Invasive species poses a serious threat to native species genetically related to the invasive species through reproductive interference, invasion and interspecific hybridization. M. stellata is a rare tree species, distributed only in Tokai region, Japan. In contrast, M. kobus is a common tree species, widely distributed mainly in Japan, but it is not distributed in the range of M. stellata. However, because M. kobus trees are well planted, if planted trees and their escaped trees (offspring) grow near the habitat of M. stellate, interspecific hybridization may occur. In this study, we evaluated the effects of a naive invasive species, M. kobus, on the persistence of a rare species, M. stellate. First, we examined the genealogical classes of adult and immature (saplings and seedlings) trees in two study sites using nuclear and chloroplast microsatellite markers. We identified F1, F2 hybrids and backcrosses to M. stellata and M. kobus, indicating that interspecific hybridization and the subsequent introgression can occur. Second, we conducted hand-pollination experiments. Interspecific crosses succeeded like intraspecies crosses. We also carried out pollination with mixed pollen from both species and determined pollen parents using paternity analysis with microsatellite markers, suggesting that reproductive interference from M. kobus can occur. Third, we examined the differences among the two species and hybrids in survival and growth of adult and immature trees for four and two years, respectively. No significant differences appeared except for survival rates of immature trees. Based on the findings, we discuss the effects of M. kobus on the persistence of M. stellata.
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