Origin of the critically endangered endemic species Scrophularia takesimensis (Scrophulariaceae) on Ulleung Island, Korea: implications for conservation

Abstract

Scrophularia takesimensis is a critically endangered endemic species of Ulleung Island, Korea. A previous molecular phylogenetic study based on nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) sequences with very limited sampling suggested that it is most closely related to the clade comprising S. alata and S. grayanoides. To determine the origin of S. takesimensis, we sampled a total of 171 accessions including S. takesimensis (9 populations and 63 individuals) and two closely related species, S. alata (11 populations and 68 individuals) and S. grayanoides (5 populations and 40 individuals) from eastern Asia and sequenced ITS and two chloroplast DNA (cpDNA) non-coding regions. Previously sequenced representative species of Scrophularia (109 taxa for ITS and 80 taxa for cpDNA) were combined with our data set and analyzed. While the global scale ITS phylogenetic tree suggests monophyly for each of the three eastern Asian species, S. takesimensis appears to be more closely related (albeit weakly) to a clade containing eastern North American/Caribbean species than to either S. alata or S. grayanoides. By contrast, the global scale cpDNA phylogenetic tree demonstrates that the eastern North America/Caribbean clade is sister to a clade comprising the three eastern Asian species. In addition, the monophyletic S. takesimensis is deeply embedded within paraphyletic S. alata, sharing its most recent common ancestor with populations from Japan/Sakhalin. Two divergent, geographically structured cp haplotype groups within S. takesimensis suggest at least two independent introductions from different source areas. A new and accurate chromosome number of S. takesimensis (2n = 94) is reported and some conservation strategies are discussed.