Genetic and ecological insights into glacial refugia of walnut (Juglans regia L.)

Authors: Aradhya, Mallikarjuna; IBRAHIMOV, ZAKIR - Azerbaijan State Agrarian University - Azerbaijan; TOKTORALIEV, BIIMYRZA - National Academy Of Science Of The Kyrgyz Republic; MAGHRADZE, DAVID - Agriculture University Of Georgia; MUSAYEV, MIRZA - Azerbaijan Academy Of Sciences; BOBOKASHVILI, ZVIADI - Agriculture University Of Georgia; VELASCO, DIANNE - University Of California; Preece, John

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/24/2017
Publication Date: 10/12/2017
Citation: Aradhya, M.K., Ibrahimov, Z., Toktoraliev, B., Maghradze, D., Musayev, M., Bobokashvili, Z., Velasco, D., Preece, J.E. 2017. Genetic and ecological insights into glacial refugia of walnut (Juglans regia L.). PLoS One. 12(10):e0185974. https://doi.org/10.1371/journal.pone.0185974.
DOI: https://doi.org/10.1371/journal.pone.0185974

Interpretive Summary: The distribution and survival of trees during the last glacial maximum (LGM) has been of interest to paleoecologists, biogeographers, and geneticists. Ecological niche models (ENMs) that associate species occurrence and abundance with climatic variables are extensively used to gain ecological to predict species distributions across landscapes over space and time. The present study deals with the glacial history of walnut to address questions related to past distributions through genetic analysis and ecological modeling of the LGM and Last Interglacial (LIG) periods. Model performance as assessed from the AUC (area under the receiver operating characteristic curve) averaged over replicate runs recorded high values for all models { the present (0.967 0.009), the LGM-CCSM (0.9530.020), LGM-MIROC (0.9590.010), and the LIG (0.9580.014) indicated the accuracy and predictive power of the models. Modeling of current climatic and species occurrence data predicted southern Caucasus, parts of West and Central Asia extending into South Asia encompassing northern Afghanistan, Pakistan, northwestern Himalayan region, and southwestern Tibet as the favorable climatic niche matching the modern distribution of walnut. The LGM hindcasts using climatic data from the CCSM and MIROC models suggested disjunct distributions of walnut with populations restricted to Transcaucasia, Central and South Asian regions extending into southwestern Tibet, northeastern India, Himalayan region of Sikkim and Bhutan, and southeastern China. Both CCSM and MIROC projections overlapped, except that MIROC projected a significant presence of walnut in the Balkan Peninsula during the LGM. In contrast, genetic analysis of the modern walnut distribution suggested a much narrower area in northern Pakistan and the surrounding areas of Afghanistan, northwestern India and southern Tajikistan as a plausible hotspot of diversity where walnut may have survived glaciations. Overall, the findings suggest that walnut perhaps survived the last glaciations in several refugia across a wide geographic area between 30 degree and 50 degree north latitude. However, humans have played a significant role in the recent history and modern distribution of walnut.

Technical Abstract: The distribution and survival of trees during the last glacial maximum (LGM) has been of interest to paleoecologists, biogeographers, and geneticists. Ecological niche models (ENMs) that associate species occurrence and abundance with climatic variables are extensively used to gain ecological to predict species distributions across landscapes over space and time. The present study deals with the glacial history of walnut to address questions related to past distributions through genetic analysis and ecological modeling of the LGM and Last Interglacial (LIG) periods. Model performance as assessed from the AUC (area under the receiver operating characteristic curve) averaged over replicate runs recorded high values for all models { the present (0.967 0.009), the LGM-CCSM (0.9530.020), LGM-MIROC (0.9590.010), and the LIG (0.9580.014) indicated the accuracy and predictive power of the models. Modeling of current climatic and species occurrence data predicted southern Caucasus, parts of West and Central Asia extending into South Asia encompassing northern Afghanistan, Pakistan, northwestern Himalayan region, and southwestern Tibet as the favorable climatic niche matching the modern distribution of walnut. The LGM hindcasts using climatic data from the CCSM and MIROC models suggested disjunct distributions of walnut with populations restricted to Transcaucasia, Central and South Asian regions extending into southwestern Tibet, northeastern India, Himalayan region of Sikkim and Bhutan, and southeastern China. Both CCSM and MIROC projections overlapped, except that MIROC projected a significant presence of walnut in the Balkan Peninsula during the LGM. In contrast, genetic analysis of the modern walnut distribution suggested a much narrower area in northern Pakistan and the surrounding areas of Afghanistan, northwestern India and southern Tajikistan as a plausible hotspot of diversity where walnut may have survived glaciations. Overall, the findings suggest that walnut perhaps survived the last glaciations in several refugia across a wide geographic area between 30 degree and 50 degree north latitude. However, humans have played a significant role in the recent history and modern distribution of walnut.