Demographical complexity within walnut species provides insights into the heterogeneity of geological and climatic fluctuations in East Asia

Abstract

East Asia constitutes one of the largest and most complex areas of plant diversity globally. This complexity is attributable to the geological history and climatic diversity of East Asia. However, controversy persists over the biogeographical hypotheses and different studies on this topic. The demographical complexity of temperate deciduous woody plants and heterogeneity of geological and climatic fluctuations in East Asia remain poorly understood. We aimed to assemble the chloroplast and nuclear genomes of 360 individuals from five walnut (Juglans) species in East Asia and comprehensively analyze the phylogenetic inference, genetic structure, population demographic, and selection pressure. The results showed that East Asian walnuts could be divided into two major groups, that is, section Juglans/Dioscaryon and sect. Cardiocaryon. Within sect. Cardiocaryon, the samples were categorized into the northern clade and southern clade, both of which were affected by Quaternary glaciation; however, the southern clade was affected to a lesser extent. The population expansion events observed in sect. Cardiocaryon and sect. Dioscaryon since the late Miocene indicated that Neogene climate cooling substantially affected the distribution of Tertiary relict plants in East Asia. Biogeographic analysis results showed that the uplift of the Qinghai–Tibetan Plateau (QTP) might have contributed to the divergence within sect. Dioscaryon. Our findings highlight the heterogeneity of climatic fluctuations in the northern and southern regions of East Asia during the late Neogene and Quaternary periods and suggest that the uplift of QTP could have facilitated the divergence of temperate deciduous woody plants.