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 in Ligularia (Asteraceae). In a previous study, Ligularia ×maoniushanensis was demonstrated as a natural hybrid species between L. duciformis and L. paradoxa based on morphological and reproductive traits.MethodsWe used three chloroplast (cpDNA) fragments (psbA-trnH, trnL-rpl32 and trnQ-5′rps16), the nuclear ribosomal internal transcribed spacer (nrITS), and co-dominant SSR and dominant ISSR markers to study natural hybridization between L. duciformis and L. paradoxa growing 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 sympatric Ligularia species by nrITS data, and L. ×maoniushanensis individuals were confirmed to be the hybrid offspring of L. duciformis and L. paradoxa. Moreover, introgression was detected among several individuals morphologically identified as L. duciformis or L. paradoxa. Analyses of the cpDNA data revealed primarily unidirectional hybridization between L. duciformis and L. paradoxa, with L. paradoxa as 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 as L. ×maoniushanensis were 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 between L. duciformis and L. paradoxa. Ligularia ×maoniushanensis was 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.
Highlights
Natural hybridization, which increasingly appears to play a key role in speciation, has been frequently reported between closely related species exhibiting sympatric or parapatric distributions, especially in plants (Abbott et al, 2010; Abbott et al, 2013; Arnold, 1997; Hegarty & Hiscock, 2005; Soltis & Soltis, 2009)
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
This study aimed to address the following questions: (1) Are morphologically intermediate individuals the hybrid progeny of L. duciformis and L. paradoxa? Does the sympatric species L. lamarum participate in hybridization; (2) If hybridization occurs, what is the directionality of this process among parental taxa; (3) Is there any evidence for introgressive hybridization, and if so what is the genetic structure of the hybrid swarms; (4) Is L. ×maoniushanensis a cohesive hybrid species, or do the individuals currently described as L. ×maoniushanensis instead constitute a recurrently formed hybrid swarm?
Summary
Natural hybridization, which increasingly appears to play a key role in speciation, has been frequently reported between closely related species exhibiting sympatric or parapatric distributions, especially in plants (Abbott et al, 2010; Abbott et al, 2013; Arnold, 1997; Hegarty & Hiscock, 2005; Soltis & Soltis, 2009). Resultant hybrids which have been adapted to habitats in the hybrid zone may further recombine parental traits to produce novel characters. Hybrid speciation often is thought not to take place instantaneously; instead, it may be a long-term, gradualistic population process In such a process, initial generations of hybrids frequently backcross to their parents, causing them to fail to establish themselves as independent lineages with stable trait combinations; this situation is termed introgressive hybridization, and such mixed taxa may form a hybrid swarm (Nolte & Tautz, 2010), in which event these hybrids generally would not be treated as new species (Zhou, Shi & Wu, 2005). Ligularia ×maoniushanensis was demonstrated as a natural hybrid species between L. duciformis and L. paradoxa based on morphological and reproductive traits. Introgression was detected in some SSR loci, and heterogeneity among loci was found in terms of detected patterns of introgression
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