Phylogeography and Population History of Eleutharrhena macrocarpa (Tiliacoreae, Menispermaceae) in Southeast Asia’s Most Northerly Rainforests

Abstract

The diversification of Tiliacoreae and the speciation of Eleutharrhena are closely linked to Southeast Asia’s most northerly rainforests which originate from the Himalayan uplift. Migration routes across biogeographical zones within the Asian clade, including those of Eleutharrhena, Pycnarrhena, and Macrococculus, and their population structures are still unexplored. We combine endocarp morphology, phylogenetic analyses, divergence time estimation, ancestral area reconstruction, as well as SCoT method to reconstruct the past diversification of Eleutharrhena macrocarpa and to understand their current distribution, rarity, and evolutionary distinctiveness. The disjunct, monospecific, and geographically restricted genera Eleutharrhena and Macrococculus both have a dry aril, a unique feature in Menispermaceae endocarps that further confirms their close relationship. Pycnarrhena and Eleutharrhena appeared during the end of the Oligocene c. 23.10 million years ago (Mya) in Indochina. Eleutharrhena speciation may be linked to climate change during this time, when humid forests became restricted to the northern range due to the Himalayan uplift. Differentiation across the Thai–Burmese range could have contributed to the isolation of the Dehong populations during the Miocene c. 15.88 Mya, when exchange between India and continental Asia ceased. Dispersal to the Lanping–Simao block and further differentiation in southeastern and southern Yunnan occurred during the Miocene, c. 6.82 Mya. The specific habitat requirements that led to the biogeographic patterns observed in E. macrocarpa contributed to a low genetic diversity overall. Population 1 from Dehong, 16 from Pu’er, and 20 from Honghe on the East of the Hua line have a higher genetic diversity and differentiation; therefore, we suggest that their conservation be prioritized.