Pollen–pistil interactions in reproductive interference: comparisons of heterospecific pollen tube growth from alien species between two native Taraxacum species

1. Reproductive interference (RI), any negative interspecific interaction during the reproductive process, has been gaining increasing attention due to its potential explanatory power for the mutually exclusive distribution of closely related species. RI in plants may occur during any of three stages: pollen transfer, pollen–pistil interactions or hybridization. Pollen–pistil interactions may be especially important as most studies of RI have suggested the involvement at this stage. Details of these interactions are required to fully explore RI and are especially relevant in considering the impact of RI in the field. 2. We present a plausible explanation of how RI functions in the pollen–pistil interaction stage using two Japanese native dandelions, Taraxacum japonicum and Taraxacum longeappendiculatum, of which only the former is vulnerable to RI from an alien congener, Taraxacum officinale. We conducted a series of hand pollinations in these native dandelions and compared pollen tube behaviour to examine differences associated with vulnerability and imperviousness to RI from the alien. 3. The two native dandelions differed in terms of the absence/presence of pollen tube elongation after heterospecific pollination (pollination with only T. officinale): pollen tubes grew through the ovaries of the vulnerable T. japonicum, but not through those of the impervious T. longeappendiculatum. In vitro hand pollination verified that the alien pollen tubes could extend into the ovaries of T. japonicum. 4. Our results show that RI from the alien dandelion consumed ovules by heterospecific pollen deposition. The pistils of the impervious native species could prevent growth of the alien pollen tubes, thereby sparing the ovules for fertilization by conspecific pollen. The pistils of the vulnerable species lacked interspecific incompatibility against the alien, and thus, the alien pollen tube entered the ovary, eliminating an opportunity for conspecific pollen fertilization. This consumption of ovules by heterospecific pollen tubes would cause a seed set failure, leading to reduced abundance and a further exertion of RI in the next generation, which explains displacement of the vulnerable species by the alien in the field.

1. 生殖干扰（Reproductive Interference, RI）指发生于生殖过程中的各类有害种间相互作用，因其能够解释近缘物种间相互排他的分布格局，正受到学界越来越多的关注。植物中的生殖干扰可发生于三个阶段：花粉传递、花粉-雌蕊相互作用或杂交过程。其中花粉-雌蕊相互作用或许尤为关键，因为多数生殖干扰相关研究均表明该阶段是干扰发生的主要位点。若要全面解析生殖干扰的作用机制，明晰该阶段的相互作用细节至关重要，且其对于评估生殖干扰在野外环境中的影响同样具有关键意义。
2. 本研究以两种日本本土蒲公英——日本蒲公英（Taraxacum japonicum）和长萼蒲公英（Taraxacum longeappendiculatum）为研究对象，针对生殖干扰在花粉-雌蕊相互作用阶段的作用机制提出了合理的解释。其中仅日本蒲公英会受到外来同属物种西洋蒲公英（Taraxacum officinale）引发的生殖干扰威胁。研究团队针对这两种本土蒲公英开展了一系列人工授粉实验，并对花粉管行为进行比较分析，以探究受干扰物种与抗干扰物种在相关性状上的差异。
3. 两种本土蒲公英在异种授粉（即仅用西洋蒲公英花粉授粉）后的花粉管伸长情况上存在显著差异：花粉管可穿透易受干扰的日本蒲公英的子房，但无法穿透具有抗性的长萼蒲公英的子房。体外人工授粉实验进一步证实，西洋蒲公英的花粉管能够侵入日本蒲公英的子房。
4. 研究结果表明，外来蒲公英引发的生殖干扰会通过异种花粉沉积消耗宿主的胚珠。具有抗性的本土物种的雌蕊能够阻断外来花粉管的生长，从而使胚珠得以保留以供同种花粉完成受精。而易受干扰的物种的雌蕊则不具备针对外来花粉的种间不亲和性，因此外来花粉管可侵入子房，剥夺了同种花粉完成受精的机会。这种由异种花粉管消耗胚珠的现象会导致结籽失败，进而造成种群丰度下降，并在下一代进一步加剧生殖干扰的影响，这也解释了野外环境中外来物种如何将易受干扰的本土物种取代的现象。