Artificial hybridization experiment with the Pinus sylvestris and Pinus mugo putative hybrid individuals and their parents revealed a selective nature of their crossability conditioned by the identical chloroplast DNA (cpDNA) haplotypes of the crossed individuals. Efficiency of hybridization between hybrid individual H1 of P . sylvestris haplotype and P . sylvestris paternal tree was relatively high, as evidenced by 78.03% of filled seeds. On the contrary, the crossing of H1 individual with P . mugo was unsuccessful. Likewise, the crossability of the H2 individual of P . mugo haplotype with the paternal tree P . mugo resulted in 31.15% of filled seeds. A relatively high crossability was characteristic also for the H1 and H2 hybrid trees of different cpDNA haplotypes. A conclusion has been drawn postulating weakened reproduction barrier between hybrid swarm individuals and the paternal species P . sylvestris and P . mugo . Differentiation of the putatively hybrid individuals from the parental species is accompanied by the variation in cpDNA inheritance.
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