Analysis on Pollen Flow in a Hybridizing Population between Farfugium hiberniflorum and F. japonicum (Asteraceae: Senecioneae): Derivation of an Empirical Formula Describing Frequency of Hybridization as a Function of Interspecific Plant Distance

Abstract The frequency of hybrids was monitored using progenies grown from seeds collected from open‐pollinated plants in mixed populations of Farfugium hiberniflorium and F. japonicum. The results showed that hybrids are more frequently derived from F. japonicum than from F. hiberniflorum. This directionality of hybridization and extraordinarily high frequency of hybrids among progenies grown from seeds from open‐pollinated plants of F. japonicum enabled us to derive an empirical equation describing the frequency of hybridization as a decreasing exponential function of interspecific plant distance. The relationship between these two variables fitted a function describing pollen flow from a particular plant. This finding suggests that interspecific pollen exchange can be viewed simply as a process dependent of interspecific plant distance, and the actual rate of hybridization is determined by the level of cross‐compatibility of a particular species.