A biogeographical study on tropical flora of southern China

Three new species of Smithiomyces from tropical Asia support an amphi-Pacific disjunct distribution in the genus

Smithiomyces is reported for the first time from tropical regions in China, thus expanding its known native geographic range from the Neotropics to tropical Asia. Phylogenetic evidence from four nuclear loci supports the monophyly of Smithiomyces and a close evolutionary relationship with the nonmonophyletic genera Melanophyllum and Cystolepiota in the Agaricaceae. Detailed morphological descriptions are provided for three newly described species from China: S. asiaticus, S. heterosporus, and S. lepiotoides. Illustrations of fresh basidiomata in the field, line drawings of key anatomical features, microscopic images of anatomical features, scanning electron microscope (SEM) images of basidiospores, and a key to known species of Smithiomyces are also provided.

Temperature and historical land connectivity jointly shape the floristic relationship between Hainan Island and the neighbouring landmasses

Present-day biodiversity in insular biota results from the interplay among geographical barriers, environmental filtering, and historical biogeography, but how these factors interact on insular biodiversity patterns is poorly understood. Here, we analysed the geographical patterns of beta diversity of seed plants between Hainan Island and the neighbouring landmasses in relation to space and the environmental factors to assess the relative effects of historical processes and ecological gradients on community assembly. We assessed beta diversity patterns by quantifying the turnover and nestedness components and used clustering and ordination to investigate the relationships between local floras from Hainan and the neighbouring landmasses. Utilising simple linear regression and linear mixed effect models, we evaluated the importance of historical processes and environmental gradients in shaping these beta diversity patterns. Our results show that the contributions of nestedness and turnover components to the total beta diversity vary across space. The flora of Hainan predominantly nests with the flora of Vietnam but shows larger species turnover with Guangdong, Guangxi, and Taiwan. Clustering and ordination analyses indicate that Hainan is first merged with Vietnam, after which it is grouped with mainland China and finally with Taiwan and the Philippines. The results of the linear mixed effect models consistently reveal that temperature, followed by the historical land connectivity, has the most important role in shaping the floristic dissimilarity. We conclude that the flora of Hainan is of continental origin and has the highest floristic affinity with Vietnam. The periodic emergence of a land bridge during Quaternary glacial cycles determines the origin of Hainan's flora, and temperature shapes the floristic dissimilarities via environmental filtering. Our study highlights the critical roles of historical sea level change and current environmental limitation in structuring the plant communities on Southeast Asian islands.

Genetic diversity and population structure in the endangered tree Hopea hainanensis (Dipterocarpaceae) on Hainan Island, China

Hopea hainanensis Merrill & Chun (Dipterocarpaceae) is an endangered tree species restricted to Hainan Island, China and a small part of Northern Vietnam. On Hainan Island, it is an important indicator species for tropical forests. However, because of its highly valued timber, H. hainanensis has suffered from overexploitation, leading to a sharp population decline. To facilitate the conservation of this species, genetic diversity and population structure were assessed using 12 SSR markers for 10 populations sampled across Hainan Island. Compared to non-threatened Hopea species, H. hainanensis exhibited reduced overall genetic diversity and increased population differentiation (AMOVA: F_ST = 0.23). Bayesian model-based clustering and principal coordinate analysis consistently assigned H. hainanensis individuals into three genetic groups, which were found to be widespread and overlapping geographically. A Mantel test found no correlation between genetic and geographical distances (r = 0.040, p = 0.418). The observed genetic structure suggests that long-distance gene flow occurred among H. hainanensis populations prior to habitat fragmentation. A recent population bottleneck was revealed, which may cause rapid loss of genetic diversity and increased differentiation across populations. Based on these findings, appropriate strategies for the long-term conservation of the endangered species H. hainanensis are proposed.

The tropical-subtropical evergreen forest transition in East Asia: An exploration

The transition from tropical to subtropical (warm temperate) evergreen forests is more clearly apparent in East Asia, from Nepal to the western Pacific coast, than elsewhere in the tropics. We review the nature of this transition and hypothesize the physical, ultimately climatic, factors that may maintain it, with a special focus on how the increasing instability and warming of climates will affect these forests. A primary climatic mediator of the transition is proposed, thereby offering a testable hypothesis for the climate-forest transition relationship. What is known of this transition is summarized in context of the primary climatic mediators of elevational zonation of forest formations in equatorial Asia to the tree line, in the Himalaya at the India-Indo-Burma northern tropical margin, and as both elevational and latitudinal zonation in southern China. Consequent secondary edaphic and other physical changes are described for the Himalaya, and hypothesized for southern China. The forest ecotones are seen to be primarily defined by tree floristic change, on which account changes in structure and physiognomy are determined. The montane tropical-subtropical transition in the Himalaya is narrow and observed to correlate with an as yet ill-defined frost line. A distinct tropical-subtropical transition forest is recognized in the southwest China mountains. There is a total change in canopy species at the Himalayan ecotone, but subcanopy tropical species persist along an elevational decline of c. 400 m. The latitudinal transition in South China is analogous, but here the tropical subcanopy component extends north over ten degrees latitude, albeit in decline. The tropical-subtropical transition is uniquely clear in East Asia because here alone a tropical wet summer-dry winter monsoon extends to 35° north latitude, encompassing the subtropical evergreen forest, whereas subtropical evergreen forests elsewhere exist under drier temperate summer climate regimes.

Circadian clock regulates hepatotoxicity of Tripterygium wilfordii through modulation of metabolism

OBJECTIVES

We aimed to determine the diurnal rhythm of Tripterygium wilfordii (TW) hepatotoxicity and to investigate a potential role of metabolism and pharmacokinetics in generating chronotoxicity.

METHODS

Hepatotoxicity was determined based on assessment of liver injury after dosing mice with TW at different circadian time points. Circadian clock control of metabolism, pharmacokinetics and hepatotoxicity was investigated using Clock-deficient (Clock^-/- ) mice.

KEY FINDINGS

Hepatotoxicity of TW displayed a significant circadian rhythm (the highest level of toxicity was observed at ZT2 and the lowest level at ZT14). Pharmacokinetic experiments showed that oral gavage of TW at ZT2 generated higher plasma concentrations (and systemic exposure) of triptolide (a toxic constituent) compared with ZT14 dosing. This was accompanied by reduced formation of triptolide metabolites at ZT2. Loss of Clock gene sensitized mice to TW-induced hepatotoxicity and abolished the time-dependency of toxicity that was well correlated with altered metabolism and pharmacokinetics of triptolide. Loss of Clock gene also decreased Cyp3a11 expression in mouse liver and blunted its diurnal rhythm.

CONCLUSIONS

Tripterygium wilfordii chronotoxicity was associated with diurnal variations in triptolide pharmacokinetics and circadian expression of hepatic Cyp3a11 regulated by circadian clock. Our findings may have implications for improving TW treatment outcome with a chronotherapeutic approach.

Exploring evolution and diversity of Chinese Dipterocarpaceae using next-generation sequencing

Tropical forests, a key-category of land ecosystems, are faced with the world's highest levels of habitat conversion and associated biodiversity loss. In tropical Asia, Dipterocarpaceae are one of the economically and ecologically most important tree families, but their genomic diversity and evolution remain understudied, hampered by a lack of available genetic resources. Southern China represents the northern limit for Dipterocarpaceae, and thus changes in habitat ecology, community composition and adaptability to climatic conditions are of particular interest in this group. Phylogenomics is a tool for exploring both biodiversity and evolutionary relationships through space and time using plastome, nuclear and mitochondrial genome. We generated full plastome and Nuclear Ribosomal Cistron (NRC) data for Chinese Dipterocarpaceae species as a first step to improve our understanding of their ecology and evolutionary relationships. We generated the plastome of Dipterocarpus turbinatus, the species with the widest distribution using it as a baseline for comparisons with other taxa. Results showed low level of genomic diversity among analysed range-edge species, and different evolutionary history of the incongruent NRC and plastome data. Genomic resources provided in this study will serve as a starting point for future studies on conservation and sustainable use of these dominant forest taxa, phylogenomics and evolutionary studies.

A biogeographical study on tropical flora of southern China

The tropical climate in China exists in southeastern Xizang (Tibet), southwestern to southeastern Yunnan, southwestern Guangxi, southern Guangdon, southern Taiwan, and Hainan, and these southern Chinese areas contain tropical floras. I checked and synonymized native seed plants from these tropical areas in China and recognized 12,844 species of seed plants included in 2,181 genera and 227 families. In the tropical flora of southern China, the families are mainly distributed in tropical areas and extend into temperate zones and contribute to the majority of the taxa present. The genera with tropical distributions also make up the most of the total flora. In terms of geographical elements, the genera with tropical Asian distribution constitute the highest proportion, which implies tropical Asian or Indo-Malaysia affinity. Floristic composition and geographical elements are conspicuous from region to region due to different geological history and ecological environments, although floristic similarities from these regions are more than 90% and 64% at the family and generic levels, respectively, but lower than 50% at specific level. These differences in the regional floras could be influenced by historical events associated with the uplift of the Himalayas, such as the southeastward extrusion of the Indochina geoblock, clockwise rotation and southeastward movement of Lanping-Simao geoblock, and southeastward movement of Hainan Island. The similarity coefficients between the flora of southern China and those of Indochina countries are more than 96% and 80% at family and generic levels, indicating their close floristic affinity and inclusion in the same biogeographically floristic unit.