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The natural hybridization between species Ligularia nelumbifolia and Cremanthodium stenoglossum (Senecioneae, Asteraceae) suggests underdeveloped reproductive isolation and ambiguous intergeneric boundary.

ABSTRACT: Natural hybridization is frequent in plants and is considered an important factor facilitating speciation. The natural intergeneric hybridization between Ligularia and Cremanthodium was previously confirmed using a couple of DNA markers. However, the mechanism of this intergeneric hybridization and the role of reproductive isolation in the process of hybridization remain unclear. Here we used double-digest restriction site-associated DNA sequencing (ddRAD-seq) to further quantify the occurrence of hybridization, the genetic structure of the hybrid population and the role of reproductive isolation between Ligularia nelumbifolia and Cremanthodium stenoglossum. The results based on the ddRAD-seq SNP data sets indicated that hybridization between L. nelumbifolia and C. stenoglossum was restricted to F₁s, and no gene introgression was identified between these two species. STRUCTURE analysis and maximum likelihood (ML) tree results showed a slightly larger genetic contribution of L. nelumbifolia to putative hybrid F₁s. We deduced that the reproductive isolation between these two parent species is not well-developed but still strong enough to maintain the genetic integrity of the species, and that their F₁s are sterile or with low fertility. Given the poorly resolved phylogenetic relationship between Ligularia and Cremanthodium, the occurrence of natural hybridization between L. nelumbifolia and C. stenoglossum may provide new insights into the re-circumscription and re-delimitation of these two genera.

SUBMITTER: Hu L

PROVIDER: S-EPMC7994929 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description: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.

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The natural hybridization between species Ligularia nelumbifolia and Cremanthodium stenoglossum (Senecioneae, Asteraceae) suggests underdeveloped reproductive isolation and ambiguous intergeneric boundary.

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