Functional transfer of mitochondrial genes to the nucleus is very common in some taxa but entirely lacking in others. Current evolutionary theories to account for this variation predict that outcrossing, which allows escape from Muller's ratchet and faster spread of beneficial mutations, should favor gene transfer. We find that functional gene transfer is more common in selfing or clonal plants than in outcrossing plants, a pattern opposite to prediction. We suggest that reproductive modes, such as selfing and vegetative reproduction, conserve adaptive mitonuclear gene combinations, allowing functional transfer, whereas outcrossing prevents transfer by breaking up these combinations.