Sporadic Genetic Connectivity among Small Insular Populations of the Rare Geoendemic Plant Caulanthus amplexicaulis var. barbarae (Santa Barbara Jewelflower).

Globally, a small number of plants have adapted to terrestrial outcroppings of serpentine geology, which are characterized by soils with low levels of essential mineral nutrients (N, P, K, Ca, Mo) and toxic levels of heavy metals (Ni, Cr, Co). Paradoxically, many of these plants are restricted to this harsh environment. Caulanthus ampexlicaulis var. barbarae (Brassicaceae) is a rare annual plant that is strictly endemic to a small set of isolated serpentine outcrops in the coastal mountains of central California. The goals of the work presented here were to 1) determine the patterns of genetic connectivity among all known populations of C. ampexlicaulis var. barbarae, and 2) estimate contemporary effective population sizes (Ne), to inform ongoing genomic analyses of the evolutionary history of this taxon, and to provide a foundation upon which to model its future evolutionary potential and long-term viability in a changing environment. Eleven populations of this taxon were sampled, and population-genetic parameters were estimated using 11 nuclear microsatellite markers. Contemporary effective population sizes were estimated using multiple methods and found to be strikingly small (typically Ne < 10). Further, our data showed that a substantial component of genetic connectivity of this taxon is not at equilibrium, and instead showed sporadic gene flow. Several lines of evidence indicate that gene flow between isolated populations is maintained through long-distance seed dispersal (e.g., >1 km), possibly via zoochory.

Transcriptome Signatures of Selection, Drift, Introgression, and Gene Duplication in the Evolution of an Extremophile Endemic Plant.

Plants on serpentine soils provide extreme examples of adaptation to environment, and thus offer excellent models for the study of evolution at the molecular and genomic level. Serpentine outcrops are derived from ultramafic rock and have extremely low levels of essential plant nutrients (e.g., N, P, K, and Ca), as well as toxic levels of heavy metals (e.g., Ni, Cr, and Co) and low moisture availability. These outcrops provide habitat to a number of endemic plant species, including the annual mustard Caulanthus amplexicaulis var. barbarae (Cab) (Brassicaceae). Its sister taxon, C. amplexicaulis var. amplexicaulis (Caa), is intolerant to serpentine soils. Here, we assembled and annotated comprehensive reference transcriptomes of both Caa and Cab for use in protein coding sequence comparisons. A set of 29,443 reciprocal best Blast hit (RBH) orthologs between Caa and Cab was compared with identify coding sequence variants, revealing a high genome-wide dN/dS ratio between the two taxa (mean = 0.346). We show that elevated dN/dS likely results from the composite effects of genetic drift, positive selection, and the relaxation of negative selection. Further, analysis of paralogs within each taxon revealed the signature of a period of elevated gene duplication (∼10 Ma) that is shared with other species of the tribe Thelypodieae, and may have played a role in the striking morphological and ecological diversity of this tribe. In addition, distribution of the synonymous substitution rate, dS, is strongly bimodal, indicating a history of reticulate evolution that may have contributed to serpentine adaptation.

Association mapping of germinability and seedling vigor in sorghum under controlled low-temperature conditions.

Sorghum is one of the world's most important food, feed, and fiber crops as well as a potential feedstock for lignocellulosic bioenergy. Early-season planting extends sorghum's growing season and increases yield in temperate regions. However, sorghum's sensitivity to low soil temperatures adversely impacts seed germination. In this study, we evaluated the 242 accessions of the ICRISAT sorghum mini core collection for seed germination and seedling vigor at 12 °C as a measure of cold tolerance. Genome-wide association analysis was performed with approximately 162,177 single nucleotide polymorphism markers. Only one marker locus (Locus 7-2) was significantly associated with low-temperature germination and none with vigor. The linkage of Locus 7-2 to low-temperature germination was supported by four lines of evidence: strong association in three independent experiments, co-localization with previously mapped cold tolerance quantitative trait loci (QTL) in sorghum, a candidate gene that increases cold tolerance and germination rate when its wheat homolog is overexpressed in tobacco, and its syntenic region in rice co-localized with two cold tolerance QTL in rice. This locus may be useful in developing tools for molecular breeding of sorghums with improved low-temperature germinability.

Allelic variants in the PRR37 gene and the human-mediated dispersal and diversification of sorghum.

KEY MESSAGE: Allele phylogenetic analysis of the sorghum flowering-time gene PRR37 provided new insight into the human-mediated selection of a key adaptive gene that occurred during sorghum's diversification and worldwide dispersal. The domestication and spread of the tropical cereal sorghum is associated with the historic movement of humans. We show that an allelic series at PRR37 (pseudo-response regulator 37), a circadian clock-associated transcription factor, was selected in long-day ecosystems worldwide to permit floral initiation and grain production. We identified a series of loss-of-function (photoperiod-insensitive) alleles encoding truncated PRR37 proteins, alleles with key amino acid substitutions in the pseudo-receiver domain, and a novel splice variant in which the pseudo-receiver domain is truncated. Each PRR37 allelic variant was traced to a specific geographic location or specialized agronomic type. We present a graphical model that shows evidence of human selection and gene flow of the PRR37 allelic variants during the global dispersal and agronomic diversification of sorghum. With the recent identification of the Ghd7 gene as an important regulator of flowering date in sorghum, we briefly examine whether loss-of-function Ghd7 allelic variants were selected prior to the human-mediated movement of sorghum from its equatorial center of origin to temperate climates worldwide.

Exploring origins, invasion history and genetic diversity of Imperata cylindrica (L.) P. Beauv. (Cogongrass) in the United States using genotyping by sequencing.

Imperata cylindrica (Cogongrass, Speargrass) is a diploid C4 grass that is a noxious weed in 73 countries and constitutes a significant threat to global biodiversity and sustainable agriculture. We used a cost-effective genotyping-by-sequencing (GBS) approach to identify the reproductive system, genetic diversity and geographic origins of invasions in the south-eastern United States. In this work, we demonstrated the advantage of employing the closely related, fully sequenced crop species Sorghum bicolor (L.) Moench as a proxy reference genome to identify a set of 2320 informative single nucleotide and insertion-deletion polymorphisms. Genetic analyses identified four clonal lineages of cogongrass and one clonal lineage of Imperata brasiliensis Trin. in the United States. Each lineage was highly homogeneous, and we found no evidence of hybridization among the different lineages, despite geographical overlap. We found evidence that at least three of these lineages showed clonal reproduction prior to introduction to the United States. These results indicate that cogongrass has limited evolutionary potential to adapt to novel environments and further suggest that upon arrival to its invaded range, this species did not require local adaptation through hybridization/introgression or selection of favourable alleles from a broad genetic base. Thus, cogongrass presents a clear case of broad invasive success, across a diversity of environments, in a clonal organism with limited genetic diversity.

Mapping and candidate genes associated with saccharification yield in sorghum.

Sorghum (Sorghum bicolor (L.) Moench) is a high-yielding, stress tolerant energy crop for lignocellulosic-based biofuel production. Saccharification is a process by which hydrolytic enzymes break down lignocellulosic materials to fermentable sugars for biofuel production, and mapping and identifying genes underlying saccharification yield is an important first step to genetically improve the plant for higher biofuel productivity. In this study, we used the ICRISAT sorghum mini core germplasm collection and 14 739 single nucleotide polymorphism markers to map saccharification yield. Seven marker loci were associated with saccharification yield and five of these loci were syntenic with regions in the maize genome that contain quantitative trait loci underlying saccharification yield and cell wall component traits. Candidate genes from the seven loci were identified but must be validated, with the most promising candidates being ß-tubulin, which determines the orientation of cellulose microfibrils in plant secondary cell walls, and NST1, a master transcription factor controlling secondary cell wall biosynthesis in fibers. Other candidate genes underlying the different saccharification loci included genes that play a role in vascular development and suberin deposition in plants. The identified loci and candidate genes provide information into the factors controlling saccharification yield and may facilitate increasing biofuel production in sorghum.

Genetic structure and linkage disequilibrium in a diverse, representative collection of the C4 model plant, Sorghum bicolor.

To facilitate the mapping of genes in sorghum [Sorghum bicolor (L.) Moench] underlying economically important traits, we analyzed the genetic structure and linkage disequilibrium in a sorghum mini core collection of 242 landraces with 13,390 single-nucleotide polymorphims. The single-nucleotide polymorphisms were produced using a highly multiplexed genotyping-by-sequencing methodology. Genetic structure was established using principal component, Neighbor-Joining phylogenetic, and Bayesian cluster analyses. These analyses indicated that the mini-core collection was structured along both geographic origin and sorghum race classification. Examples of the former were accessions from Southern Africa, East Asia, and Yemen. Examples of the latter were caudatums with widespread geographical distribution, durras from India, and guineas from West Africa. Race bicolor, the most primitive and the least clearly defined sorghum race, clustered among other races and formed only one clear bicolor-centric cluster. Genome-wide linkage disequilibrium analyses showed linkage disequilibrium decayed, on average, within 10-30 kb, whereas the short arm of SBI-06 contained a linkage disequilibrium block of 20.33 Mb, confirming a previous report of low recombination on this chromosome arm. Four smaller but equally significant linkage disequilibrium blocks of 3.5-35.5 kb were detected on chromosomes 1, 2, 9, and 10. We examined the genes encoded within each block to provide a first look at candidates such as homologs of GS3 and FT that may indicate a selective sweep during sorghum domestication.

Genetic analyses of nickel tolerance in a North American serpentine endemic plant, Caulanthus amplexicaulis var. barbarae (Brassicaceae).

PREMISE OF THE STUDY: The evolution of metal tolerance in plants is an important model for studies of adaptation to environment, population genetics, and speciation. Here, we investigated nickel tolerance in the North American serpentine endemic Caulanthus amplexicaulis var. barbarae in comparison with its nonserpentine sister taxon C. amplexicaulis var. amplexicaulis. We hypothesized that the serpentine endemic would have a heritable growth advantage on nickel-containing substrates. METHODS: We employed an artificial growth assay to quantify biomass accumulation. Study plants were crossed to create an F(2:3) population that was used to determine the heritability of nickel tolerance and to map quantitative trait loci (QTL). Nickel accumulation in both laboratory populations and native specimens was examined using energy-dispersive x-ray fluorescence (EDXRF). KEY RESULTS: The serpentine endemic had a dramatic growth advantage at concentrations of nickel >30 µmol/L. Caulanthus amplexicaulis var. barbarae and its nonserpentine sister taxon both accumulated nickel to substantial levels. Nickel tolerance was highly heritable (h(2) = 0.59) and not associated with accumulation. The QTL analyses identified two major loci for nickel tolerance, on linkage group 2 (LG2) and linkage group 9 (LG9). CONCLUSIONS: In our study, nickel tolerance was determined by two major loci with large effects. At both loci, alleles from the serpentine parent conferred positive effects on nickel tolerance, suggesting that they are adaptive in the natural serpentine environment. The mechanism of nickel tolerance in the serpentine plant was not exclusion of nickel. Nickel tolerance may have an inducible component in C. amplexicaulis var. barbarae.

A comparative genomic map for Caulanthus amplexicaulis and related species (Brassicaceae).

Adaptation to environment is the cornerstone of ecological genetics. The subject of this study is a wild relative of the sequenced and annotated model plant species, Arabidopsis thaliana. Caulanthus amplexicaulis var. barbarae lives on serpentine soils, known for high concentrations of heavy metals and low concentrations of essential plant macronutrients, and provides a compelling example of an organism's adaptation to environment. We constructed an F(2) linkage map, using a cross to the nonserpentine sister taxon, C. amplexicaulis var. amplexicaulis. C. amplexicaulis is a member of a highly diverse set of taxa (within the tribe Thelypodieae), described here as the 'Streptanthoid Complex' that are adapted to a broad range of environments, yet share a common n = 14 chromosome number and likely arose by a recent radiation. The linkage map consists of 97 polymorphic microsatellite markers, and 40 exon-primed intron-crossing markers based on A. thaliana exon sequences and Brassica ESTs. The map covers 14 linkage groups and has a total length of 1513 cM. Both the patterns of marker segregation and the comparative map indicate that C. amplexicaulis is a diploid organism with a compact genome. All exon-primed intron-crossing markers, and an unexpectedly large number of microsatellite markers (83%), had significant similarity to the A. thaliana genome, facilitating the development of a comparative genome map. As a proof of principle, we used the comparative map to identify candidate genes underlying differences in sepal colour between the two parent taxa. We demonstrate that the genomic tools developed here will be portable throughout the Streptanthoid Complex.