Genetic Diversity and Genetic Structure of Three Sympatric Oak Species in Serbian Landscape of Outstanding Features "Kosmaj" Assessed by Nuclear Microsatellites

Quercus petraea (Matt.) Liebl., Q. pubescens Willd., and Q. frainetto Ten. grow naturally in sympatry in the Landscape of Outstanding Features "Kosmaj" (Mt. Kosmaj) in Serbia, in the Western Balkans. The levels of genetic diversity and genetic structure in populations of these species (160 trees in total) was assessed by means of 14 nuclear microsatellites. The number of alleles detected in overall sample was 314, with the locus QrZAG90 being the most informative one in all three species (31, 35 and 36 alleles in Q. frainetto, Q. pubescens and Q. petraea, respectively). The levels of genetic diversity of all three species were relatively high (HE = 0.824, 0.834, and 0.794 in Q. petraea, Q. pubescens and Q. frainetto, respectively). Low but statistically significant inbreeding was detected in all three species (F = 0.100, 0.131 and 0.065 in Q. petraea, Q. pubescens and Q. frainetto, respectively), which, however, most likely reflects population substructure, which was observed in the STRUCTURE analysis. The optimal number of genetic groups revealed by Bayesian clustering analysis did not coincide with the number of analyzed species as it amounted to four (one gene pool was observed in each of the two species, Q. petraea and Q. pubescens, while two gene pools were observed in Q. pubescens). Furthermore, a higher level of hierarchical genetic structure, with six gene pools was found, with Q. frainetto being the only oak species in which population substructure was not observed. Genetic differentiation of three sympatric oak species was relatively low but statistically significant, with the highest FST value found among Q. petraea and Q. frainetto (FST = 0.047, P = 0.001), and the lowest between Q. petraea and Q. pubescens (FST = 0.032, P = 0.001). Our results reveal rather high levels of genetic diversity in all three studied oak species, which retained their genetic integrity despite sympatric distribution, indicating low levels of interspecific hybridization, and pronounced genetic structure of Q. pubescens and Q. petraea.