Phenotypic plasticity and local adaptation in two extreme populations of Acacia karroo

Abstract

Phenotypic plasticity is the effect of growth variability in response to different environmental conditions. Adaptive phenotypic plasticity is more specific; it is the ability of a single genotype to produce an array of phenotypes depending on the environmental extent. We investigated differences in adaptive phenotypic plasticity as well as local adaptations measured by reaction norms of two extreme populations (Leeu Gamka, arid Karoo and Richards Bay, subtropical coastal forest) of the extremely phenotypically plastic tree species Acacia karroo in a common-garden experiment. In addition, we used allozymes to determine whether these populations were genetically differentiated. Seeds were grown at different levels of water availability with respect to their original environments and browsing by mammals was simulated in the greenhouse by clipping plants. We found that the populations are adaptively phenotypically plastic in response to these factors. The slow-growing population (Leeu Gamka) was phenotypically more plastic with regard to defence-related traits (spines, tannin) and the fast-growing population (Richards Bay) was phenotypically more plastic regarding growth-related traits (height, leaves). The interactions between populations and environments in some traits indicated genetic differentiation in plastic responses between populations and, consequently, that phenotypic plasticity is locally adaptive and not merely due to environmental differences. The two populations appear to have pure strategies; when environmental conditions were improved by addition of water, the forest population increased investment in growth but not defence, whereas the arid population increased defence production but not growth. Principal coordinate analysis (PCoA) of allozymes revealed little genetic overlap among populations.