From nine to two: Ultra-barcoding develops a rigorous species
delimitation framework in Paris section Axiparis (Melanthiaceae)
Species delimitation is a difficult task in traditional morphology-based
taxonomy. The ultra-barcoding approach, which uses whole plastid genomes
(plastomes) and nuclear ribosomal DNA (nrDNA) regions as extended DNA
barcodes for species identification and delimitation, has been
recommended as one of the candidate techniques for plant barcoding 2.0.
Yet, the efficacy of this approach in delineating species boundaries
remains poorly understood. Here, we attempt to decipher species
delimitation in the taxonomically challenging clade, Paris L. section
Axiparis H. Li, using phylogenetic inference and multiple sequence-based
species delimitation methods (ABGD, SDP, and mPTP) utilizing complete
plastomes and nrDNA clusters from multiple accessions per described
species. The results suggest that only two species-level taxonomic units
that possess not only morphological uniqueness but also genetic
distinctiveness and evolutionary independence can be recognized in P.
sect. Axiparis. Therefore, previous taxonomic work overemphasized minor
intraspecific morphological differences to establish species, and thus
resulted in proliferation of as many as seven synonyms in this clade.
Inferred from ultra-barcoding analyses, we propose a taxonomic revision
of P. sect. Axiparis which will help inform future decisions regarding
species conservation in the commercially valuable and severely
threatened genus Paris. This case study indicates that the
ultra-barcoding approach has great promise for developing a rigorous
species delimitation framework that will facilitate credible taxonomic
revision especially in taxonomically difficult plant taxa.