Pollen-pistil interaction and asymmetric reproductive interference in Veronica species

Reproductive interference is a negative interspecific interaction that can drive mutually exclusive distributions of closely related species. While recent research indicates that reproductive interference in plants frequently occurs during pollen-pistil interactions, comprehensive descriptions of these interactions are scarce. Understanding the mechanisms underlying reproductive interference requires studies integrating empirical observations with interaction analyses. This study investigates pollen-pistil interactions between three Veronica species recently observed to exhibit asymmetric reproductive interference.Our experiments revealed similar pollen tube behavior in V. polita var. lilacina and V. cymbalaria pistils, irrespective of whether they were pollinated with conspecific or V. persica pollen. Conversely, in V. persica pistils, the number of pollen tubes significantly decreased following heterospecific pollination compared to conspecific pollination. Furthermore, half of the pollen grains on V. persica stigmas, presumably heterospecific pollen grains, occasionally appeared non-luminous under the fluorescence microscope after mixed pollination. Conspecific pollen tubes appeared to grow faster within V. persica pistils; however, statistical analysis did not support this trend.These results suggest that V. polita var. lilacina and V. cymbalaria pistils exhibit limited discrimination against V. persica pollen, resulting in ovule discounting by the heterospecific pollen. This mechanism explains the empirical observation that these species experience reduced seed set after mixed pollination, even when conspecific pollen is applied first. In contrast, V. persica pistils demonstrate a degree of discrimination between conspecific and heterospecific pollen, particularly when conspecific pollen arrives on the stigma prior to heterospecific pollen. This selectivity accounts for the empirical findings that V. persica exhibits reduced seed set only when surrounded by V. polita var lilacina or after mixed pollination with V. cymbalaria pollen preceding conspecific pollen.