Ecotypes of an ecologically dominant prairie grass (Andropogon gerardii) exhibit genetic divergence across the U.S. Midwest grasslands' environmental gradient

Abstract

Big bluestem (Andropogon gerardii) is an ecologically dominant grass with wide distribution across the environmental gradient of U.S. Midwest grasslands. This system offers an ideal natural laboratory to study population divergence and adaptation in spatially varying climates. Objectives were to: (i) characterize neutral genetic diversity and structure within and among three regional ecotypes derived from 11 prairies across the U.S. Midwest environmental gradient, (ii) distinguish between the relative roles of isolation by distance (IBD) vs. isolation by environment (IBE) on ecotype divergence, (iii) identify outlier loci under selection and (iv) assess the association between outlier loci and climate. Using two primer sets, we genotyped 378 plants at 384 polymorphic AFLP loci across regional ecotypes from central and eastern Kansas and Illinois. Neighbour-joining tree and PCoA revealed strong genetic differentiation between Kansas and Illinois ecotypes, which was better explained by IBE than IBD. We found high genetic variability within prairies (80%) and even fragmented Illinois prairies, surprisingly, contained high within-prairie genetic diversity (92%). Using Bayenv2, 14 top-ranked outlier loci among ecotypes were associated with temperature and precipitation variables. Six of seven BayeScanFST outliers were in common with Bayenv2 outliers. High genetic diversity may enable big bluestem populations to better withstand changing climates; however, population divergence supports the use of local ecotypes in grassland restoration. Knowledge of genetic variation in this ecological dominant and other grassland species will be critical to understanding grassland response and restoration challenges in the face of a changing climate.

The role of ecotypic variation and the environment on biomass and nitrogen in a dominant prairie grass

“…Kansas contains more contiguous prairie dominated by this species than Illinois, where .99.9% of this ecosystem has been converted to row-crop agriculture (Samson and Knopf 1994). Genetic studies of A. gerardii demonstrate that small, highly fragmented populations contain high genetic diversity (Gustafson et al 1999, Gray et al 2014, and the genetic structure of the Illinois populations used in this study was distinct from the KS populations (Gray et al 2014). …”

“…Biomass production of C 4 grasses, including Andropogon gerardii, is significantly related to precipitation across the Great Plains (Epstein et al 1998) and in response to water availability within tallgrass prairie (Knapp et al 2001). Further, there is evidence that aboveground tissue chemistry, leaf morphology, and genetics vary among ecotypes of A. gerardii reciprocally planted along a longitudinal gradient with a two-fold increase in mean annual precipitation (MAP; Zhang et al 2012, Olsen et al 2013, Gray et al 2014. There is little consensus on how precipitation affects belowground NPP (BNPP; Hayes and Seastedt 1987, McCulley et al 2005, and no knowledge of ecotypic variation in A. gerardii belowground, where the majority of plant biomass in tallgrass prairie resides Matchett 2001, Nippert et al 2012).…”

Phenotypic distribution models corroborate species distribution models: A shift in the role and prevalence of a dominant prairie grass in response to climate change

“…Will A. gerardii remain an important species of the central grassland ecosystem? Given that phenotypic variation in this species is in large part genetically controlled (Gray et al., ; Johnson et al., ; Mendola et al., ) and not because of phenotypic plasticity, for several reasons we expect that A. gerardii will likely not be able to acclimate or adapt quick enough to future climate conditions. First, most reproduction in this species is asexual through rhizomes’ production of clones (Weaver & Fitzpatrick, ).…”

“…Tellingly, a reciprocal common garden experiment established across a ~1200 km transect from western Kansas to Illinois demonstrated local adaptation and strong genetic control over growth‐related traits in this species (e.g., biomass, height; Johnson et al., ; Mendola et al., ). The strong correlation between growth and climatic gradients (Gray et al., ; McMillan, ) suggests that adaptive, genetically controlled phenotypic differences could play an important role in predicting the distribution of A. gerardii in response to climate change.…”

Environmental variation shapes genetic variation in Bouteloua gracilis: Implications for restoration management of natural populations and cultivated varieties in the southwestern United States

“…Colonization history and isolation by distance (IBD) may also play a role in determining how genetic variation is shaped and ultimately the detection of loci that may be under divergent selection (Lotterhos & Whitlock, 2014). Incorporating this approach can be challenging (Nadeau, Meirmans, Aitken, Ritland, & Isabel, 2016), but may yield additional insight into how environmental variation shapes genetic variation in widespread species, including grasses (e.g., Gray et al, 2014). Incorporating this approach can be challenging (Nadeau, Meirmans, Aitken, Ritland, & Isabel, 2016), but may yield additional insight into how environmental variation shapes genetic variation in widespread species, including grasses (e.g., Gray et al, 2014).…”