DUVALL, MELVIN R.*, DAYLE E. SAAR, W. SCOTT GRAYBURN, and GABRIEL P. HOLBROOK. Plant Molecular Biology Center and Department of Biological Sciences, Northern Illinois University, DeKalb, IL 60115-2861. - Phylogenetics of the C3 - C4 intermediate species Steinchisma hians (= Panicum milioides, Poaceae): Evidence for multiple origins and loss of the C4 syndrome in one taxonomic tribe.
Plant species, such as Steinchisma hians, that exhibit
intermediacy between the C3 and C4
photosynthetic types offer the opportunity to study the evolution of
adaptive complexes. We conducted a molecular phylogenetic study of
this and related species with two markers: 1) the grass-specific
insert sequence in the chloroplast RNA polymerase locus
(rpoC2); and 2) the externally transcribed spacer region in the
nuclear ribosomal repeat (ETS). To our knowledge this is the first
report of the use of ETS in grass phylogenetics. Separate analyses
indicated no substantial incongruence between these two loci so data
combination was performed. Combined phylogenetic analysis resolved
exclusive clades of two photosynthetic subtypes. A reversion of
C4 to C3 photosynthesis during the evolution of
the lineage that includes S. hians was identified. This and
other recent results suggest that C4 to C3
reversions are more common than previously supposed. The C3
species, Panicum laxum, had a strongly supported sister group
relationship to S. hians. The most parsimonious interpretation
is that S. hians represents an incipient reversal from
C3 back to C4 photosynthesis beginning with the
capacity to compartmentalize photorespiratory metabolism in the bundle
sheath tissue. Our results further confirmed the multiple origins of
C4 photosynthesis within panic grasses.
Key words: C3 - C4 intermediacy, ETS, external transcribed spacer, Paniceae, photosynthesis, rpoC2, Steinchisma hians