Evolutionary constraint on low elevation range expansion: Defense‐abiotic stress‐tolerance trade‐off in crosses of the ecological model Boechera stricta

Abstract

Most transplant experiments across species geographic range boundaries indicate that adaptation to stressful environments outside the range is often constrained. However, the mechanisms of these constraints remain poorly understood. We used extended generation crosses from diverged high and low elevation populations. In experiments across low elevation range boundaries, there was selection on the parental lines for abiotic stress‐tolerance and resistance to herbivores. However, in support of a defense‐tolerance trade‐off, extended generation crosses showed nonindependent segregation of these traits in the laboratory across a drought‐stress gradient and in the field across the low elevation range boundary. Genotypic variation in a marker from a region of the genome containing a candidate gene (MYC2) was associated with change in the genetic trade‐off. Thus, using crosses and forward genetics, we found experimental genetic and molecular evidence for a pleiotropic trade‐off that could constrain the evolution of range expansion.

Adaptation across geographic ranges is consistent with strong selection in marginal climates and legacies of range expansion

“…Furthermore, if range‐edge sites are uniquely challenging, adaptation might also be hampered by reduced genetic variation following strong and persistent directional selection (Hoffmann and Blows 1994), or by genetic constraints arising from antagonistic genetic correlations (Levins 1968; Antonovics 1976; Bradshaw and McNeilly 1991; see Olsen et al. 2019 for an empirical test). Together, this body of theory predicts that the magnitude of local adaptation decreases from geographically central to peripheral populations (Fig.…”

Variation in gene expression patterns across a conifer hybrid zone highlights the architecture of adaptive evolution under novel selective pressures

“…Additionally, we highlight the environment dependent impact of interspecific gene flow towards adaptive evolution in the P. strobiformis-P. flexilis hybrid zone populations. Our work stands among recent efforts to evaluate adaptive potential of species at or beyond their climate edge (Geber & Eckhart, 2005; Olsen et al 2019) and highlights the utility of transcriptomics to assess the architecture aiding evolution towards a novel climate optimum.…”

Effects of species interactions on the potential for evolution at species' range limits

“…Trade-offs can arise when the abiotic and biotic environments impose antagonistic selection on a common trait. For example, Olsen et al [54] created experimental crosses between high-and lowelevation populations of the small perennial plant Boechera stricta to expose genetic variation in trait combinations, then transplanted crosses across the species' low-elevation range limit. Both drought and herbivores imposed strong selection beyond the range, but did so antagonistically, resulting in a genetic trade-off that would prevent evolutionary niche and range expansion [54].…”

“…For example, Olsen et al [54] created experimental crosses between high-and lowelevation populations of the small perennial plant Boechera stricta to expose genetic variation in trait combinations, then transplanted crosses across the species' low-elevation range limit. Both drought and herbivores imposed strong selection beyond the range, but did so antagonistically, resulting in a genetic trade-off that would prevent evolutionary niche and range expansion [54]. In another example, the distribution of the Trinidadian guppy Poecilia reticulata appears to be confined to freshwater by competition with a closely related competitor, which inhabits brackish water [55].…”