Functional drought response in the Brachypodiumdistachyon species complex

Some polyploid species show enhanced physiological tolerance to drought compared with their progenitors. However, very few studies have examined the consistency of physiological drought response between genetically differentiated natural polyploid populations, which is key to evaluating the importance of adaptive evolution after polyploidization in those systems where drought exerts a selective pressure.

Martínez et al. used a comparative functional approach to investigate differentiation of drought-tolerance-related traits in the Brachypodium species complex, a model system for grass polyploid adaptive speciation and functional genomics that comprises three closely related annual species: the two diploid parents, B. distachyon and B. stacei, and the allotetraploid derived from them, B. hybridum. They also analysed the Differentiation of drought-tolerance-related traits between ten genetically distinct B. hybridumpopulations and its ecological correlates.

The authors found the functional drought response is overall well differentiated between Brachypodium species. B. hybridum allotetraploids showed a transgressive expression pattern in leaf phytohormone content in response to drought. In contrast, otherB. hybridum physiological traits correlated to B. stacei ones. Particularly, proline and water content were the traits that best discriminated these species from B. distachyon under drought.

The authors conclude: “Here we have shown that part of the drought response of allopolyploids is transgressive, which is concordant with the expected benefits derived from ancient heterosis for allopolyploids. On the other hand, we have also demonstrated that, at species level, B. hybridum leaf-level functional responses to water stress are essentially correlated to one of its ancestors, B. stacei, which concurs with post-polyploidization adaptive evolution.”