Molecular Evidence for Natural Hybridization between Cotoneaster dielsianus and C. glaucophyllus

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.