Life history interactions with environmental conditions in a host-parasiterelationship and the parasite's mode of transmission

The microsporidian parasite Edhazardia aedis is capable of vertical or hori zontal transmission among individuals of its host, the mosquito Aedes aegyp ti, and either mode of transmission may follow the other. We show that foll owing the horizontal infection of host larvae, the parasite's subsequent mo de of transmission largely depends on host life history traits and their re sponses to different environmental conditions. In two experiments the inten sity of larval exposure to infection and the amount of food available to th em were simultaneously manipulated. One experiment followed the dynamics of host development and the parasite's production of spores while the other e stimated the outcome of their relationship. Host life history traits varied widely across treatment conditions while those of the parasite did not. Of particular importance was the host's larval growth rate. Horizontal rather than vertical transmission by the parasite was more likely as low food and high dose conditions favoured slower larval growth rates. This pattern of transmission behaviour with host growth rate can be considered in terms of reproductive value: the potential vertical transmission success that female mosquitoes offer the parasite decreases as larval growth rates slow and ma kes them more attractive to exploitation for horizontal transmission (requi ring host mortality). However, the lack of variation in the parasite's life history traits gave rise in some conditions to low estimates for both its vertical and horizontal transmission success. We suggest that the unrespons ive behaviour of the parasite's life history traits reflects a bet-hedging strategy to reduce variance in its overall transmission success in the unpr edictable environmental conditions and host larval growth rates that this p arasite encounters in nature.