum) with Synchytrium endobioticum pathotypes 1, 2, 6 and 18

The obligate biotrophic, soil-borne fungus Synchytrium endobioticum causes wart disease of potato (Solanum tuberosum), which is a serious problem for crop production in countries with moderate climates. S. endobioticum induces hypertrophic cell divisions in plant host tissues leading to the formation of tumor-like structures. Potato wart is a quarantine disease and chemical control is not possible. From 38 S. endobioticum pathotypes occurring in Europe, pathotypes 1, 2, 6 and 18 are the most relevant. Genetic resistance to wart is available but only few current potato varieties are resistant to all four pathotypes. The phenotypic evaluation of wart resistance is laborious, time-consuming and sometimes ambiguous, which makes breeding for resistance difficult. Molecular markers diagnostic for genes for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 would greatly facilitate the selection of new, resistant cultivars. Two tetraploid half-sib families (266 individuals) segregating for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 were produced by crossing a resistant genotype with two different susceptible ones. The families were scored for five different wart resistance phenotypes. The distribution of mean resistance scores was quantitative in both families. Resistance to pathotypes 2, 6 and 18 was correlated and independent from resistance to pathotype 1. DNA pools were constructed from the most resistant and most susceptible individuals and screened with genome wide simple sequence repeat (SSR), inverted simple sequence region (ISSR) and randomly amplified polymorphic DNA (RAPD) markers. Bulked segregant analysis identified three SSR markers that were linked to wart resistance loci (Sen). Sen1-XI on chromosome XI conferred partial resistance to pathotype 1, Sen18-IX on chromosome IX to pathotype 18 and Sen2/6/18-I on chromosome I to pathotypes 2,6 and 18. Additional genotyping with 191 single nucleotide polymorphism (SNP) markers confirmed the localization of the Sen loci. Thirty-three SNP markers linked to the Sen loci permitted the dissection of Sen alleles that increased or decreased resistance to wart. The alleles were inherited from both the resistant and susceptible parents

Identifying Molecular Markers Suitable For Frl Selection in Tomato Breeding

Modern plant breeding heavily relies on the use of molecular markers. In recent years, next generation sequencing (NGS) emerged as a powerful technology to discover DNA sequence polymorphisms and generate molecular markers very rapidly and cost effectively, accelerating the plant breeding programmes. A single dominant locus, Frl, in tomato provides resistance to the fungal pathogen Fusarium oxysporum f. sp. radicis-lycopersici (FORL), causative agent of Fusarium crown and root rot. In this study, we describe the generation of molecular markers associated with the Frl locus. An F2 mapping population between an FORL resistant and a susceptible cultivar was generated. NGS technology was then used to sequence the genomes of a susceptible and a resistant parent as well the genomes of bulked resistant and susceptible F2 lines. We zoomed into the Frl locus and mapped the locus to a 900 kb interval on chromosome 9. Polymorphic single-nucleotide polymorphisms (SNPs) within the interval were identified and markers co-segregating with the resistant phenotype were generated. Some of these markers were tested successfully with commercial tomato varieties indicating that they can be used for marker-assisted selection in large-scale breeding programmes