Evolution in Stressful Environments. I. Phenotypic Variability, Phenotypic Selection, and Response to Selection in Five Distinct Environmental Stresses

Abstract

Abstract. Considerable debate has accompanied efforts to integrate the selective impacts of environmental stresses into models of life-history evolution. This study was designed to determine if different environmental stresses have consistent phenotypic effects on life-history characters and whether selection under different stresses leads to consistent evolutionary responses. We created lineages of a wild mustard (Sinapis arvensis) that were selected for three generations under five stress regimes (high boron, high salt, low light, low water, or low nutrients) or under near-optimal conditions (control). Full-sibling families from the six selection histories were divided among the same six experimental treatments. In that test generation, lifetime plant fecundity and six phenotypic traits were measured for each plant. Throughout this greenhouse study, plants were grown individually and stresses were applied from the early seedling stage through senescence. Although all stresses consistently reduced lifetime fecundity and most size-and growth-related traits, different stresses had contrasting effects on flowering time. On average, stress delayed flowering compared to favorable conditions, although plants experiencing low nutrient stress flowered earliest and those experiencing low light flowered latest. Contrary to expectations of Grime's triangle model of life-history evolution, this ruderal species does not respond phenotypically to poor environments by flowering earlier. Most stresses enhanced the evolutionary potential of the study population. Compared with near-optimal conditions, stresses tended to increase the opportunity for selection as well as phenotypic variance, although both of these quantities were reduced in some stresses. Rather than favoring traits characteristic of stress tolerance, such as slow growth and delayed reproduction, phenotypic selection favored stress-avoidance traits: earlier flowering in all five stress regimes and faster seedling height growth in three stresses. Phenotypic correlations reinforced direct selection on these traits under stress, leading to predicted phenotypic change under stress, but no significant selection in the control environment. As a result of these factors, selection under stress resulted in an evolutionary shift toward earlier flowering. Environmental stresses may drive populations of ruderal plant species like S. arvensis toward a stress-avoidance strategy, rather than toward stress tolerance. Further studies will be needed to determine when selection in stressful environments leads to these alternative life-history strategies.

Phenotypic response of plants to simulated climate change in a long-term rain-manipulation experiment: a multi-species study

“…Vice versa, it could also be a passive stress response with no adaptive value. For example, delayed flowering was also observed in other drought experiments (Fox 1990b;Blum 1996;Stanton et al 2000;Peñuelas et al 2004) and has been attributed to a severe growth limitation. Our own observations in the field indeed indicated that most species developed faster in the wetter treatments (ESM, Supplementary Table 1) as suggested by Sachs (1999).…”

“…However, the treatment effects were surprisingly inconsistent. In three species in the Mediterranean site, we indeed found an earlier onset of reproduction in the drought treatment, which could be indicative of a drought-escape strategy (Geber and Dawson 1990;Stanton et al 2000;McKay et al 2003;Sherrard and Maherali 2006;Franks 2011). However, this pattern did not occur in the other species and it was even reverted in one of them (B. didyma).…”

Natural variation for drought-response traits in the Mimulus guttatus species complex

“…Many studies suggest that trade-oVs may exist between traits involved in dehydration-avoidance and dehydrationescape strategies (Geber and Dawson 1990;Stanton et al 2000;McKay et al 2003; but see Sherrard and Maherali 2006). In particular, higher WUE is expected to be favored in water-limited environments, where minimizing water loss allows the plant to extend its growth over a longer period.…”

“…It is well established that the interaction of many environmental and genetic factors can contribute to Xowering time variation in plants (reviewed in Simpson and Dean 2002;Koornneef et al 2004), and that the optimal timing of Xowering may vary tremendously across diVerent habitats in response to external cues, including day length (Weinig et al 2002), light levels (Stanton et al 2000), temperature (Eckhart et al 2004), nutrient level (Stanton et al 2000), and water availability (Fox 1990;Eckhart et al 2004;Franke et al 2006). While we are still in the early stages of identifying the speciWc environmental cues that inXuence Xowering time patterns in Mimulus, recent studies suggest that local adaptive Xowering-time diVerences are a response to the diVering intensity of seasonal drought in coast and inland habitats of M. guttatus (Hall and Willis 2006;Lowry et al 2008).…”

“…Alternately, plants may forego this high WUE, and instead maximize early growth to Xower and reproduce early, thereby avoiding the negative Wtness eVects of drought, particularly when its onset is seasonally predictable. Indeed, several studies in annual systems suggest that trade-oVs exist between dehydrationavoidance traits such as higher WUE and dehydrationescape traits such as rapid growth and early Xowering Dawson 1990, 1997;Stanton et al 2000;McKay et al 2003; but see Sherrard and Maherali 2006). Additionally, we used relative water content (RWC) to assess the plants' level of water deWcit and capacity for dehydration avoidance.…”