Environmentally induced and (epi-)genetically based physiological trait differentiation between Heliosperma pusillum and its polytopically evolved ecologically divergent descendent, H. veselskyi (Caryophyllaceae: Sileneae)

Abstract

Evolution is driven by natural selection, favouring individuals adapted in morphology and physiology to the environmental conditions at their growing site. Here, we studied environmentally induced and (epi-)genetically based components of divergence of photosynthesis in response to irradiance and temperature and cellular characteristics reflecting water availability in two reciprocally non-monophyletic, parapatric and interfertile, but morphologically and ecologically strongly divergent species in their natural habitat and in a common garden. Heliosperma pusillum occurs in alpine wet rocky habitats, whereas H. veselskyi is restricted to montane shady habitats with dry soils below overhanging rocks. Microclimatic divergence of natural habitats was evident in photosynthetically active photon flux density and leaf temperature. Photosynthetic light response was in agreement with differences in irradiance climate, suggesting that different photosynthetic parameters are favoured at particular growing sites. Photosynthesis was adapted to lower daily minimum temperatures at the alpine site, but did not differ at high temperatures. In the common garden, both species adjusted their functional traits in response to the shift in environmental conditions. The different light response of photosynthesis suggests an (epi-)genetically based component affecting photosynthetic parameters. Early-stage speciation between H. pusillum and H. veselskyi is probably environmentally induced, as the close connection between trait values and microclimatic conditions suggests that functional differentiation is adaptive. This is in line with the independent evolution of similar phenotypes under similar ecological conditions in different regions.