Introgression of Asian germplasm into natural populations of Castanea sativa Mill. in southern Europe

At the beginning of the 20^th century, Asian Castanea crenata and C. mollissima trees were introduced into European forests. Due to spontaneous hybridization and later backcrossing, evidence of introgression of this non-native germplasm into wild C. sativa populations has been documented in northwestern Spain. However, the extent of this introgression has not been monitored in chestnut populations of southern Europe. This study aimed to detect introgression of Asian germplasm in 14 natural C. sativa populations across southern Europe. The analysis conducted using STRUCTURE software identified four distinct groups representing C. sativa individuals based on their geographic origin and a separate group representing the Asian species. Six interspecific hybrids were detected in Chambre (SP03), Médulas (SP07) and Chantada (SP13) populations, in northern Spain. Absence of C. crenata and C. mollissima introgression into C. sativa populations in northern Portugal, southern France, and northern Italy is reported. In terms of genetic variation, a total of 11 private alleles were detected: six specific to C. crenata, three specific to C. mollissima, and two present in both species. This study provides robust evidence of introgression of Asian germplasm into C. sativa populations in southern Europe. Moreover, the work highlights the utility of microsatellite markers to evaluate hybridization in chestnut forests.