Species delimitation in Orychophragmus (Brassicaceae) based on chloroplast and nuclear DNA barcodes

Abstract

AbstractSpecies delimitation is fundamental to the basic understanding of biodiversity because of the central role that the concept of species plays in most branches of biology. However, few studies have been designed to test conflicting delineations of plant species under an integrated species concept using DNA barcodes in combination with other lines of evidence. Such an approach may deliver more objective, testable and uniform species units as subjects for a range of studies. Here we aim to examine competing hypotheses of species delimitation in Orychophragmus, a member of the mustard family, based on these methods and principles. Two to seven species have previously been recognized in this genus by different taxonomists. We sequenced five commonly used DNA barcodes (nuclear ribosomal ITS and chloroplast matK, rbcL, trnH‐psbA, and trnL‐F) for 160 individuals collected across the major distribution ranges of all taxa currently recognized in the genus. Based on the monophyletic clusters produced by analysis of the combined nrITS and cpDNA sequence variations, we recovered nine independent evolutionary lineages that were further supported by diagnosable morphological traits, distinct inter‐cluster genetic gaps, reproductive isolation, and geographical distribution. These lineages may be treated as nine species. We also found substantial differences in the capacity of nrITS and cpDNA barcodes to discriminate between closely related species in two clades of the genus. Our empirical study of Orychophragmus highlights the importance of applying both chloroplast DNA and nrITS barcodes for species delimitation in plants.