Phylogenetic relationships of enigmatic Sphingiphila (Bignoniaceae) based on molecular and wood anatomical data

Abstract

AbstractSphingiphila is a monospecific genus, endemic to the Bolivian and Paraguayan Chaco, a semi‐arid lowland region. The circumscription of Sphingiphila has been controversial since the genus was first described. Sphingiphila tetramera is perhaps the most enigmatic taxon of Bignoniaceae due to the presence of very unusual morphological features, such as simple leaves, thorn‐tipped branches, and tetramerous, actinomorphic flowers, making its tribal placement within the family uncertain. Here we combined molecular and wood anatomical data to determine the placement of Sphingiphila within the family. The analyses of a large ndhF dataset, which included members of all Bignoniaceae tribes, placed Sphingiphila within Bignonieae. A second, smaller ndhF and pepC dataset, which included only members of tribe Bignonieae, placed the genus within Tanaecium. Unlike most macro‐morphological traits, Sphingiphila is not unusual within Bignoniaceae from a wood anatomical point of view. Sphingiphila shares the presence of narrow vessels and vasicentric to aliform confluent parenchyma with the rest of the family. In addition, Sphingiphila has several specific wood anatomical traits, such as vessels in a diagonal to tangential arrangement, small intervessel pits, and non‐storied heterocellular rays with occasional perforated ray cells. These features suggest that the genus is best placed either in Tecomeae s.str. or Bignonieae, with a better placement in Bignonieae due to its abundant parenchyma, despite lacking the cambial variant synapomorphic for this group. Sphingiphila and Tanaecium form a clade that is strongly supported by molecular characters, including two indels that are molecular synapomorphies of this clade. In addition, careful morphological inspections show that S. tetramera shares with Tanaecium the subulate, bromeliad‐like prophylls, the most evident synapomorphy of this genus apart from long, tubular, villose corollas, and a lepidote ovary. Given the molecular phylogenetic placement of S. tetramera and its macro‐morphological and wood anatomical similarities with Tanaecium, we propose the new combination Tanaecium tetramerum.