Within the legume family, the taxonomic status of subtribe Glycyrrhizinae of tribe Galegeae and of the genus Adinobotrys has been re-assessed. Based on genome skimming data, we conducted phylogenomic analyses of the inverted repeat-lacking clade within subfamily Papilionoideae. The results support the sister relationship between Glycyrrhizeae and Adinobotrys. Glycyrrhizeae is resurrected based on Glycyrrhiza and Glycyrrhizopsis, and a new tribe, Adinobotryeae, is proposed to accommodate Adinobotrys.

Adinobotrys, Fabaceae, genome skimming, Glycyrrhiza, IRLC legumes, the GAW clade, Wisterieae

Within subfamily Papilionoideae of Leguminosae, a series of molecular phylogenetic studies have supported a monophyletic group that lost one copy of the 25-kilobase inverted repeat (IR) in the plastid genome, known as the “IR-lacking clade” or IRLC (Lavin et al. 1990; Sanderson and Liston 1995; Sanderson and Wojciechowski 1996; Doyle et al. 2000; Wojciechowski 2003). The IRLC harbours many economically important plants, for example, alfalfa, beans, clovers, lentils, peas, vetches, chickpeas and liquorice (Lewis et al. 2005). Recently, phylogeneticists have revealed an early branching clade, the Glycyrrhizeae-Adinobotrys-Wisterieae clade [i.e. the GAW clade named by Duan et al. (2021)], as sister to the remaining IRLC taxa (LPWG 2013, 2017; Duan et al. 2020, 2021; Xia et al. 2021).

Within the GAW clade, the temperate herbaceous genera Glycyrrhiza L., with Meristotropis Fisch. & C.A.Mey. being treated within Glycyrrhiza, and Glycyrrhizopsis Boiss. are sister taxa (Duan et al. 2020, 2021; also see Fig. 1), Tribe Glycyrrhizeae was proposed by Rydberg (1917) to accommodate Glycyrrhiza, which was degraded as subtribe Glycyrrhizinae of tribe Galegeae by Rydberg (1923) and was then slightly emended by Polhill (1981) with Glycyrrhizopsis species treated as a synonym of Glycyrrhiza. Although Glycyrhizeae was not widely accepted by taxonomists, the taxonomic position of Glycyrrhizinae is also questionable. Galegeae is a polyphyletic and morphologically diverse tribe (Sanderson and Liston 1995; Lock and Schrire 2005; Duan et al. 2015), and our prior phylogenetic studies have shown that Glycyrrhizinae is only distantly related to core Galegeae (Duan et al. 2015, 2021). In consequence, the taxonomic rank of Glycyrrhizeae/Glycyrrhizinae needs to be reconsidered.

Figure 1. 

Representative plants of Adinobotryeae and Glycyrrhizeae. Inflorescences (A) and fruits (B) of Adinobotrys atropurpureus; inflorescence (C) and infructescence (D) of Glycyrrhiza pallidiflora; inflorescence (E) and infructescence (F) of Glycyrrhiza uralensis; fruits (G) of Glycyrrhiza inflata; inflorescence (H) and part of dried infructescence [I; photographed on herbarium specimen: A. Eustace 31 (E!)] of Glycyrrhizopsis flavescens.

Apart from Glycyrrhizinae, the remaining taxa of the GAW clade included some woody genera, formerly placed in tribe Millettieae s.l., which were subsequently assigned into two clades: the liana (rarely climbing shrubs) tribe Wisterieae and the tree genus Adinobotrys Dunn (Compton et al. 2019; Duan et al. 2021; also see Fig. 1). Previous phylogenetic results excluded Adinobotrys from Wisterieae or Millettieae (Hu et al. 2000; Duan et al. 2021), but further taxonomic study is required to re-assess the placement of this genus.

In the present study, we used the genome skimming method (Straub et al. 2012; Zhang et al. 2015) to obtain 75 chloroplast (cp) coding sequences (CDSs), and constructed a phylogenomic framework of the GAW clade to test the taxonomic position and status of genus Adinobotrys and tribe Glycyrrhizeae. Our study provides a taxonomic foundation for future evolutionary, systematic and biogeographical studies of the IRLC legumes.

All the sequenced plastomes were successfully assembled into complete circular configurations. The sizes of the cp genomes ranged from 122,310 to 156,702 bp and the GC contents were between 33.7% and 35.9% (see Suppl. material 1 for details). The alignment length of concatenated cp CDSs for the analyses in MrBayes and IQ-TREE was 72,106 bp. Our ML results were congruent in topology with the corresponding BI trees, and the support values of the former (as “LBS” hereafter) were thus labelled on the corresponding branches of the latter (as “PP” hereafter). Phylogenetic reconstructions (Fig. 2) supported the monophyly of the IRLC (PP = 1, LBS = 100%), and the GAW clade (PP = 1, LBS = 71%) was sister to the rest of the IRLC taxa (PP = 1, LBS = 100%). Within the GAW clade, Adinobotrys and the well-supported Glycyrrhizeae (PP = 1, LBS = 100%) formed a clade (PP = 1, LBS = 80%), which was sister to tribe Wisterieae (PP = 1, LBS = 100%).

Figure 2. 

Bayesian maximum clade credibility tree of the GAW clade and related groups based on chloroplast CDSs. Bayesian posterior probabilities are given above branches, Maximum Likelihood bootstrap values below branches. Asterisks indicate PP = 1 and LBS = 100%. W. nieuwenhuisii indicates Whitfordiodendron nieuwenhuisii.

Within the GAW clade, some tropical/subtropical (rarely temperate) woody liana/tree species, formerly belonging to Millettieae s.l., clustered with the temperate herbaceous Glycyrrhiza-Glycyrrhizopsis clade (Compton et al. 2019; Duan et al. 2020, 2021; also see Fig. 2). Recently, Compton et al. (2019) and Duan et al. (2021) assigned the above-mentioned liana/tree group into two non-sister clades: genus Adinobotrys and tribe Wisterieae (14 genera), corroborated by our cp CDSs trees (Fig. 2). Herein, we propose a monogeneric tribe Adinobotryeae based on Adinobotrys, and resurrect the tribe Glycyrrhizeae corresponding to the Glycyrrhiza-Glycyrrhizopsis clade (see Taxonomic Treatment) for the following reasons:

A. Our phylogenomic analyses (Fig. 2) validated that Adinobotrys does not belong to Wisterieae; and the Glycyrrhiza-Glycyrrhizopsis clade is phylogenetically distant from both of the core groups of Galegeae [i.e. the Erophaca-Astragalean clade; see Lock and Schrire (2005)] and the type genus (i.e. Galega L.) (also see Wojciechowski et al. 2004; Duan et al. 2015, 2021; Compton et al. 2019). It is thus reasonable to divide the GAW clade into three independent groups to ensure monophyly of Adinobotrys and the Glycyrrhiza-Glycyrrhizopsis clade with respect to tribe Wisterieae.

B. Our recent analyses indicated that a chloroplast capture event may have occurred in the GAW clade, and the common ancestors of Adinobotrys and Wisterieae were hypothesised to be the putative paternal and maternal parents of the Glycyrrhiza-Glycyrrhizopsis clade, respectively (Duan et al. 2021). Since Wisterieae was already known as a well-defined tribe in the GAW clade, its counterpart groups (Adinobotrys and the Glycyrrhiza-Glycyrrhizopsis clade) should also be recognised at the tribal rank. Such an evolutionary event-linked taxonomy was also noted in previous studies (e.g. Degtjareva et al. 2012).

C. As far as morphological differentiation is concerned, Glycyrrhizeae, containing the widely distributed Glycyrrhiza and the Anatolian endemic Glycyrrhizopsis Boiss. (Meng 2005; Duan et al. 2020; see Fig. 1C–I), is easily distinguished from Wisterieae and Adinobotrys by its herbaceous habit and temperate distribution. Adinobotrys, which has often been treated as part of Callerya Endl. within Wisterieae (Polhill 1981; Schot 1994; Schrire 2005), has a unique evergreen tree habit in contrast to the predominantly liana habit of Wisterieae (Fig. 1A, B; also see Compton et al. 2019; Compton and Schrire 2020).

Despite the tribal revision above, taxonomic questions still remain. Since Glycyrrhizeae, and Astragaleae sensu Duan et al. (2021), corresponding to the aforementioned Erophaca-Astragalean clade, were removed from tribe Galegeae, the remaining genus Galega was found to be nested within the Vicioid clade (Steele and Wojciechowski 2003; Wojciechowski et al. 2004; Duan et al. 2021). Therefore, tribe Galegeae needs further taxonomic consideration before either being merged into a larger tribe corresponding to the Vicioid clade, or re-instated in its earlier, narrower sense, i.e. as the monogeneric tribe circumscribed by Hutchinson (1964). In addition, the recent work of Compton and Schrire (2020) expanded the genus Adinobotrys sensu Compton et al. (2019) to contain four species, although its infra-generic relationships need to be studied in more detail.

Thanks to the curators of the following herbaria for providing specimen photos, leaf samples, seeds or DNA samples: A, CSH, E, IBSC, K, KNYA, KUN, LE, MA, MO, P, PE, TASH, US, WUK and XJBI. We thank Artem Leostrin for literature searches; and Cheng Du, Hiroaki Hayashi, Imalka Kahandawala, Jia-Hui Lu, Jing-Zhi Gong, Kathryn Richardson, Liang Zhao, Pei-Liang Liu, Shi-Jin Li, Shi-Min Duan, Virginia Valcárcel, Wei Shi, Zhao-Yang Chang, Zhen-Hai Wu and Zhuo Zhou for sample collecting. The project was supported by the General Program of National Natural Science Foundation of China (Grant No. 32070229) and the Forestry Flora and Fauna Conservation Project of Guangdong Province (Grant no. E036011002).