The Plant Press

From The Plant Press, Vol. 27, No. 1, January 2024.

Tracking the climate-influenced north temperate radiation of the grape genus using genomic data

By Jun Wen, Richard Hodel, Zelong Nie, and Gary Krupnick

As a research botanist and curator for almost 20 years at the National Museum of Natural History (NMNH), Jun Wen has been studying the biogeography of the northern hemisphere and the biogeographic relationships between temperate and tropical elements. She has been actively studying the origin and evolution of eastern Asian and eastern North American disjunct biogeographic pattern in flowering plants since her graduate school days. Wen and her team of colleagues have shown a special interest in the grape genus Vitis (Vitaceae), from which she and her collaborators have been collecting specimens during recent field expeditions in the United States, Mexico, and China. The broad geographic sampling of the grape genus has enabled using this group of plants as a model for improving our understanding of plant biogeography and evolution in temperate North America.

The north temperate region was characterized by a warm climate and a rich thermophilic flora before the Eocene, but the early diversifications of the temperate biome under global climate change and biome shift remain uncertain. A recent paper by Wen and colleagues from the Chinese Academy of Sciences, Huazhong Agricultural University, Jishou University, University of Alaska Fairbanks, and NMNH used the grape genus Vitis as a model to unravel the diversification of this economically highly important lineage of plants and provide insights into the temporal and spatial evolution of plants in temperate North America. The paper, “Climate‐influenced boreotropical survival and rampant introgressions explain the thriving of New World grapes in the north temperate zone,” was published in Journal of Integrative Plant Biology (65: 1183-1203; https://doi.org/10.1111/jipb.13466) and authored by the grape team including Ze-Long Nie, Richard Hodel, Zhi-Yao Ma, Gabriel Johnson, Chen Ren, Ying Meng, Stefanie Ickert-Bond, Xiu-Qun Liu, Elizabeth Zimmer, and Jun Wen. 

The grape genus Vitis has about 75 species constituting a charismatic component of the north temperate flora, with a few species also extending to the tropical zone. The grape genus is also sister to a lineage of tropical relatives from Central America. Hence the grape genus represents an attractive model to explore how temperate radiation occurred from its tropical origin and to track the diversification history of more than 50 million years since the geological epoch of the Eocene when the earth was relatively warm (i.e., the boreotropics).

The grape team applied various analyses ranging from phylogenetics, biogeography, and phylogenetic networks using extensive genomic data with an emphasis on the New World Vitis and its Eurasian relatives. The biogeographic inference and fossil evidence support that Vitis originated in North America and diverged from its closest sister group around 60 million years ago in the Paleocene geological epoch. The grape genus was widely distributed from North America to Europe during the Paleocene to the Eocene, followed by widespread extinction and survival of relict populations in the tropical New World. During the warming climate of the early to mid Miocene (20-15 million years ago), a Vitis ancestor migrated northward from the refugia with subsequent diversification across the North American region (see figure on the right). 

The grape research team also found strong evidence for different evolutionary signals encoded in different genes within both recent and ancient lineages of the New World Vitis. In other words, the different nuclear genes are telling different histories of the plant evolution, which signals hybridization and even introgression, with the latter as a type of more complex hybridization involving repeated backcrossing with one of the parents. Furthermore, the evolutionary tree inferred using the chloroplast genomes showed strong conflicts with the evolutionary tree inferred from the nuclear genomes, another common pattern as a result of hybridization and introgression. Extensive analyses based on genomic datasets hence strongly support the important role of wide occurrence of hybridization and introgression in driving the diversification of North American Vitis in the last 5-20 million years. 

The study highlighted that hybridization potentially represents one of the most important mechanisms for the diversification of Vitis species in temperate North America and even the entire temperate Northern Hemisphere in a changing climate. The scenario reported in this study may be a common model of temperate diversification of flowering plants that adapted to the global climate cooling and fluctuation in the last 50 million years. The study also supports the hypothesis that modern temperate biomes originated and expanded in relatively recent times in Earth's history when global climate cooling intensified after the warming period of the late Eocene. 

This study has important implications for our understanding of how the effects of past climatic changes on plant biogeography may impact the future responses of plants, including grapes, to ongoing severe climatic changes. The grape team led by Wen is now using the biogeographic and speciation history they learned on grapes to further advocate the importance on understanding the evolutionary history for classifying, conservation, and utilization of this important lineage of plants.