Rehmannieae or Rehmanniaceae? Evidence from plastome sequences and floral morphology

Abstract

Rehmannia and Triaenophora form a clade and are widely accepted as being closely allied with Orobanchaceae in Lamiales. However, the taxonomic rank of this clade and its geographical origins are understudied and poorly known. The genera have together been recognized as Orobanchaceae tribe Rehmannieae or Rehmanniaceae. To re-evaluate the taxonomy and systematic position and affinities of these genera, we reconstructed a phylogenety using complete plastid genomes (plastomes) and performed an analysis of comparative floral development. Additionally, we investigated the geographical distributions of Rehmannia and Triaenophora to determine the centre of diversity and origin. The phylogenetic analyses showed that Rehmannia and Triaenophora formed a clade that is sister to all other Orobanchaceae. This relationship was also supported by fruit and floral morphological traits, such as capsules partly or half exserted from the persistent calyx tubes, reticulate testa of seeds, cochlear ascending corolla (rhinanthoid aestivation), late sympetalous corolla tube and axile placentation. In addition, the loss of the adaxial stamen leaving a gap at its expected initiation site and two lateral bracteoles in Rehmannia and Triaenophora may be derived or represent a phylogenetic transitional form in Lamiales. We suggest that Rehmannia and Triaenophora clade should be treated as Orobanchaceae tribe Rehmannieae rather than as a distinct family of Lamiales. The distribution pattern of Rehmannia and Triaenophora indicate that the Shennongjia-Wushan region in central China represents the geographical origin and centre of diversification of the genera.