Abstract
Delimitating species boundaries is the primary aim of biological classification and could be critical for evaluating the evolving process of species and conserving biodiversity. Rhododendron is an iconic group with an extraordinary diversity in southwest China. However, it remains unknown whether the recorded species therein comprise independently evolving lineages or artificially delimitated morphological entities. In this study, we carried out species delimitation of four Rhododendron species in the R. vernicosum-R. decorum species complex based on morphological analyses and population genetic data from nuclear simple sequence repeats (SSR) markers. We randomly selected a total of 105 specimens of different individuals identified as four species across their distributional ranges to examine the statistically distinct phenotypic clusters based on 19 morphological traits. Similarly, we genotyped 55 individuals of four species from 21 populations using 15 SSR markers. The morphological analyses sorted R. decorum and the other three species into two different phenotypic clusters. The genetic clusters were consistent with the morphological clusters. However, we also recovered the third genetic cluster, comprising six R. vernicosum populations and containing the admixed genetic compositions of the other two distinct genetic clusters. This hybrid group was morphologically similar to the typical R. vernicosum (including the samples from its type specimen locality and both R. verruciferum and R. gonggashanense) but with more genetic ancestry from R. decorum. Based on our findings, we identify two distinct species and one putative hybrid group due to introgression in the R. vernicosum-R. decorum species complex. We propose to merge R. verruciferum and R. gonggashanense into R. vernicosum based on genetic compositions and our morphological analyses. The hybrid group inferred from our findings, however, needs further investigations.
Highlights
Accurate species delimitation is crucial for biodiversity conservation because an error in species recognition will result in wasted effort in species conservation and may cause erroneous scientific inference when using species as the basic analysis unit (Mallet, 1995; De Queiroz, 2007; Wiens, 2007)
We explored all the recorded populations and collected representative individuals to address the following two questions: (1) Do R. verruciferum and R. gonggashanense comprise two independent morphological and genetic clusters? and (2) Did hybridization and cryptic introgression occur under the similar morphology in this species complex?
The test of variances among 19 morphological traits using ANOVA (P < 0.05) in PAST showed a significant difference between two groups (Supplementary Table 2)
Summary
Accurate species delimitation is crucial for biodiversity conservation because an error in species recognition will result in wasted effort in species conservation and may cause erroneous scientific inference when using species as the basic analysis unit (Mallet, 1995; De Queiroz, 2007; Wiens, 2007). This is difficult for some taxonomically complicated plant groups that comprise numerous closely related species (Rieseberg et al, 2006). Despite that multiple approaches under different species concepts were proposed (Sokal and Sneath, 1963; Cracraft, 1983; Wiens and Penkrot, 2002; Ramsey et al, 2003; Rissler and Apodaca, 2007; Craven et al, 2008; Yang and Rannala, 2010; Niemiller et al, 2011; Reeves and Richards, 2011; Hu et al, 2015; Liu et al, 2015), it is better to carry out species delimitations based on the consensus that different species should represent different evolving lineages, and morphological statistics and population genotyping seem to be the two convenient but necessary approaches for such a study (Liu, 2016)
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