Plastome phylogenomic and biogeographic study on Thuja (Cupressaceae)

Abstract

Background: Investigating the biogeographical disjunction of East Asia and North America flora is the key to understanding the formation and dynamic biodiversity in the Northern Hemisphere. The small Cupressaceae genus Thuja, comprising five species, exhibits a typical disjunct distribution in East Asia and North America. Owing to the obscure relationships, the biogeographical history of the genus remains controversial. Here, complete plastomes were employed to investigate the plastome evolution, phylogenetic relationships and biogeographic history of Thuja. Results: All plastomes of Thuja share the same gene contents arranged in the same order. The loss of an IR was evident in all Thuja plastomes, and the B-arrangement as previously recognized was detected. Phylogenomic analyses resolved two sister pairs, T. standishii-T. koraiensis and T. occidentalis-T. sutchuenensis, with T. plicata sister to T. occidentails-T. sutchuenensis. Molecular dating and biogeographic result suggest the diversification of Thuja occurred in the Middle Miocene, and the ancestral area of extant species was located in northern East Asia. Incorporating the fossil record, we inferred that Thuja likely originated from the high-latitude areas of North America in the Paleocene with a second diversification center in northern East Asia. Conclusions: The dispersal events attributed to the Bering Land Bridge in the Miocene and subsequent vicariance events accompanying climate cooling may have shaped the current geographic distributions of Thuja. Given most lineages of gymnosperm, including Thuja, have encountered massive extinction resulted from climatic oscillations, we here advocate that the potential effect of extinction should be re-evaluated in exploring the evolutionary history of gymnosperms.