Cytonuclear Genomic Interactions and Hybrid Breakdown

Reduced fitness in interpopulation hybrids can be a first indication of genetic incompatibilities that may ultimately lead to reproductive isolation and speciation. A growing number of cases of hybrid breakdown have been traced to incompatibilities between the nuclear genome and the organellar genomes of the mitochondria and chloroplasts. Although these organellar genomes derive from ancient bacterial endosymbioses, they have been vastly reduced in size and now encode relatively few genes. The remaining genes are necessary but not sufficient for organelle function. In fact, most proteins functioning in the organelles are encoded in the nuclear genome and need to be imported after synthesis in the cytosol. The necessary interactions between organelle and nuclear genomes have resulted in some degree of coadaptation within all natural populations. Hybridization brings together previously untested allelic combinations and can disrupt intergenomic coadaptation, resulting in organelle dysfunction and, consequently, hybrid breakdown.