Molecular evidence of introgression of Asian germplasm into a natural Castanea sativa forest in Spain

Abstract Evidence has been documented in Europe of introgression, the transfer of genetic material, between Asian chestnut species (Castanea crenata and C. mollissima) and the native C. sativa through spontaneous hybridization and subsequent backcrossing. However, the extent of this introgression has not been monitored in a particular forest stand or in adult and juvenile trees simultaneously. We assessed introgression in a natural C. sativa forest in northwest Spain and developed a reliable method to detect the presence of Asian germplasm of Castanea spp. A total of 34 adult trees and 42 saplings were genotyped at 13 SSRs in a forest where ink-disease-resistant C. crenata and C. mollissima seedlings had been planted in the 1940s. The 13 SSR loci selected were highly polymorphic and 115 different alleles were detected for the individuals sampled. Bayesian clustering identified two groups for C. sativa and one group each for C. mollissima and C. crenata. Within the forest, 70.6 per cent of adults and 28.6 per cent of juveniles were classified as pure C. sativa. Most juveniles were C. sativa × C. crenata (>40 per cent) and ca. 10 per cent were C. crenata × C. mollissima hybrids. Six new alleles private to C. crenata are reported here. The study quantifies, for the first time in Europe, introgression of Asian germplasm into a natural C. sativa forest. It also examines the extent of introgression in offspring and provides a method to detect exotic germplasm in C. sativa forests. We discuss why the forest studied may benefit from transfer of alleles involved in ink-disease resistance and why introgression will be detrimental to drought tolerance in offspring.