Microgeographic adaptation corresponds with elevational distributions of congeneric montane grasshoppers

Local adaptation can occur at small spatial scales relative to the
dispersal capacity of species. Alpine ecosystems have sharp environmental
clines that offer an opportunity to investigate the effects of fine scale
shifts in species’ niche breadth on adaptive genetic processes. Here we
examine two grasshopper species endemic to the Australian Alps
(Kosciuscola spp.) that differ in elevational niche breadth; one broader,
K. usitatus (1400-2200m), and one narrower, K. tristis (1600-2000m). We
examine signatures of selection with respect to environmental and
morphological variables in two mountain regions using FST outlier tests
and environmental association analyses (EAA) applied to Single Nucleotide
Polymorphism data (K. usitatus: 9,017 SNPs, n = 130; K. tristis: 7,363
SNPs, n = 135). Stronger genetic structure was found in the more narrowly
distributed K. tristis, which showed almost twice the number of SNPs under
putative selection (10.8%) compared with K. usitatus (5.3%). When
examining SNPs in common across species (n = 3,058), 260 SNPs (8.5%) were
outliers shared across species, and these were mostly associated with
elevation, a proxy for temperature, suggesting parallel adaptive processes
in response to climatic drivers. Additive polygenic scores (an estimate of
the cumulative signal of selection across all candidate loci) were
non-linearly and positively correlated with elevation in both species.
However, a steeper correlation in K. tristis indicated a stronger signal
of spatially varying selection towards higher elevations. Our study
illustrates that the niche breadth of co-occurring and related species
distributed along the same environmental cline is associated with
differences in patterns of microgeographic adaptation.