Host genotype and environment affect the trade-off between horizontal and vertical transmission of the parasite Edhazardia aedis

Background: If a parasite is able to transmit horizontally or vertically, which transmission mode will it choose?
We investigated how the growth conditions and the genotype of the mosquito Aedes aegypti affect the transmission mode of the parasite Edhazardia aedis.
Results: In poor conditions the parasites were more likely to be transmitted horizontally, whereas in favourable conditions they were more likely to be transmitted vertically. This plasticity appears to be adaptive. Unfavourable conditions delayed emergence, giving the parasite more time to produce its horizontally transmitted stage; in more favourable conditions mosquitoes have greater reproductive success, increasing the effectiveness of vertical transmission. In addition, the parasite's ability to transmit vertically was influenced by the genetic background of the host (i.e., its full-sib family), giving a genetic correlation between the host's life-history and which of the parasite's transmission mode it enables. In par...