On the hybrid origin of the C-2 Salsola divaricata agg. (Amaranthaceae) from C-3 and C-4 parental lineages

C-2 photosynthesis is characterised using recapturing photorespiratory CO2 by RuBisCo in Kranz-like cells and is therefore physiologically intermediate between C-3 and C-4 photosynthesis. C-2 can be interpreted as an evolutionary precursor of C-4 and/or as the result of hybridisation between a C-3 and C-4 lineage. We compared the expression of photosynthetic traits among populations of the Salsola divaricata agg. (C-2) from humid subtropical to arid habitats on the coasts of the Canary Islands and Morocco and subjected them to salt and drought treatments. We screened for enhanced C-4-like expression of traits related to habitat or treatment. We estimated species trees with a transcriptome dataset of Salsoleae and explored patterns of gene tree discordance. With phylogenetic networks and hybridisation analyses we tested for the hybrid origin of the Salsola divaricata agg. We observed distinct independent variation of photosynthetic traits within and among populations and no clear evidence for selection towards C-4-like trait expression in more stressful habitats or treatments. We found reticulation and gene tree incongruence in Salsoleae supporting a putative hybrid origin of the Salsola divaricata agg. C-2 photosynthesis in the Salsola divaricata agg. combines traits inherited from its C-3 and C-4 parental lineages and seems evolutionarily stable, possibly well adapted to a wide climatic amplitude.