Phylogenetic placement of enigmatic Astianthus (Bignoniaceae) based on molecular data, wood and bark anatomy

Abstract

Background: Astianthus is a monospecific arborescent genus of Bignoniaceae that occur in the Pacific Coast of central Mexico and northern Central America, where it grows in dense populations along riversides. Its phylogenetic placement has remained controversial since Astianthus has unusual morphological characters such as a four-loculed ovary, and simple, pulvinate, verticillate leaves.
 Methods: Here we used three plastid markers ndhF, rbcL, and trnL-F, wood, and bark anatomical data to investigate the phylogenetic placement of Astianthus and assign it to one of Bignoniaceae’s main clades.
 Results: Our molecular phylogenetic analyses indicated that Astianthus belongs in tribe Tecomeae s.s., where other charismatic Neotropical Bignoniaceae genera such as Campsis and Tecoma are currently placed. Wood and bark anatomy support this placement, as Astianthus reunites a unique combination of features only known from members of Tecomeae s.s., such as storied axial parenchyma, the co-occurrence of homo- and heterocellular rays, septate fibers, and scattered phloem fibers in the bark.
 Conclusions: The placement of Astianthus within Tecomeae s.s. provides further support to previous proposals for the Neotropical origin of this Pantropical tribe.

Bark anatomy of lianescent Bignoniaceae: a generic synopsis

“…The first study aiming to describe Bignoniaceae bark structure is that of Roth (1981), where the phloem is described as extremely homogeneous, marked by a stratified structure, with an alternation of fiber bands. Subsequent works, on the other hand, have disputed this statement, showing that although a stratified phloem is prevalent, some genera show a non-stratified phloem, with either scattered fibers (e.g., Rajput et al 2017; Pace et al 2021), or fibers forming the entire background tissue (Pace et al 2015a). Here we confirm that although the bark tends to be homogeneous within some clades, it is not homogeneous in the family as a whole and can be extremely informative in sorting genera.…”

“…; Lohmann 2004; Heywood et al 2007; Pace et al 2021). The bark of Bignoniaceae has been increasingly studied by Roth (1981) and Pace et al (2011, 2015a, 2021). The former described rather similar bark types in two species of Jacaranda and three species of Tabebuia Gomes ex DC.…”

“…The family includes many species of economical importance, especially due to their high-quality wood used for timber (e.g., Paratecoma peroba (Record) Kuhlm., several Handroanthus Mattos; Record & Hess 1972) and their ornamental value (e.g., Tecoma stans Juss., Campsis radicans (L.) Seem. ; Lohmann 2004; Heywood et al 2007; Pace et al 2021). The bark of Bignoniaceae has been increasingly studied by Roth (1981) and Pace et al (2011, 2015a, 2021).…”

“…A different distribution of these fibers, scattered in the phloem instead of stratified, served as a basis to support the inclusion of Astianthus D.Don. within Tecomeae s.s ., and its position as sister to Campsis Lour., a taxon that shows identical fiber distribution (Pace et al 2021; also explored here for Campsis ). In turn, Pace et al (2011, 2015a, 2021) studied differences between the regular and variant secondary phloem in lianas and explored how some variable key cell types evolved within Bignonieae, the largest Bignoniaceae tribe.…”

“…Characters from stem and wood anatomy (Dos Santos 1995) support clades within this tribe (Lohmann 2003, 2006). The importance of wood anatomy in systematic studies of the Bignoniaceae has grown substantially during the past three decades (Dos Santos & Miller 1992, 1997; Dos Santos 1995; Pace & Angyalossy 2013; Pace et al 2015b), contributing important information for an improved circumscription of enigmatic genera within the family (Pace et al 2016, 2021) and allowing to place fossil wood at the generic rank (Moya & Brea 2018; Franco et al 2021). Here we aim to: i) describe the phloem and periderm anatomy of all 20 genera currently recognized within tribe Bignonieae, plus the three most cultivated lianescent genera of Tecomeae s.s.…”