Adaptation to a local environment is expected to arise after a long history in a given location, especially in the case of plants using sexual reproduction. In the shorter term, phenotypic plasticity is presumed to be more important, particularly for asexual species, and exclusively clonal propagation is thought to reduce a species’ capacity for local adaptation. Invasive and introduced species present a useful system with which to study this phenomenon, because they must cope with novel conditions in their environments. Many invasive species combine both sexual and asexual modes of propagation, but field tests that include obligately asexual plants are rare.
In a new article published in Annals of Botany, lead author Violeta I. Simón-Porcar and colleagues studied the relative contributions of adaptive evolution and phenotypic plasticity in the successful establishment of two closely-related invasive species in the United Kingdom: Mimulus guttatus, which uses both sexual and asexual reproduction, and M. x robertsii, which is exclusively asexual. The researchers compared the growth and morphology of populations of each species kept in growth chambers mimicking the environmental conditions at the latitudinal extremes of the UK. They then tested local adaptation in the invasive portion of the two species’ ranges with a transplant experiment between two field sites at the same latitudinal extremes, located in the Shetland Isles and on the Isle of Wight.
Overall, sexual populations demonstrated local adaptation through fruit production while asexual populations adapted via stolon production. Temperature differences had the greatest effect on the phenology of individual plants and on plant height, though not biomass. Long days affected sexual organ production as well as all measured growth traits. The net effect of higher temperatures in the south outweighed that of longer days in the north however, as individuals grew bigger and produced more flowers on the Isle of Wight versus Shetland. The results suggested that there is indeed greater phenotypic plasticity in asexual versus sexual species, but that stolon production specifically showed similar plasticity in both species. Furthermore, reciprocal transplant experiments showed robust evidence of local adaptation in both species.
“Selection on clonal taxa could occur through genotypic selection in genetically diverse founding populations (clonal selection), or, perhaps, through other mechanisms including epigenetic modification,” write the authors. “Although further comparisons between sexual and asexual taxa in other suitable natural systems are needed for inferences on the evolutionary rates and mechanisms of asexual taxa across plant lineages, our study provides a starting point for understanding the early evolutionary trajectory of invasive asexual plant populations.”