Abstract
Areas just across species range boundaries are often stressful, but even with ample genetic variation within and among range-margin populations, adaptation towards stress tolerance across range boundaries often does not occur. Adaptive trans-generational plasticity should allow organisms to circumvent these problems for temporary range expansion; however, range boundaries often persist. To investigate this dilemma, we drought stressed a parent generation of Boechera stricta (A.Gray) A. Löve & D. Löve, a perennial wild relative of Arabidopsis, representing genetic variation within and among several low-elevation range margin populations. Boechera stricta is restricted to higher, moister elevations in temperate regions where generalist herbivores are often less common. Previous reports indicate a negative genetic correlation (genetic tradeoff) between chemical defence allocation and abiotic stress tolerance that may prevent the simultaneous evolution of defence and drought tolerance that would be needed for range expansion. In growth chamber experiments, the genetic tradeoff became undetectable among offspring sib-families whose parents had been drought treated, suggesting that the stress-induced trans-generational plasticity may circumvent the genetic tradeoff and thus enable range expansion. However, the trans-generational effects also included a conflict between plastic responses (environmental tradeoff); offspring whose parents were drought treated were more drought tolerant, but had lower levels of glucosinolate toxins that function in defence against generalist herbivores. We suggest that either the genetic or environmental tradeoff between defence allocation and stress tolerance has the potential to contribute to range limit development in upland mustards.
Highlights
A central question in evolutionary ecology is what are the factors and processes that contribute to the development of species range limits (Parmesan et al 2005; Gaston 2009; Sexton et al 2009; Wiens 2011; Anderson et al 2012)
We examined the effect of drought-induced trans-generational plasticity on this genetic tradeoff, and we determined whether there were any conflicts among drought-induced trans-generational plastic responses that might contribute to range limit development
Shoot size was positively correlated with reproductive output across the drought treatments in the parent generation, and we know from previous work (Siemens et al 2009) that shoot size is correlated with over-winter survivorship across the range boundary in the field; plant size in this system can be used as an indicator of fitness and for evolutionary inferences
Summary
A central question in evolutionary ecology is what are the factors and processes that contribute to the development of species range limits (Parmesan et al 2005; Gaston 2009; Sexton et al 2009; Wiens 2011; Anderson et al 2012). Most transplant experiments show that areas just across geographic range boundaries are stressful (Sexton et al.2009 for review), suggesting that organisms are often at their physiological limits at range edges. Adaptation for stress tolerance would be required to occupy these stressful environments. Since range boundaries exist, several genetic constraints apparently prevent this adaptation. We asked why trans-generational plasticity does not allow for temporary range expansion by overcoming any genetic constraints that may limit range
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