Hybridization and introgression in sympatric and allopatric populations of four oak species

BACKGROUND: Hybridization and introgression are vital sources of novel genetic variation driving diversification during reticulated evolution. Quercus is an important model clade, having extraordinary diverse and abundant members in the Northern hemisphere, that are used to studying the introgression of species boundaries and adaptive processes. China is the second-largest distribution center of Quercus, but there are limited studies on introgressive hybridization. RESULTS: Here, we screened 17 co-dominant nuclear microsatellite markers to investigate the hybridization and introgression of four oaks (Quercus acutissima, Quercus variabilis, Quercus fabri, and Quercus serrata) in 10 populations. We identified 361 alleles in the four-oak species across 17 loci, and all loci were characterized by high genetic variability (H(E) = 0.844–0.944) and moderate differentiation (F(ST) = 0.037–0.156) levels. A population differentiation analysis revealed the following: allopatric homologous (F(ST) = 0.064) < sympatric heterogeneous (F(ST) = 0.071) < allopatric heterogeneous (F(ST) = 0.084). A Bayesian admixture analysis determined four types of hybrids (Q. acutissima × Q. variabilis, Q. fabri × Q. serrata, Q. acutissima × Q. fabri, and Q. acutissima × Q. variabilis × Q. fabri) and their asymmetric introgression. Our results revealed that interspecific hybridization is commonly observed within the section Quercus, with members having tendency to hybridize. CONCLUSIONS: Our study determined the basic hybridization and introgression states among the studied four oak species and extended our understanding of the evolutionary role of hybridization. The results provide useful theoretical data for formulating conservation strategies. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12870-021-03007-4.