The role of nucleocytoplasmic interactions in the genesis of post-zygotic isolation has been given little attention by plant evolutionists. I present evidence from reciprocal crosses, cytoplasmic substitution lines, and cell fusion lines that hybrid weakness and sterility often arise from interactions between the nuclear genome and the chloroplast and mitochondrial genomes. These interactions are much more important in the origin and isolation of species than we appreciate. The strength of the post-zygotic barriers tends to be a function of cytoplasmic divergence. I also review evidence indicating that the properties and evolutionary potential of allopolyploids and diploid hybrid derivatives may be influenced by cytoplasmic factors.

Communicating Editor: John V. Freudenstein