Résumé
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 particular, genotypes with large bodies (and therefore
high fecundity) were more likely to enable vertical transmission than
genotypes with small bodies. This led to a trade-off among the host's
families (which can be interpreted as a genetic correlation) for the
parasite's transmission mode. Conclusions: Since horizontal
transmission is linked to higher virulence than vertical transmission, the
host's contribution to transmission mode has important consequences
for the evolution of parasites with mixed-mode transmission.