What creates Gentiana diversity in mountains?

The Qinghai-Tibetan Plateau (QTP) is extremely rich in species, but why? One recent explanation is the mountain‐geobiodiversity hypothesis (MGH) by Mosbrugger et al. The argument is that biodoversity happens when you have a large range of elevations, in close proximity, with climatic change that acts as a ‘species-pump’. In a recent paper, Peng-Cheng Fu and colleagues investigated the evolutionary history of three closely related Gentiana endemic species, to see if it was compatible with this hypothesis.

Gentiana is found around the world in montane habitats. It’s often prized by gardeners for its deep trumpet flowers with petals of striking blue. There are now over three hundred and fifty species of Gentiana around the world, but it’s a puzzle why there are so many, say the authors. “Although the biogeography and diversification of Gentiana at deeper times are well known, the actual mechanisms by which speciation has occurred within the genus have been overlooked. That is why, in this study, we focused on three closely related species endemic to the QTP: G. veitchiorum Hemsley, G. lawrencei var. farreri T. N. Ho and G. dolichocalyx T. N. Ho.”

The team examined the population genetic structure using chloroplast DNA (cpDNA) and nuclear microsatellite loci. Looking for differences in the DNA acts like a molecular clock. If you can compare how the plants diverged with climactic conditions when the diverge you can start making inferences about what might be driving those changes.

“Gentiana veitchiorum and G. lawrencei var. farreri are sister species that can be easily distinguished from each other morphologically,” write Fu and colleagues. “Based upon cpDNA, the two lineages diverged around 4.89 Ma… during the final extension of the QTP and the uplift of the Hengduan Mountains. This time frame also overlaps with climate fluctuations. Considering their overlapping distribution range distribution patterns, their low genetic differentiation and recent divergence time, it is realistic to hypothesize that the two species arose from a recent speciation event possibly induced by a combination of climatic and geological changes, as is suggested in several other alpine groups.”

The team also found that there was evidence of hybridization. This is expected with he mountain‐geobiodiversity hypothesis, as species diverge and then meet with secondary contact. Yet there was something else suggested by the DNA. “Our data, however, seem to indicate that hybridization occurred within a refugium and was followed by a range expansion of the mixed genomes,” write Fu and colleagues. “We therefore suggest that hybridization may play a role throughout climatic cycles, and not only upon secondary contact as suggested by the MGH.”The team also found evidence in the DNA to clarify the status of G. dolichocalyx which is very similar to G. lawrencei var. farreri. “Our results clearly show that the two species have only little genetic differentiation, indicating either that they diverged recently or that hybridization had a homogenizing effect. However, if hybridization was so common, then chloroplast capture could be expected. We do not observe such a phenomenon since the two species are clearly distinct in the plastome phylogeny. Therefore, G. dolichocalyx should be considered as a geographically limited and distinct species with only little genetic differentiation from G. lawrencei var. Farreri.”