Artificial crosses and hybridization frequency in five-needle pines

The capability of conifers for interspecific hybridization is well known. Five-needle pines from the section Quinquefoliae of the subgenus Strobus play an important role in ecosystems and have great economic significance. Interspecific hybridization that occurred in nature or under controlled pollination suggests that reproductive isolation is relative, being expressed to different degrees. Controlled pollination allows us to gather knowledge about reproductive compatibility and provides valuable material for breeding work. Artifi- cial crosses of five-needle pines began with a purely practical purpose to increase blister rust resistance and it was subsequently suggested that species crossability should be taken into account in their classification. We carried out a number of controlled pollinations using Pinus sibirica, P. koraiensis and natural P. sibirica × P. pum- ila hybrid clones as maternal trees along with pollen of 11 species and four hybrids of the five-needle pines. For the first time, seeds were obtained from the interspecific crosses P. koraiensis × P. armandii, P. koraiensis × P. monticola, P. koraiensis × P. sibirica, P. sibirica × P. ayacahuite, P. sibirica × P. armandii, and P. sibirica × P. wallichiana, as well as from some combinations with hybrids. Based on the results and available information on the crossability of the species, we concluded that complete reproductive isolation does not exist among five-needle pines, and confirmed that interspecific gene flow is common in this section.