Résumé
1)Co-infections may modify parasite transmission opportunities directly as
a consequence of interactions in the within host environment, but also
indirectly through changes in host life-history. Furthermore, host and
parasite traits are sensitive to the abiotic environment with variable
consequences for parasite transmission in co-infections. (2)We investigate
how co-infection of the mosquito Aedes aegypti with two microsporidian
parasites (Vavraia culicis and Edhazardia aedis) at two levels of larval
food availability affects parasite transmission directly, and indirectly
through effects on host traits (3)In a laboratory infection experiment we
compared how co-infection, at low and high larval food availability,
affected the probability of infection, within-host growth and the
transmission potential of each parasite, compared to single infections.
Horizontal transmission was deemed possible for both parasites when
infected hosts died harbouring horizontally-transmitting spores. Vertical
transmission was judged possible for E. aedis when infected females
emerged as adults. We also compared the total input number of spores used
to seed infections with output number, in single and co-infections for
each parasite. (4)The effects of co-infection on parasite fitness were
complex, especially for V. culicis. In low larval food conditions,
co-infection increased the chances of mosquitoes dying as larvae or pupae,
thus increasing opportunities for V. culicis’ horizontal transmission.
However, co-infection reduced larval longevity and hence time available
for V. culicis spore production. Overall there was a negative net effect
of co-infection on V. culicis whereby the number of spores produced was
less than the number used to seed infection. Co-infections also negatively
affected horizontal transmission of the more virulent parasite, E. aedis,
through reduced longevity of pre-adult hosts. However, its potential
transmission suffered less relative to V. culicis. (5)Our results show
that co-infection can negatively affect parasite transmission
opportunities, both directly as well as indirectly via effects on host
life-history. We also find that transmission is contingent on the combined
effect of the abiotic environment.