C 3 -C 4 intermediates may be of hybrid origin - a reminder.

The currently favoured model of the evolution of C-4 photosynthesis relies heavily on the interpretation of the broad phenotypic range of naturally growing C-3-C-4 intermediates as proxies for evolutionary intermediate steps. On the other hand, C-3-C-4 intermediates had earlier been interpreted as hybrids or hybrid derivates. By first comparing experimentally generated with naturally growing C-3-C-4 intermediates, and second summarising either direct or circumstantial evidence for hybridisation in lineages comprising C-3, C-4 andC(3)-C-4 intermediates, we conclude that a possible hybrid origin of C-3-C-4 intermediates deserves careful examination. While we acknowledge that the current model of C-4 photosynthesis evolution is clearly the best available, C-3-C-4 intermediates of hybrid origin, if existing, should not be used for further analysis of this model. However, experimental C-3 x C-4 hybrids potentially are excellent systems to analyse the genetic differences between C-3 and C-4 species and, also using segregating progeny, to study the relationship between individual photosynthetic traits and environmental factors.