Endosperm-based postzygotic hybridization barriers: developmental mechanisms and evolutionary drivers

The endosperm is a nourishing tissue that serves to support embryo growth. Failure of endosperm development will ultimately cause embryo arrest and seed lethality, a phenomenon that is frequently observed upon hybridization of related plant species or species that differ in ploidy. Endosperm-based interspecies or interploidy hybridization barriers depend on the direction of the hybridization, causing nonreciprocal seed defects. This reveals that the parental genomes are not equivalent, implicating parent-of-origin specific genes generating this type of hybridization barrier. Recent work revealed that endosperm-based hybridization barriers are rapidly evolving. In this review, we discuss the developmental mechanisms causing hybrid seed lethality in angiosperms as well as the evolutionary forces establishing endosperm-based postzygotic hybridization barriers.