The historical biogeography of the Northern Hemisphere has attracted the attention of botanists and biogeographers. A striking feature of the geographical distribution of plants in this hemisphere is the existence of closely related taxa that are disjunctly distributed in eastern Asia, Europe, and North America.
The common explanation offered for this distributional pattern used to be that itresulted from the fragmentation of a once more widely distributed mixed mesophytic forest during the mid-Cenozoic. This once continuously distributed forest is thought to have become fragmented and restricted to refugia in North America and Eurasia due to climatic cooling, aridification, and geological changes that occurred during the mid-Cenozoic.
Besides this hypothesis, long-distance dispersal is also an alternative explanation for intercontinental disjunction of some plants in the Northern Hemisphere. Despite the different mechanisms hypothesized to explain current intercontinental disjunction, previous studies generally assumed that taxa showing such disjunct distribution originated in Laurasia. However, some plants that are now distributed in the Northern Hemisphere may have originated in the Southern Hemisphere.
Biotic exchange between Laurasia and Gondwana could have occurred via long-distance dispersal or via links provided by islands, land bridges and geological movements during the late Cretaceous and Cenozoic. Floral and faunal exchange between Laurasia and the Gondwanan fragments could have occurred between Eurasia and Africa due to the (Africa) Arabia-Eurasia collision during the Neogene.
Between North America and South America via the Isthmus of Panama link during the Miocene, between the Asian continent and Australia via islands of southeastern Asia from the Oligocene onwards. In addition, India may also have played a significant role in biotic exchange between Laurasia and Gondwana.
According to the “Out-of-India” hypothesis some Asian biotic elements may have an ancient Gondwanan origin and reached Asia by “rafting” on the Indian subcontinent after the breakup of Gondwana.
Paliurus Mill. (Rhamnaceae) is a small genus with a narrow and disjunct distribution in East Asia and the Mediterranean. However, fossil evidence from the late cretaceous to the present suggests it once had a broad geographic distribution, encompassing parts of India, North America, Asia, and Europe. Paliurus appears to be an ideal plant group for testing these mechanism which we have mentioned above.
To reconstruct the evolutionary history of Paliurus and understand the origin of the disjunction observed today in the Northern Hemisphere. Prof. SUN Hang ’s group from Kunming Institute of Botany, Chinese Academy of Sciences (KIB/CAS) reconstructed the diversification history of Paliurus by the cooperation with Dr. Yu-Sheng (Christopher) LIU from California state university, Dr. Andrea S. MESEGUER, Dr. Martin GODEFROID from French National Institute for Agricultural Research revealed thatbiogeographical reconstruction integrated with fossil information indicates the origin and dispersal pattern of the genus coincide with the “Out-of-India” hypothesis.
The genus may have originated in India or other parts of Gondwana and then dispersed to East Asia after the collision of the Indian subcontinent with Eurasia, and to North America via the Bering land bridge. Paliurus appears to have reached the Mediterranean region in the late Oligocene following closure of the Turgai Strait. The uplift of the Qinghai-Tibet Plateau and the advent of drying and cooling climates during the Miocene may have fragmented the distribution of the genus, shaping the biogeographical patterns observed today.
Paliurus populations in different regions either became extinct or adapted to changes in local ecological conditions following global climatic shifts through geological time. This research provides new insight for us to understand the formation mechanism of disjunct distribution of plants in the Northern Hemisphere.
Results published in Taxon etilted "Out-of-India dispersal of Paliurus (Rhamnaceae) indicated by combined molecular phylogenetic and fossil evidence".
This research was supported by the Major Research Plan of National Natural Science Foundation of China (NSFC), the Strategic Priority Research Program of CAS and NSFC-Yunnan Joint Fund.
Fig 1: Hypothetical original area and dispersal routes of Paliurus. A, Late Cretaceous (66 Ma); B, Eocene (44 Ma); C, Oligocene (28 Ma); D, Miocene(14 Ma); E, Pliocene (3 Ma). — Arrows indicate possible migratory directions of Paliurus between different areas: A, from India to Asia; B, from East Asia to North America; C, from East Asia to Mediterranean areas. (Image by KIB)
Key Laboratory for Plant Diversity and Biogeography of East Asia,
Kunming Institute of Botany, Chinese Academy of Sciences
Prof. Dr. SUN Hang, email: email@example.com