Host benefit and the evolution of specialization in symbiosis

Various animals and plants benefit from symbiotic microorganisms, but the e ffectiveness of the microbial symbionts (i.e. the amount of benefit that th e animal/plant host derives from the symbiosis) varies among symbiont genot ypes in natural populations. Recent studies on symbioses with horizontal tr ansmission suggest that three factors may contribute to this variation: (a) selection pressure on the microbial symbionts to exploit the host, resulti ng in reduced host benefit (b) variation with environmental circumstances i n the amount of benefit derived by a host from different symbiont genotypes and (c) unpredictable or low abundance of the microbial partner available to infect hosts from the free-living environment. The latter two factors wo uld counter the selection pressure on hosts to specialize (i.e. to form sym bioses exclusively with highly effective symbiont genotypes), despite varia tion in symbiont effectiveness. Vertical transmission is identified as a ke y route to host specialization on effective symbionts.