Data from: Strength and variability of postmating reproductive isolating barriers between four European white oak species

The identification and quantification of the relative importance of reproductive isolating barriers is of fundamental importance to understand species maintenance in the face of interspecific gene flow between hybridizing species. Yet, such assessments require extensive experimental fertilisations that are particularly difficult when dealing with more than two hybridizing and long generation time species such as oaks. Here we quantify the relative contribution of four postmating reproductive isolating barriers consisting of two prezygotic barriers (gametic incompatibility, conspecific pollen precedence) and two postzygotic barriers (germination rate, early survival) from extensive controlled pollinations between four oak species (Quercus robur, Q. petraea, Q. pubescens and Q. pyrenaica) that have been shown to frequently hybridize in natural populations. We found high variation in the strength of total reproductive isolation between species, ranging from total reproductive isolation to advantage toward hybrid formation. As previously found, Q. robur pollen was unable to fertilize Q. petraea due to a strong reproductive isolating mechanism. On the contrary, Q. pubescens pollen was more efficient at fertilizing Q. petraea than conspecific pollen. Overall, prezygotic barriers contribute far more than postzygotic barriers to isolate species reproductively, suggesting a role for reinforcement in the development of prezygotic barriers. Conspecific pollen precedence reduced hybrid formation when pollen competition was allowed, however presence of conspecific pollen did not totally prevent hybridization. Our results suggest that pollen competition depends on multiple ecological and environmental parameters, including species local abundance, and that it may be of uppermost importance to understand interspecific gene flow among natural multispecies populations.

鉴定并量化生殖隔离障碍（reproductive isolating barriers）的相对重要性，对于理解杂交物种间存在种间基因流（interspecific gene flow）时的物种维持机制具有核心意义。然而，此类评估需要开展大量人工控制授粉（controlled pollinations）实验，而当涉及超过两种杂交类群且世代周期较长的物种（如栎属植物）时，这类实验的实施难度极大。本研究针对4种在自然种群中频繁发生杂交的栎属物种——夏栎（Quercus robur）、无梗花栎（Q. petraea）、柔毛栎（Q. pubescens）以及比利牛斯栎（Q. pyrenaica）——开展了大规模控制授粉实验，量化了4种交配后生殖隔离障碍的相对贡献：其中2种为合子前障碍（prezygotic barriers），即配子不亲和（gametic incompatibility）与同种花粉优先（conspecific pollen precedence）；另外2种为合子后障碍（postzygotic barriers），即花粉萌发率（germination rate）与早期存活率（early survival）。研究发现，不同物种间的总生殖隔离强度存在显著差异，其跨度从完全生殖隔离到有利于杂交形成的不同状态。正如此前研究所示，由于存在较强的生殖隔离机制，夏栎的花粉无法完成对无梗花栎的受精。与之相反，柔毛栎的花粉在为无梗花栎受精时，其效率要高于其同种花粉。总体而言，合子前障碍在物种生殖隔离中的贡献远高于合子后障碍，这暗示强化作用（reinforcement）在合子前障碍的演化过程中发挥了重要作用。当存在花粉竞争条件时，同种花粉优先效应会降低杂交形成的概率，但即便有同种花粉存在，也无法完全阻止杂交事件的发生。本研究结果表明，花粉竞争受多种生态与环境参数影响，包括物种的本地种群丰度，而阐明这一过程对于理解自然多物种种群间的种间基因流具有至关重要的意义。