On the persistence of reproductive barriers in<i>Eucalyptus</i>: the bridging of mechanical barriers to zygote formation by<i>F</i>₁hybrids is counteracted by intrinsic post-zygotic incompatibilities

Abstract

The removal of strong mechanical barriers between E. nitens and E. globulus allows F1 hybrids to act as a bridge for bi-directional gene flow between these species. However, strong and persistent post-zygotic barriers exist, meaning that wherever F1 hybridization does occur, intrinsic post-zygotic barriers will be responsible for most reproductive isolation in this system. This potential transient nature of mechanical barriers to zygote formation due to additive inheritance in hybrids appears under-appreciated, and highlights the often important role that intrinsic post-mating barriers play in maintaining species boundaries at zones of secondary contact.