Abstract
Natural hybridization has been considered to represent an important factor influencing the high diversity of the genus Ligularia Cass. in the Hengduan Mountains, China. Natural hybridization has been confirmed to occur frequently in Ligularia. To date, however, it has been demonstrated only within a single population. In this paper, we present evidence of natural hybridization in Ligularia from four different locations. The internal transcribed spacer (ITS) region of the nuclear ribosomal DNA and three chloroplast intergenic spacers (trnK-rps16, trnL-rpl32 and trnQ-5'rps16) of 149 accessions of putative hybrids and their putative parents (L. cymbulifera and L. tongolensis) were analyzed for evidence of hybridization. The ITS data clearly distinguished two putative parental species and sympatric L. vellerea and supported the hypothesis that those morphological intermediates were products of natural hybridization between L. cymbulifera and L. tongolensis. Moreover, several identified morphological parents were actual introgressed products. Because of hybridization and introgression, chloroplast DNA sequences generated a poorly resolved network. The present results indicate that varying degrees of hybridization and introgression occur differently depending on the habitat context. We conclude that gene flow caused by natural hybridization in Ligularia indeed plays an important role in the species diversity.
Highlights
The importance of hybridization in plant speciation and evolution has been debated for decades, with opposing views of hybridization as either a creative evolutionary force or evolutionary noise [1,2,3,4,5,6,7,8]
All the putative natural hybrids exhibited more than 10 double peaks, and the cloned sequences mainly included two sequence types; other haplotypes not included in the pure sequences may have resulted from PCR and plasmid amplification error
The putative hybrids shared the same copies with the putative parents L. cymbulifera and L. tongolensis
Summary
The importance of hybridization in plant speciation and evolution has been debated for decades, with opposing views of hybridization as either a creative evolutionary force or evolutionary noise [1,2,3,4,5,6,7,8]. The high species diversity of the genus Ligularia is considered to have arisen from rapid and continuous allopatric speciation in small and isolated populations, combined with interspecific diploid hybridization in the QTP and adjacent areas of eastern Asia [11, 14]. A study on natural hybridization between L. subspicata and L. vellerea (Franchet) Handel-Mazzetti confirmed this conjecture [16]. Based on morphological and molecular data (cpDNA and ITS sequences, ISSR markers), bidirectional but asymmetric hybridization and introgression were detected between L. subspicata and L. nelumbifolia (Bureau & Franchet) Handel-Mazzetti. Hybridization is very common in Ligularia, with various forms and progenies These instances were all found in a single plot, and there has been no natural hybridization reported from multiple sites to date
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