Fine-scale spatial genetic structure in a locally abundant native bunchgrass (Achnatherum thurberianum) including distinct lineages revealed within seed transfer zones

Analyses of the factors shaping genetic variation in widespread plant
species are important for understanding evolutionary history and local
adaptation and have applied significance for guiding conservation and
restoration decisions. Thurber’s needlegrass (Achnatherum thurberianum) is
a widespread, locally abundant grass that inhabits heterogeneous arid
environments of western North America and is of restoration significance.
It is a common component of shrubland steppe communities in the Great
Basin Desert, where drought, fire, and invasive grasses have degraded
natural communities. Using a reduced representation sequencing approach,
we generated SNP data at 5,677 loci across 246 individuals from
17 A. thurberianum populations spanning five previously
delineated seed zones from the western Great Basin. Analyses revealed
pronounced population genetic structure, with individuals forming
consistent geographical clusters across a variety of population genetic
analyses and spatial scales. Low levels of genetic diversity within
populations, as well as high population estimates of linkage
disequilibrium and relatedness, were consistent with self-fertilization as
a contributor to population differentiation. Variance partitioning and
partial redundancy analysis (pRDA) indicated local adaptation to
environment as additionally influencing the spatial distribution of
genetic variation. The environmental variables driving these results were
similar to those implicated in recent genecological work which inferred
local adaptation for seed zone delineation. Our analyses also revealed a
complex evolutionary history of A. thurberianum in the
Great Basin, where previously delineated seed zones contain distantly
related populations. Our results indicate evolutionary history, mating
system, and differentiation across distinct geographic and environmental
scales have shaped genetic variation in A.
thurberianum and illustrate how numerous aspects of population
genetic variation might require consideration for restoration planning.