Abstract
BackgroundAlthough allopatric speciation is viewed as the most common way in which species originate, allopatric divergence among a group of closely related species has rarely been examined at the population level through phylogeographic analysis. Here we report such a case study on eight putative cypress (Cupressus) species, which each have a mainly allopatric distribution in the Qinghai-Tibetan Plateau (QTP) and adjacent regions. The analysis involved sequencing three plastid DNA fragments (trnD-trnT, trnS-trnG and trnL-trnF) in 371 individuals sampled from populations at 66 localities.ResultsBoth phylogenetic and network analyses showed that most DNA haplotypes recovered or haplotype-clustered lineages resolved were largely species-specific. Across all species, significant phylogeographic structure (NST > GST, P < 0.05) implied a high correlation between haplotypes/lineages and geographic distribution. Two species, C. duclouxiana and C. chengiana, which are distributed in the eastern QTP region, contained more haplotypes and higher diversity than five species with restricted distributions in the western highlands of the QTP. The remaining species, C. funebris, is widely cultivated and contained very little cpDNA diversity.ConclusionsIt is concluded that the formation of high mountain barriers separating deep valleys in the QTP and adjacent regions caused by various uplifts of the plateau since the early Miocene most likely promoted allopatric divergence in Cupressus by restricting gene flow and fixing local, species-specific haplotypes in geographically isolated populations. The low levels of intraspecific diversity present in most species might stem from population bottlenecks brought about by recurrent periods of unfavorable climate and more recently by the negative impacts of human activities on species' distributions. Our findings shed new light on the importance of geographical isolation caused by the uplift of the QTP on the development of high plant species diversity in the QTP biodiversity hotspot.
Highlights
Allopatric speciation is viewed as the most common way in which species originate, allopatric divergence among a group of closely related species has rarely been examined at the population level through phylogeographic analysis
We aimed to address the following questions based on surveys of sequence variation for three plastid DNA fragments within and between populations at the species level: (1) Do plastid DNA haplotypes or clusters of such haplotypes correspond with morphological differentiation and the allopatric distributions of the Cupressus species examined? (2) Does intraspecific DNA diversity correlate with the natural population size of a species and, in turn, provide guidance for conservation management?
No intraspecific cpDNA variation was recorded in C. gigantea, C. jiangeensis, C. cashmeriana, C. austrotibetica and C. torulosa
Summary
Allopatric speciation is viewed as the most common way in which species originate, allopatric divergence among a group of closely related species has rarely been examined at the population level through phylogeographic analysis We report such a case study on eight putative cypress (Cupressus) species, which each have a mainly allopatric distribution in the Qinghai-Tibetan Plateau (QTP) and adjacent regions. No previous studies have attempted to examine genetic differentiation in this region among species of a closely related group, and in so doing shed light on their respective evolutionary histories We report such a case study of a group of cypress species (Cupressus L.) that occurs on the QTP and adjacent areas of China and the Himalayas. Most of these Asian species occur allopatrically in the QTP and adjacent regions, and only a few are parapatric in parts of their range (Fig. 1) (Additional File 1) [10,15]
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