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Abstract
BackgroundDuring adaptive radiation events, characters can arise multiple times due to parallel evolution, but transfer of traits through hybridization provides an alternative explanation for the same character appearing in apparently non-sister lineages. The signature of hybridization can be detected in incongruence between phylogenies derived from different markers, or from the presence of two divergent versions of a nuclear marker such as ITS within one individual.Methodology/Principal FindingsIn this study, we cloned and sequenced ITS regions for 30 species of the genus Rheum, and compared them with a cpDNA phylogeny. Seven species contained two divergent copies of ITS that resolved in different clades from one another in each case, indicating hybridization events too recent for concerted evolution to have homogenised the ITS sequences. Hybridization was also indicated in at least two further species via incongruence in their position between ITS and cpDNA phylogenies. None of the ITS sequences present in these nine species matched those detected in any other species, which provides tentative evidence against recent introgression as an explanation. Rheum globulosum, previously indicated by cpDNA to represent an independent origin of decumbent habit, is indicated by ITS to be part of clade of decumbent species, which acquired cpDNA of another clade via hybridization. However decumbent and glasshouse morphology are confirmed to have arisen three and two times, respectively.ConclusionsThese findings suggested that hybridization among QTP species of Rheum has been extensive, and that a role of hybridization in diversification of Rheum requires investigation.
Highlights
Adaptive radiation events are a significant source of new species, ecological diversity and morphological innovation [1,2,3,4]
Most species samples were collected in the Qinghai–Tibetan Plateau (QTP), and voucher specimens were deposited in the herbaria of Northwest Plateau Institute of Biology (HNWP), the Chinese Academy of Science, and School of life Sciences, Lanzhou University, China (Table 1)
We firstly sequenced the amplification products directly. Most of these sequences can not be identified exactly, especially for seven species, i.e. R. hotaoense, R. officinale, R. tanguticum, R. pumilum, R. likiangense, R. franzenbachii and R. reticulatum, mainly because there are a lot of impurity peaks
Summary
Adaptive radiation events are a significant source of new species, ecological diversity and morphological innovation [1,2,3,4]. Such events are well known on oceanic islands [3,5,6], they may occur on continental landmasses if significant climatic and/or geological upheavals have created new ecological niches [4,7,8]. Hybrid speciation can confer tolerance of new habitats [26,27], and may contribute to radiation events where many new niches are available to colonise [28,29,30,31,32] or even trigger them [24]. The signature of hybridization can be detected in incongruence between phylogenies derived from different markers, or from the presence of two divergent versions of a nuclear marker such as ITS within one individual
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