Chloroplast DNA inversions and the origin of the grass family (Poaceae).

The phylogenetic affinities of the grass family (Poaceae) have long been debated. The chloroplast genomes of at least some grasses have been known to possess three inversions relative to the typical gene arrangement found in most flowering plants. We have surveyed for the presence of these inversions in grasses and other monocots by polymerase chain reaction amplification with primers constructed from sequences flanking the inversion end points. Amplification phenotypes diagnostic for the largest inversion (28 kilobase pairs) were found in genera representing all grass subfamilies, and in the nongrass families Restionaceae, Ecdeiocoleaceae, and Joinvilleaceae, but not in any other monocots--notably, Flagellariaceae, Anarthriaceae, Cyperaceae, or Juncaceae. This finding is consistent with one of the two principal views of grass phylogeny in suggesting that Poaceae and Cyperaceae (sedges) are not closest relatives. A second (approximately 6 kilobases) inversion appears to occur in a subset of the families possessing the 28-kilobase inversion and links Joinvilleaceae and Poaceae, while the smallest inversion appears unique to grasses. These inversions thus provide a nested set of phylogenetic characters, indicating a hierarchy of relationships in the grasses and allies, with Joinvilleaceae identified as the likely sister group to the Poaceae.