Evidence for continual hybridization rather than hybrid speciation betweenLigularia duciformisandL. paradoxa(Asteraceae)

Abstract

BackgroundHybrids possess phenotypic traits that are often intermediate between their parental taxa, which commonly serves as evidence of hybridization in morphological analyses. Natural hybridization has been shown to occur frequently inLigularia(Asteraceae). In a previous study,Ligularia×maoniushanensiswas demonstrated as a natural hybrid species betweenL. duciformisandL.paradoxabased on morphological and reproductive traits.MethodsWe used three chloroplast (cpDNA) fragments (psbA-trnH,trnL-rpl32 andtrnQ-5′rps16), the nuclear ribosomal internal transcribed spacer (nrITS), and co-dominant SSR and dominant ISSR markers to study natural hybridization betweenL. duciformisandL. paradoxagrowing sympatrically in two locations. Parental taxa were inferred using network analyses of cpDNA and nrITS haplotypes. Admixture among individuals was examined using the Bayesian clustering programs STRUCTURE and NewHybrids based on the SSR and ISSR data; and potential introgression in the SSR loci was assessed using the INTROGRESS package.ResultsThe putative parental species were clearly distinguished from other sympatricLigulariaspecies by nrITS data, andL.×maoniushanensisindividuals were confirmed to be the hybrid offspring ofL.duciformisandL.paradoxa. Moreover, introgression was detected among several individuals morphologically identified asL.duciformisorL. paradoxa. Analyses of the cpDNA data revealed primarily unidirectional hybridization betweenL. duciformisandL. paradoxa, withL.paradoxaas the maternal parent in Mt. Maoniu, whereas bidirectional but asymmetrical hybridization was inferred to occur in Heihai Lake. The STRUCTURE analyses based on the SSR data detected two distinct clusters among the three taxa. The NewHybrids analyses showed that individuals circumscribed asL.×maoniushanensiswere dominated by early- and later-generation and backcrossing hybrids. The NewHybrids results based on the ISSR data were congruent with SSR results. In addition, introgression was detected in some SSR loci, and heterogeneity among loci was found in terms of detected patterns of introgression.ConclusionsOur data provide strong evidence for hybridization and introgression betweenL.duciformisandL.paradoxa.Ligularia×maoniushanensiswas demonstrated to be of hybrid origin. Since no evident reproductive isolation was found between the two parental species, detected hybrids appear to be part of hybrid swarms resulting from frequent and ongoing gene flow, which might impede the formation of a new hybrid species.