A Pleiotropic Flowering Time QTL Exhibits Gene-by-Environment Interaction for Fitness in a Perennial Grass.

Appropriate flowering time is a crucial adaptation impacting fitness in natural plant populations. Although the genetic basis of flowering variation has been extensively studied, its mechanisms in nonmodel organisms and its adaptive value in the field are still poorly understood. Here, we report new insights into the genetic basis of flowering time and its effect on fitness in Panicum hallii, a native perennial grass. Genetic mapping in populations derived from inland and coastal ecotypes identified flowering time quantitative trait loci (QTL) and many exhibited extensive QTL-by-environment interactions. Patterns of segregation within recombinant hybrids provide strong support for directional selection driving ecotypic divergence in flowering time. A major QTL on chromosome 5 (q-FT5) was detected in all experiments. Fine-mapping and expression studies identified a gene with orthology to a rice FLOWERING LOCUS T-like 9 (PhFTL9) as the candidate underlying q-FT5. We used a reciprocal transplant experiment to test for local adaptation and the specific impact of q-FT5 on performance. We did not observe local adaptation in terms of fitness tradeoffs when contrasting ecotypes in home versus away habitats. However, we observed that the coastal allele of q-FT5 conferred a fitness advantage only in its local habitat but not at the inland site. Sequence analyses identified an excess of low-frequency polymorphisms at the PhFTL9 promoter in the inland lineage, suggesting a role for either selection or population expansion on promoter evolution. Together, our findings demonstrate the genetic basis of flowering variation in a perennial grass and provide evidence for conditional neutrality underlying flowering time divergence.

Geographic patterns of genomic diversity and structure in the C4 grass Panicum hallii across its natural distribution.

Geographic patterns of within-species genomic diversity are shaped by evolutionary processes, life history and historical and contemporary factors. New genomic approaches can be used to infer the influence of such factors on the current distribution of infraspecific lineages. In this study, we evaluated the genomic and morphological diversity as well as the genetic structure of the C4 grass Panicum hallii across its complex natural distribution in North America. We sampled extensively across the natural range of P. hallii in Mexico and the USA to generate double-digestion restriction-associated DNA (ddRAD) sequence data for 423 individuals from 118 localities. We used these individuals to study the divergence between the two varieties of P. hallii, P. hallii var. filipes and P. hallii var. hallii as well as the genetic diversity and structure within these groups. We also examined the possibility of admixture in the geographically sympatric zone shared by both varieties, and assessed distribution shifts related with past climatic fluctuations. There is strong genetic and morphological divergence between the varieties and consistent genetic structure defining seven genetic clusters that follow major ecoregions across the range. South Texas constitutes a hotspot of genetic diversity with the co-occurrence of all genetic clusters and admixture between the two varieties. It is likely a recolonization and convergence point of populations that previously diverged in isolation during fragmentation events following glaciation periods.

The genomic landscape of molecular responses to natural drought stress in Panicum hallii.

Environmental stress is a major driver of ecological community dynamics and agricultural productivity. This is especially true for soil water availability, because drought is the greatest abiotic inhibitor of worldwide crop yields. Here, we test the genetic basis of drought responses in the genetic model for C4 perennial grasses, Panicum hallii, through population genomics, field-scale gene-expression (eQTL) analysis, and comparison of two complete genomes. While gene expression networks are dominated by local cis-regulatory elements, we observe three genomic hotspots of unlinked trans-regulatory loci. These regulatory hubs are four times more drought responsive than the genome-wide average. Additionally, cis- and trans-regulatory networks are more likely to have opposing effects than expected under neutral evolution, supporting a strong influence of compensatory evolution and stabilizing selection. These results implicate trans-regulatory evolution as a driver of drought responses and demonstrate the potential for crop improvement in drought-prone regions through modification of gene regulatory networks.

Population genomics and climate adaptation of a C4 perennial grass, Panicum hallii (Poaceae).

BACKGROUND: Understanding how and why genetic variation is partitioned across geographic space is of fundamental importance to understanding the nature of biological species. How geographical isolation and local adaptation contribute to the formation of ecotypically differentiated groups of plants is just beginning to be understood through population genomic studies. We used whole genome sequencing combined with association study of climate to discover the drivers of differentiation in the perennial C4 grass Panicum hallii. RESULTS: Sequencing of 89 natural accessions of P.hallii revealed complex population structure across the species range. Major population genomic separation was found between subspecies P.hallii var. hallii and var. filipes as well as between at least four major unrecognized subgroups within var. hallii. At least 139 genomic SNPs were significantly associated with temperature or precipitation across the range and these SNPs were enriched for non-synonymous substitutions. SNPs associated with temperature and aridity were more often found in or near genes than expected by chance and enriched for putative involvement in dormancy processes, seed maturation, response to hyperosmosis and salinity, abscisic acid metabolism, hormone metabolism, and drought recovery. CONCLUSIONS: Both geography and climate adaptation contribute significantly to patterns of genome-wide variation in P.hallii. Population subgroups within P.hallii may represent early stages in the formation of ecotypes. Climate associated loci identified here represent promising targets for future research in this and other perennial grasses.

Promises and Challenges of Eco-Physiological Genomics in the Field: Tests of Drought Responses in Switchgrass.

Identifying the physiological and genetic basis of stress tolerance in plants has proven to be critical to understanding adaptation in both agricultural and natural systems. However, many discoveries were initially made in the controlled conditions of greenhouses or laboratories, not in the field. To test the comparability of drought responses across field and greenhouse environments, we undertook three independent experiments using the switchgrass reference genotype Alamo AP13. We analyzed physiological and gene expression variation across four locations, two sampling times, and three years. Relatively similar physiological responses and expression coefficients of variation across experiments masked highly dissimilar gene expression responses to drought. Critically, a drought experiment utilizing small pots in the greenhouse elicited nearly identical physiological changes as an experiment conducted in the field, but an order of magnitude more differentially expressed genes. However, we were able to define a suite of several hundred genes that were differentially expressed across all experiments. This list was strongly enriched in photosynthesis, water status, and reactive oxygen species responsive genes. The strong across-experiment correlations between physiological plasticity-but not differential gene expression-highlight the complex and diverse genetic mechanisms that can produce phenotypically similar responses to various soil water deficits.

Colombian froze biodiversity: 16 years of the tissue collection of the humboldt intiitute / La biodiversidad congelada de Colombia: 16 años de la colección de tejidos del Instituto Humboldt

Collections of frozen tissue samples stand as keystone sources of molecular information to construct biodiversity knowledge, and are particularly challenged if they focus on megadiverse countries. In 1998 the Humboldt Institute (Instituto de Investigación de Recursos Biológicos Alexander von Humboldt) began a tissue collection of Colombian biodiversity (IAvH-CT) and the aim of this work is to present a diagnostic and an historical perspective for that collection, constructed by compiling information and experiences on its management as well as by organizing and curating the information of each catalogued sample. After 16 years, the IAvH-CT harbors 16,469 samples, which represent around 2530 species from 1289 genera, and 323 families of the Colombian biodiversity. Samples are biased toward plants (44 %) and birds (40 %), but also include other animal taxa. Geographically, IAvH-CT includes samples from all Colombian departments, but there is broad variation in their coverage. When compared with other international collections, IAvH-CT fulfills several standards of sample storage and data management, but its major weakness is that several tissues seem to lack a vouchered specimen. Tissues housed at IAvH-CT have been included in at least 48 studies published in several scientific journals. IAvH-CT is implementing strategies to improve curatorial standards, fill-in taxonomic gaps, and to explore the potential of its samples to understand the outstanding Colombian biota in a cooperative research framework among institutions.

Las colecciones de tejidos son fuentes fundamentales de información molecular para el conocimiento de la biodiversidad, y son particularmente desafiantes si están enfocadas en países megadiversos. En 1998 el Instituto Humboldt (Instituto de Investigación de Recursos Biológicos Alexander von Humboldt) inició una colección de tejidos de la biodiversidad colombiana (IAvH-CT). El objetivo de este trabajo es presentar un diagnóstico y una perspectiva histórica de esta colección, mediante la compilación de información y de experiencias sobre su manejo y organizando y curando la información de cada muestra catalogada. Después de 16 años IAvH-CT resguarda 16,469 muestras, que representan alrededor de 2530 especies de 1289 géneros y 323 familias de la biodiversidad colombiana. El número de muestras está sesgado hacia plantas (44 %) y aves (40 %), pero también incluyen otros taxones animales. Geográficamente, IAvH-CT incluye muestras de todos los departamentos colombianos, pero hay una gran variación en su cobertura. Al ser comparada con otras colecciones internacionales se encuentra que IAvH-CT cumple varios estándares de almacenamiento de muestras y de manejo de datos, pero su gran debilidad es que varios tejidos aparentemente carecen de un ejemplar de referencia. Varios tejidos almacenados en IAvH-CT han sido incluidos en al menos 48 estudios publicados en varias revistas científicas. IAvH-CT está implementando estrategias para mejorar la curaduría, llenar vacíos taxonómicos y explorar el potencial de sus muestras para entender la impresionante biota colombiana en un marco de investigación en cooperación con otras instituciones.

Integrating transcriptional, metabolomic, and physiological responses to drought stress and recovery in switchgrass (Panicum virgatum L.).

BACKGROUND: In light of the changes in precipitation and soil water availability expected with climate change, understanding the mechanisms underlying plant responses to water deficit is essential. Toward that end we have conducted an integrative analysis of responses to drought stress in the perennial C4 grass and biofuel crop, Panicum virgatum (switchgrass). Responses to soil drying and re-watering were measured at transcriptional, physiological, and metabolomic levels. To assess the interaction of soil moisture with diel light: dark cycles, we profiled gene expression in drought and control treatments under pre-dawn and mid-day conditions. RESULTS: Soil drying resulted in reduced leaf water potential, gas exchange, and chlorophyll fluorescence along with differential expression of a large fraction of the transcriptome (37%). Many transcripts responded differently depending on time of day (e.g. up-regulation pre-dawn and down-regulation mid-day). Genes associated with C4 photosynthesis were down-regulated during drought, while C4 metabolic intermediates accumulated. Rapid changes in gene expression were observed during recovery from drought, along with increased water use efficiency and chlorophyll fluorescence. CONCLUSIONS: Our findings demonstrate that drought responsive gene expression depends strongly on time of day and that gene expression is extensively modified during the first few hours of drought recovery. Analysis of covariation in gene expression, metabolite abundance, and physiology among plants revealed non-linear relationships that suggest critical thresholds in drought stress responses. Future studies may benefit from evaluating these thresholds among diverse accessions of switchgrass and other C4 grasses.