Yellow Cress hybrids combine the best of their parents in responses to flooding

Hybrids provide a unique opportunity to study ecological differentiation. The naturally hybridizing Yellow Cress species Rorippa amphibia (RA) and R. sylvestris (RS) occupy different habitats: RA grows in sites with more stable water tables (eg oxbow lakes), mostly as emergent plants, while RS occurs in temporarily submerged sites (eg river floodplains) that often dry out in summer. We addressed whether their hybrid, R. x anceps, exhibits intermediate responses to flooding, or combines the best of both parents.

In a greenhouse, reciprocal F1 hybrids and their parents were exposed to 3 treatments: submerged, waterlogged and well drained. After two weeks, we recorded changes in characters that are known to be under selection by flooding: leaf orientation, petiole length and biomass allocation.

When waterlogged, RA produced more biomass and shorter petioles than RS. RS always developed more leaves and reacted strongest to waterlogging and submergence by aborting leaves and orientating its leaves more upright. Interestingly, hybrids seemed to combine the best of both parents by producing more biomass, many leaves (RS trait) and large leaves (RA trait). Hybrid responses to flooding mostly resembled those of RA, suggesting that flooding traits inherit dominantly. Thus, we conclude that hybrids perform best under waterlogged conditions and could be successful particularly in the RA habitat.