Complex transitions between C3 and C4 photosynthesis during the evolution of paniceae:: A phylogenetic case study emphasizing the position of Steinchisma hians (Poaceae), a C3-C4 intermediate

A two-tiered, nested molecular phylogenetic study of panicoid grasses to explore character state transitions between the C-3 and C-4 adaptive syndromes is presented. A broad survey of 92 panicoid species was sampled for the grass-specific insert sequence in the chloroplast RNA polymerase locus (rpoC2), combining published and unpublished sequences. This portion of the study also included an intensive phylogenetic investigation of one clade of seven species that included Steinchisma hians, which is notable for exhibiting intermediacy between the C-3 and C-4 photosynthetic types. Both rpoC2 data and previously published sequences of the F subunit of an NADH-dependent dehydrogenase were analyzed together for this small group. A rigorous phylogenetic investigation of S. hians and 13 other species of Panicoideae included in the broad survey was then performed with sequences of both rpoC2 and the externally transcribed spacer region of the nuclear ribosomal repeat. These 14 species were selected to maximize representation among photosynthetic subtypes. Combined analysis resolved single origins of two photosynthetic subtypes. A reversion of C-4 to C-3 photosynthesis during the evolution of the lineage that includes S. hians is identified. These and other recent results indicate that repeated reversions from C-4 to C-3 have occurred. The C-3 species Panicum laxum has a strongly supported sister group relationship to S. hians (C-3-C-4). The most parsimonious interpretation is that S. hians represents an incipient reversal from C-3 to C-4 photosynthesis, beginning with the capacity to compartmentalize photorespiratory metabolism in the bundle sheath tissue.