Rethinking the history of common walnut (Juglans regia L.) in Europe: Its origins and human interactions

Abstract

Common walnut (Juglans regia L) is an economically important species cultivated worldwide for its high-quality wood and nuts. It is generally accepted that after the last glaciation J. regia survived and grew in almost completely isolated stands in Asia, and that ancient humans dispersed walnuts across Asia and into new habitats via trade and cultural expansion. The history of walnut in Europe is a matter of debate, however. In this study, we estimated the genetic diversity and structure of 91 Eurasian walnut populations using 14 neutral microsatellites. By integrating fossil pollen, cultural, and historical data with population genetics, and approximate Bayesian analysis, we reconstructed the demographic history of walnut and its routes of dispersal across Europe. The genetic data confirmed the presence of walnut in glacial refugia in the Balkans and western Europe. We conclude that human-mediated admixture between Anatolian and Balkan walnut germplasm started in the Early Bronze Age, and between western Europe and the Balkans in eastern Europe during the Roman Empire. A population size expansion and subsequent decline in northeastern and western Europe was detected in the last five centuries. The actual distribution of walnut in Europe resulted from the combined effects of expansion/contraction from multiple refugia after the Last Glacial Maximum and its human exploitation over the last 5,000 years.

Advances in genotyping microsatellite markers through sequencing and consequences of scoring methods for Ceratonia siliqua (Leguminosae)

“…SSR variation has been the most widely used molecular marker for population genetics and molecular ecology since the 1990s (Guichoux et al., ). Due to their high mutation rate and their potential to screen hundreds to thousands of individuals, microsatellite markers have been recently used in several studies focusing on the evolutionary history of fruit trees (Besnard et al., ; Cornille et al., ; Diez et al., ; Pollegioni et al., ). In this study, we developed 18 new polymorphic SSR markers in C. siliqua .…”

Walnut: past and future of genetic improvement

“…Although Persian walnut was introduced into Europe and the Balkans before the last glacial period (~50.000 ya) (Carrion and Sanchez-Gomez 1992), it is still debated how trees have reached Europe. In relation to that, a very recent study focused on the history of its spread across Europe has integrated various data such as fossil pollen and microsatellite markers (Pollegioni et al 2017) and came to the conclusion that natural populations existed before human intervention. This piece of work also confirmed the presence of walnut in glacial refugia in Western Europe and in the Balkans.…”

“…to~395 A.D.). Evidence suggests human-mediated dispersal during the Roman Empire between Western Europe and the Balkans in Eastern Europe (Pollegioni et al 2017). In the meantime, Chinese archives suggest a form of domestication of walnut in Southern Tibetan and the Yunnan region during the Han dynasty (~206 B.C.…”

A new genomic tool for walnut (Juglans regia L.): development and validation of the high‐density Axiom™ J. regia 700K SNP genotyping array

“…The discovery and large-scale genotyping of thousands of genomewide molecular markers are the first steps in this transition to molecular breeding (Sehgal et al, 2016). In walnut, most genetic studies have been based on a handful of molecular markers, notably Random Amplified Polymorphic DNA (RAPD- Nicese et al, 1998;Woeste et al, 1996) and Single Sequence Repeats (SSRs- Pollegioni et al, 2017;Woeste et al, 2002). However, these types of molecular markers are laborious, time-consuming, not evenly distributed across the genome and, therefore, of limited use in implementing new genome-based approaches such as genome selection (GS) and association mapping (AM; Rasheed et al, 2017).…”