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

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