Taxonomy & Evolution

Evolutionary history of Pseudotaxus: old lineage, young populations

In southeast China there is a puzzle. Pseudotaxus chienii lives as a relict, a plant that used to have a much wider geographic range. Now it’s limited to comparatively few locations. This is common for many plants that have found past climate change a challenge. The puzzle is that these relict populations appear to be the same species. If these plants have been isolated, why hasn’t distance, and evolution, driven a wedge between them? Yixuan Kou and colleagues have been examining the evolutionary history of P. chienii to find out.

Distribution of three clusters (W, red; C, yellow; E, blue) revealed by STRUCTURE analysis using K = 3, which was determined to be the optimal K value using the ΔK method. Source Kou et al. 2020.

The team examined fourteen nuclear loci as well as two chloroplast genomic regions from twelve P. chienii populations. They then applied statistical tests to see if the plants were, in fact, still exchanging DNA through wind-based pollen or else recent colonisers from an earlier shared lineage. A third possibility they considered is that the populations had been isolated for a long while, but under very similar conditions, leading to little divergence.

What they found is that the populations appeared to diverge from each other about and a half million years ago. This was consistent with the hypothesis that the populations are colonisers from a common source.

“Although there are wide confidence intervals around the time estimates,” the authors write, “the recent time estimates by two different approaches suggests that: (1) Pseudotaxus might have existed in south-east China and other locations initially – however, the lineage of P. chienii branched recently in south-east China and other members and populations of Pseudotaxus elsewhere went extinct; and (2) P. chienii might have broader ranges (perhaps including south-east China) but south-east China could have been colonized (never appeared in south-east China) or re-colonized (appeared in south-east China initially but disappeared later) recently, while populations in other locations went extinct. Whichever scenario is more plausible, the results of this study suggest that the extant P. chienii lineage is relatively young and the populations of P. chienii started to differentiate in south-east China recently (possibly started in the late Pliocene).”

“The reason that these monotypic gymnosperms have a relict status in China is that the prerequisites (enough time for strict isolation and divergent selection) for allopatric speciation in south-east China have not been fulfilled even though the opportunities for allopatric speciation prevail. This study may provide a new perspective for understanding the evolution of relict gymnosperm flora in south-east China as well as the evolution of the Sino-Japanese Flora.”