The distinct plastid genome structure of Maackia fauriei (Fabaceae: Papilionoideae) and its systematic implications for genistoids and tribe Sophoreae

Traditionally, the tribe Sophoreae sensu lato has been considered a basal but also heterogeneous taxonomic group of the papilionoid legumes. Phylogenetic studies have placed Sophoreae sensu stricto (s.s.) as a member of the core genistoids. The recently suggested new circumscription of this tribe involved the removal of traditional members and the inclusion of Euchresteae and Thermopsideae. Nonetheless, definitions and inter- and intra-taxonomic issues of Sophoreae remain unclear. Within the field of legume systematics, the molecular characteristics of a plastid genome (plastome) have an important role in helping to define taxonomic groups. Here, we examined the plastome of Maackia fauriei, belonging to Sophoreae s.s., to elucidate the molecular characteristics of Sophoreae. Its gene contents are similar to the plastomes of other typical legumes. Putative pseudogene rps16 of Maackia and Lupinus species imply independent functional gene loss from the genistoids. Our overall examination of that loss among legumes suggests that it is common among all major clades of Papilionoideae. The M. fauriei plastome has a novel 24-kb inversion in its large single copy region, as well as previously recognized 50-kb and 36-kb inversions. The 36-kb inversion is shared by the core genistoids. The 24-kb inversion is present in the eight genera belonging to three tribes: Euchresteae, Sophoreae s.s., and Thermopsideae. The phylogenetic distribution of this 24-kb inversion strongly supports the monophyly of members of Sophoreae s.s. with Euchresteae and Thermopsideae. Hence, it can be used as a putative synapomorphic characteristic for the newly circumscribed Sophoreae, including Euchresteae and Thermopsideae. However, plastome conformation suggests a slightly narrower taxonomic group because of heterogeneous results from Bolusanthus and Dicraeopetalum. The phylogenetic analysis, based on plastome sequences from 43 legumes, represents well our understanding of legume systematics while resolving the genistoid clade as a sister group to an Old World clade. It also demonstrates the value that plastomes are powerful marker for systematic studies of basal papilionoid legumes.