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...