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

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 that associate species occurrence and abundance with climatic variables are widely used to gain ecological and evolutionary insights and to predict species distributions 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 present, LGM and Last Interglacial (LIG) periods. A maximum entropy method was used to project the current walnut distribution model on to the LGM (21–18 kyr BP) and LIG (130–116 kyr BP) climatic conditions. Model tuning identified the walnut data set filtered at 10 km spatial resolution as the best for modeling the current distribution and to hindcast past (LGM and LIG) distributions of walnut. The current distribution model 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 hindcast of distributions suggested the occurrence of walnut during LGM was somewhat limited to southern latitudes from southern Caucasus, 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 current 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° and 45° North latitude. However, humans probably played a significant role in the recent history and modern distribution of walnut.

The complex evolutionary history of apricots: Species divergence, gene flow and multiple domestication events

“…We confirmed in this study previous results showing genetic structure within the wild Prunus armeniaca populations in Central Asia, with a highly differentiated Kyrgyz population in the South (red cluster, Decroocq et al, 2016). Such genetic structure is probably related to glacial refugia for P. armeniaca in South Central Asia (Aradhya et al, 2017), followed by a recolonization of the Northern territories, Thanks to more extensive sampling compared to previous studies, we could infer the wild apricot evolutionary history in China. We found high genetic differentiation between the different Armeniaca species, P. armeniaca, Prunus sibirica and Prunus mume.…”

“…We confirmed in this study previous results showing genetic structure within the wild Prunus armeniaca populations in Central Asia, with a highly differentiated Kyrgyz population in the South (red cluster, Decroocq et al, ). Such genetic structure is probably related to glacial refugia for P. armeniaca in South Central Asia (Aradhya et al, ), followed by a recolonization of the Northern territories, including the Kazakh Tian Shan ranges, upon global warming. ABC analyses further confirmed that the yellow wild P. armeniaca populations in Central Asia recently diverged from the red wild P. armeniaca populations in Kyrgyzstan.…”

What is a tree in the Mediterranean Basin hotspot? A critical analysis

“…The difficulty of estimating the native distribution arises also in other cultivated medio-European trees that are not only present in the Mediterranean region. It exists indeed conflicting results between an ecological niche model projecting a significant presence of walnut (Juglans regia) in the Balkan Peninsula during the LGM in contrast to genetic analysis that suggest refugia in Southwest Asia (Aradhya et al 2017). For another key cultivated tree, the chestnut (Castanea sativa), a paleodistribution modelling approach validated with pollen and charcoal records (Roces-Díaz et al 2018) seems to be in agreement with a phylogeographical analysis (Mattioni et al 2013): LGM refugia were suggested in the north of the Iberian and Italian peninsulas, in Greece, and along the southern coast of the Black Sea.…”

The Phytogeographic History of Common Walnut in China

“…The native range of common walnut is uncertain, but (apparently) wild populations grow in isolated favorable habitats across a wide geographical range from China to the Iberian Peninsula ( Manning, 1978 ; Draine and Hiden, 1998 ; Beer et al, 2008 ; Martínez-García et al, 2016 ; Pollegioni et al, 2017 ). Because of continued uncertainty concerning the number and locations of refugia for J. regia , current populations may or may not have been derived from refugia in Southern Europe, the Balkans, the Carpathian mountains, Anatolia, the Zagros mountains, and even China ( Martínez-García et al, 2016 ; Aradhya et al, 2017 ).…”

“…This road led westward along the northern foothills of the Eastern Tien Shan Mountains. It connected Eastern China (Shandong province) and Western China (Eastern Tien Shan mountains; Urumqi; Xinjiang province), facilitating cultural and agrarian exchange ( Christian, 2000 ; Aradhya et al, 2017 ; Pollegioni et al, 2017 ).…”