When do different C4 leaf anatomies indicate independent C4 origins? Parallel evolution of C4 leaf types in Camphorosmeae (Chenopodiaceae)

Broad-scale phylogenetic studies give first insights in numbers, relationships, and ages of C-4 lineages. They are, however, generally limited to a model that treats the evolution of the complex C-4 syndrome in different lineages as a directly comparable process. Here, we use a resolved and well-sampled phylogenetic tree of Camphorosmeae, based on three chloroplast and one nuclear marker and on leaf anatomical traits to infer a more detailed picture of C-4 leaf-type evolution in this lineage. Our ancestral character state reconstructions allowed two scenarios: (i) Sedobassia is a derived C-3/C-4 intermediate, implying two independent gains of C-4 in Bassia and Camphorosma; or (ii) Sedobassia is a plesiomorphic C-3/C-4 intermediate, representing a syndrome ancestral to the Bassia/Camphorosma/Sedobassia lineage. In Bassia, a kochioid leaf type (Bassia muricata and/or Bassia prostrata type) is ancestral. At least three independent losses of water-storage tissue occurred, resulting in parallel shifts towards an atriplicoid leaf type. These changes in leaf anatomy are adaptations to different survival strategies in steppic or semi-desert habitats with seasonal rainfall. In contrast, Camphorosma shows a fixed C-4 anatomy differing from Bassia types in its continuous Kranz layer, which indeed points to an independent origin of the full C-4 syndrome in Camphorosma, either from an independent C-3 or from a common C-3/C-4 intermediate ancestor, perhaps similar to its C-3/C-4 intermediate sister genus Sedobassia. The enlarged bundle sheath cells of Sedobassia might represent an important early step in C-4 evolution in Camphorosmeae.