Abstract
Natural hybridization is common in plants and results in different evolutionary consequences to hybridizing species. Pre- and post-zygotic reproductive isolating barriers can impede hybridization between closely related species to maintain their species integrity. In Northwest Yunnan, three Ligularia species (Ligularia cyathiceps, L. duciformis and L. yunnanensis) and four types of morphologically intermediate individuals were discovered growing together in an area subject to human disturbance. In this study, we used three low-copy nuclear loci to test the natural hybridization hypothesis and the hybridization direction was ascertained by three chloroplast DNA fragments. The results indicated there were two hybridization groups at the study site, L. cyathiceps × L. duciformis and L. duciformis × L. yunnanensis, and two types of morphologically intermediate individuals were produced by L. cyathiceps and L. duciformis, and another two types were produced by L. duciformis and L. yunnanensis, while no hybrids between L. cyathiceps and L. yunnanensis were observed. Both hybridizing groups showed bidirectional but asymmetric hybridization and the factors influencing the symmetry are discussed. Most hybrids produced by the two hybridization groups seemed to be F1 generation. Hybrids with different morphologies within the same hybridization group showed similar genetic components. The results suggest that although human disturbance may promote natural hybridization among the three Ligularia species bringing them together, hybrids are limited to F1s and therefore species boundaries might be maintained.
Highlights
Natural hybridization is common across plants, in rapidly radiating groups (Mallet 2005), and can result in both positive and negative evolutionary outcomes (Barton 2001)
A total of six haplotypes were derived, and low levels of haplotype polymorphism were observed in L. cyathiceps, L. duciformis and L. yunnanensis which had two, two and two haplotypes, respectively (Table 2; Fig. 2A)
Individuals S8 and S16 were homozygous for a L. duciformis haplotype, while the other four individuals H12, S1, S4 and S32 were homozygous for a L. yunnanensis haplotype
Summary
Natural hybridization is common across plants, in rapidly radiating groups (Mallet 2005), and can result in both positive and negative evolutionary outcomes (Barton 2001). Species integrity is maintained by pre- and post-zygotic reproductive isolating barriers preventing. Human disturbance is regarded as an important promoter of hybridization (Anderson 1948; Bleeker and Hurka 2001), and previously isolated species may come into contact and hybridize due to human alterations to the environment (Rhymer and Simberloff 1996; Seehausen et al 2008; Crispo et al 2011; Bohling et al 2016). The direction of hybridization is affected by both pre- and post-zygotic barriers and asymmetric hybridization frequently occurs in plants as a result of differences in the strength of reproductive barriers between hybridizing species (Bacilieri et al 1996; Arnold 1997; Ma et al 2014; Zhang et al 2016)
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