Why are mycorrhizal fungi and symbiotic nitrogen‐fixing bacteria not genetically integrated into plants?

Abstract

The bacteria and fungi involved in diazotrophic and mycorrhizal symbioses with plants have varying degrees of genetic recombination. Those with very limited, or no, recombination are at risk from the progressive accumulation of mutations (‘Müller's ratchet'). This accumulation could be alleviated by complete genetic integration of the parts of the symbiont genomes relating to their functionin hospice(nitrogen fixation and acquisition of a range of nutrients, respectively) into the nuclear genome of the (usually) sexually reproducing exhabitant. This integration is not known to have occurred, and the diazotrophic and mycorrhizal inhabitants with very restricted recombination survive ‘Müller's ratchet', as do some other symbioses, free‐living organisms that have reproduced asexually for many generations and the organelle genomes of mitochondria and plastids. An evolutionary and environmental possibility is that it was difficult to maintain genetically integrated diazotrophs and mycorrhizas as environments changed with respect to the availability of combined nitrogen and of phosphorus. Genetic modification techniques could be used to attempt to genetically integrate diazotrophs and mycorrhizas; even granted that this could be done, a careful evaluation is required of whether the benefits of such manipulation outweigh whatever costs are entailed.