Abstract
Southwestern (SW) China is an area of active tectonism and erosion, yielding a dynamic, deeply eroded landscape that influences the genetic structure of the resident populations of plants and animals. Iron walnut (Juglans regia subsp. sigillata) is a deciduous tree species endemic to this region of China and cultivated there for its edible nuts. We sampled 36 iron walnut populations from locations throughout the species' range in SW China and genotyped a total of 765 individuals at five chloroplast DNA regions and 22 nuclear microsatellite loci. Species distribution models were produced to predict the evolution and historical biogeography of iron walnut and to estimate the impacts of climate oscillations and orographic environments on the species' demography. Our results indicated that J. regia subsp. sigillata had relatively low genetic diversity, high interpopulation genetic differentiation, and asymmetric interpopulation gene flow. Based on DIYABC analysis, we identified two lineages of J. sigillata in southwestern China. The lineages (subpopulations) diverge during the last glacial period (~1.34 Ma). Southwestern China was a glacial refuge during the last glacial period, but increasingly colder and arid climates might have fostered the fragmentation of J. regia subsp. sigillata within this refugium. Finally, we found that recent habitat fragmentation has led to a reduction in population connectivity and increased genetic differentiation by genetic drift in isolated populations. Our results support a conclusion that geological and climatic factors since the Miocene triggered the differentiation, evolutionary origin, and range shifts of J. sigillata in the studied region.
Highlights
Southwestern (SW) China is a unique alpine area with a range of climates, complex topography, and a high proportion of endemic and relict flora
To examine the connections among geography and climatic oscillations and the processes that drive genetic differentiation, we examined the biogeography of the perennial, woody species iron walnut (Juglans regia subsp. sigillata)
Analysis of EST-SSRs in populations where J. regia and J. sigillata are sympatric in SW China revealed that the J. sigillata and J. regia populations were divided into two genetic clusters with frequent gene introgression (Yuan, Zhang, Peng, & Ge, 2008), but we found that iron walnut should be considered a subspecies or landrace of J. regia based on genotype by sequencing and EST-SSRs data (Feng et al, 2018)
Summary
Southwestern (SW) China is a unique alpine area with a range of climates, complex topography, and a high proportion of endemic and relict flora (about 29% of species). With regard to the systematic status of J. sigillata, there is a longterm, heated debate (Kuang & Lu, 1979; Wang, Pei, Gu, & Wang, 2008; Wu, Pei, Xi, & Li, 2000; Yang & Xi, 1989) It has been regarded as an ecotype of J. regia, but numerous botanists have viewed it as a different species (Aradhya, Potter, Gao, & Simon, 2007; Kuang & Lu, 1979). We used 22 simple sequence repeat (EST-SSR) markers and sequence variation at five chloroplast fragments (cpDNAs) to (a) determine the genetic diversity and population structure of J. sigillata at chloroplast DNA sequences and nuclear markers; (b) estimate the degree of population differentiation and gene flow of J. sigillata populations from the different regions; and (c) determine divergence times of two populations within J. sigillata and of J. sigillata from J. regia; (d) deduce current and past population and range dynamics [during the last interglacial (LIG; 130–116 kyr BP) and the last glacial maximum (LGM; 21–18 kyr BP)], and understand their possible underlying environmental causes
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