Species must rapidly adapt or go extinct. Rapid adaptation has been driven in the past by large genomic rearrangements like hybridization and whole genome duplication (WGD), and these processes continue to drive contemporary evolution. Scientists have developed theoretical frameworks for how hybridization and WGD may each be associated with novel traits and adaptation, but few have given equal attention to both processes. The failure to unify theory is a consequence of a lack of experiments that successfully isolate the independent effects of hybridization and WGD, a challenge because the two processes have arisen simultaneously in many lineages (allopolyploidy). In addition, for some taxa, the genomic consequences of either hybridization or WGD have been noted, but phenotypic and ecological effects remain unmeasured. We performed the most complete test to date of the independent effects of hybridization and WGD on phenotypes. We conducted a common garden experiment comparing diploids, autotetraploids, and allotetraploid hybrids of Arabidopsis thaliana and Arabidopsis arenosa. To make inferences about how phenotypic variation may be associated with adaptation, we manipulated stress gradients in a factorial treatment design. We imposed nutrient and salt treatments, two stressors relevant to the biology of these stress-tolerant/ruderal taxa.
Results/Conclusions
We found substantial variation in phenotypes and stress responses across taxa, with some variation corresponding to hybridity and WGD. Compared to their parents, hybrids showed a combination of trait values that were intermediate, transgressive, and dominant for the paternal parent, A. arenosa. Dominance for A. arenosa was consistent with previous research showing that these hybrids preferentially silence the A. thaliana genome. We saw transgressively slow phenology in tetraploids compared to their diploid progenitors, a pattern commonly observed as a consequence of WGD. Some taxa showed plasticity in response to salt stress, and plasticity was sometimes adaptive (associated with increased fitness). Tetraploids were not particularly plastic--notable given that other researchers have associated WGD with increased plasticity. However, for tetraploids, plasticity in growth and biomass was adaptive, while plasticity in hybrids was not associated with fitness consequences. Our separation of the adaptive consequences of hybridization and WGD will provide a significant theoretical advancement in evolutionary ecology. Our findings will contribute to the understanding of how these processes have driven diversification and adaptation of lineages on both ancient and modern timescales.