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Abstract
Hybridization accompanied by polyploidization and apomixis has been demonstrated as a driving force in the evolution and speciation of many plants. A good example to study the evolutionary process of hybridization associated with polyploidy and apomixis is the genus Cotoneaster (Rosaceae), which includes approximately 150 species, most of which are polyploid apomicts. In this study, we investigated all Cotoneaster taxa distributed in a small region of Malipo, Yunnan, China. Based on the morphological characteristics, four Cotoneaster taxa were identified and sampled: C. dielsianus, C. glaucophyllus, C. franchetii, and a putative hybrid. Flow cytometry analyses showed that C. glaucophyllus was diploid, while the other three taxa were tetraploid. A total of five low-copy nuclear genes and six chloroplast regions were sequenced to validate the status of the putative hybrid. Sequence analyses showed that C. dielsianus and C. glaucophyllus are distantly related and they could be well separated using totally 50 fixed nucleotide substitutions and four fixed indels at the 11 investigated genes. All individuals of the putative hybrid harbored identical sequences: they showed chromatogram additivity for all fixed differences between C. dielsianus and C. glaucophyllus at the five nuclear genes, and were identical with C. glaucophyllus at the six chloroplast regions. Haplotype analysis revealed that C. dielsianus possessed nine haplotypes for the 11 genes, while C. glaucophyllus had ten, and there were no shared haplotypes between the two species. The putative hybrid harbored two haplotypes for each nuclear gene: one shared with C. dielsianus and the other with C. glaucophyllus. They possessed the same chloroplast haplotype with C. glaucophyllus. Our study provided convincing evidence for natural hybridization between C. dielsianus and C. glaucophyllus, and revealed that all hybrid individuals were derivatives of one initial F1 via apomixes. C. glaucophyllus served as the maternal parent at the initial hybridization event. We proposed that anthropological disturbance provided an opportunity for hybridization between C. dielsianus and C. glaucophyllus, and a tetraploid F1 successfully bred many identical progenies via apomixis. Under this situation, species integrity could be maintained for these Cotoneaster species, but attentions should be kept for this new-born hybrid.
Highlights
Hybridization, previously viewed as a mere side branch or noise of evolution, is recognized as a major evolutionary force and a significant portion of speciation (e.g., Arnold, 1997; Rieseberg and Willis, 2007; Soltis and Soltis, 2009; Soltis et al, 2014)
As early as 1917, Winge first introduced a theory linking the formation of hybridization and the development of polyploids, proposing that reproductive isolation could be rapidly established between new polyploids and their parental species, so that new polyploid hybrid species could arise in just a few generations (Winge, 1917)
We focused our objectives on a limited area of Malipo county, Yunnan, China, where a species with erect red petals, a species with erect pink petals (C. franchetii), a species with spreading white petals (C. glaucophyllus), and an unidentified taxon with intermediate characteristics between C. dielsianus and C. glaucophyllus can be found (Figure 1; Table 1)
Summary
Hybridization, previously viewed as a mere side branch or noise of evolution, is recognized as a major evolutionary force and a significant portion of speciation (e.g., Arnold, 1997; Rieseberg and Willis, 2007; Soltis and Soltis, 2009; Soltis et al, 2014). The process of hybridization can help us understand the origin of adaptations, the maintenance of plant diversity, and the formation of new species. It was proposed that “allopolyploidy, perhaps more than any other process, has played a major role in the origin of many species and has driven and shaped the evolution of vascular plants” (Feldman and Levy, 2005) and many angiosperms are of ancient polyploid origin (Wagner and Wagner, 1980). A potential evolutionary solution to this problem is asexual reproduction, i.e., apomixis or agamospermy (Asker and Jerling, 1992; Sochor et al, 2015). With usually uniparental reproduction, lowered cost of sex, maintenance of adapted genotypes and occasional seed reproduction (Hörandl, 2006), many apomictic plants can achieve great ecological and evolutionary success. Apomixis has been well documented in numerous genera of Rosaceae, Cotoneaster (Nybom and Bartish, 2007), Crataegus (Lo et al, 2009), Rubus (Sochor et al, 2015), Sorbus (Robertson et al, 2010; Ludwig et al, 2013), and Potentilla sensu lato (Morgan et al, 1994)
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