Morphological and molecular evidence clarifies the taxonomic position of Indosasa solearis and reveals the hybrid origin of Sinobambusa (Poaceae: Bambusoideae: Arundinarieae)

The three-branched bamboos (Arundinarieae) form a taxonomically difficult group due to recent rapid radiation and frequent hybridization, with generic and species delimitations of many taxa problematic. Indosasa solearis , described from North Vietnam based on flowering materials, was later transferred to Sinobambusa . But its taxonomic position was controversial due to limited vegetative data. After a detailed morphological study of this bamboo at its type locality, we found that the overall morphology of this species is surprisingly similar to I. parvifolia and I. jinpingensis . Thus, we conducted morphological and phylogenomic analyses to ascertain the taxonomic position of I. solearis and its relationship with those two species. Morphological examination confirmed I. solearis exhibits diagnostic characters of Indosasa , like leptomorph rhizomes, diffuse culms, strongly and asymmetrically raised branch supra-nodal ridges, 4–9 foliage leaves per ultimate branch, pseudospikelets, and each floret with six stamens and three stigmas. No essential difference was found among the three Indosasa species both on macro- and micromorphology. Phylogenies based on single-copy orthologous nuclear gene datasets and single nucleotide polymorphism (SNP) data strongly support I. solearis as an Indosasa member, closely related to I. parvifolia and I. jinpingensis . Based on the combined morphological and molecular evidence, we confirmed that I. solearis belongs to Indosasa , and is conspecific with I. parvifolia and I. jinpingensis . We therefore reinstate I. solearis and synonymize I. parvifolia and I. jinpingensis with it. Furthermore, our gene flow analyses by SNP data revealed the hybrid origin of the genus Sinobambusa , with genomic signatures suggesting an ancient cross between Indosasa and Pleioblastus . This finding highlights the reticulate evolutionary history in the three-branched bamboos.