Competition × drought interactions change phenotypic plasticity and the direction of selection on Arabidopsis traits

Abstract

Populations often exhibit genetic diversity in traits involved in responses to abiotic stressors, but what maintains this diversity is unclear. Arabidopsis thaliana exhibits high within-population variation in drought response. One hypothesis is that competition, varying at small scales, promotes diversity in resource use strategies. However, little is known about natural variation in competition effects on Arabidopsis physiology. We imposed drought and competition treatments on diverse genotypes. We measured resource economics traits, physiology, and fitness to characterize plasticity and selection in response to treatments. Plastic responses to competition differed depending on moisture availability. We observed genotype-drought-competition interactions for relative fitness: competition had little effect on relative fitness under well-watered conditions, whereas competition caused rank changes in fitness under drought. Early flowering was always selected. Higher d 13 C was selected only in the harshest treatment (drought and competition). Competitive context significantly changed the direction of selection on aboveground biomass and inflorescence height in wellwatered environments. Our results highlight how local biotic conditions modify abiotic selection, in some cases promoting diversity in abiotic stress response. The ability of populations to adapt to environmental change may thus depend on small-scale biotic heterogeneity.

Natural selection on traits and trait plasticity in Arabidopsis thaliana varies across competitive environments

“…Earlier flowering in Arabidopsis was selected under competition-present but not competition-absent conditions. Selection favoring earlier flowering (negative β ) has been also reported in recent studies 61 , 62 . In annual plants, the transition from vegetative growth to reproductive maturity is strongly influenced by resource availability 63 – 65 and biotic interactions 14 , 66 thus flowering earlier can be favored when doing so reduces the negative effects of factors such as competition and seed predation.…”

“…We show that plastic responses are more selectively important under competition-present than competition-absent conditions, although this is not evident across all measured traits. The standard statistical approach for determining the role of selection on trait plasticity 17 , 19 , 62 , 76 – 78 , which we also used here, has been revised on multiple occasions highlighting its complexities and limitations 38 , 44 , 45 , 79 . One of the most commonly discussed limitations of this approach is the inclusion of both a trait and its plasticity in the same model despite the non-independence of these values 44 .…”

“…Reporting results from both the simple and full models allows us to account for the potential effects of traits correlated with plasticity on relative fitness. Inferences from the literature about selection on plasticity typically use models similar to our simple model 19 , 38 , 62 , 77 , 111 , but ignoring correlated traits may generate misleading inferences regarding the role of selection on plasticity. Trait correlations vary across different environments therefore the covariance with trait plasticity can as well 112 , 113 .…”

Plasticity in response to plant–plant interactions and water availability

“…Trait plasticity may “enhance species’ ability to coexist” (Pfennig et al 2006), but this depends on other factors (see review by Turcotte and Levine 2016). Because plasticity has the potential to reduce niche overlap, we might expect competitors to exhibit plasticity in response to severe conditions, such as drought (Lak et al 2020, Lorts and Lasky 2020). We found plasticity in all traits for Potentilla in intraspecific or interspecific competition, and for root mass and root : shoot ratios for Leucanthemum in intraspecific competition.…”

What processes must we understand to forecast regional-scale population dynamics?

“…Biotic constraints such as resource competition may limit organisms' ability to take advantage of favourable conditions, such as greater resource supply (e.g. higher rainfall for plants) [68,69]. Abiotic conditions may determine the nature of species interactions, such as when stress shifts the balance between competition and facilitation [70].…”