Figure 7: A model for altered stress-responsive gene expression and growth vigour in hybrids.

(a) An example of an F1 (ColXC24) hybrid showing growth vigour compared with the parents, Col and C24 (scale bars, 1 cm). Natural variation of circadian rhythms and stress responses is associated with the adaptation of each ecotype to local environments. (b) Parent 1 and parent 2 who have adapted to different environments require high levels of gene expression in response to abiotic (parent 1) or biotic (parent 2) stress, which is partly mediated by circadian rhythms (clock symbols). Parent 1 and parent 2 do not reach the full growth potential, probably because of the fitness cost in response to the stress in respective adaptive environments. In F1 hybrids, the expression of both biotic and abiotic stress-responsive genes is compromised, which we propose leads to increased levels of growth vigour.