Experimental regimes and selection protocol
To study the interplay between sexual selection (SS) and pathogen
presence (P), we used a factorial design that manipulated the presence
or absence of SS (polygamous versus monogamous mating systems) and the
presence or absence of our pathogen, P. entomophila, resulting in
4 experimental regimes (+SS +P, +SS –P, -SS +P, -SS -P). Within each
experimental regime, 3 replicate populations were established.
Generation 0 adults were obtained by amplifying flies from the IV base
population stock, collecting virgin flies, and randomly assigning 80
males and 80 females to each of the 12 populations. At 5-6 days old,
virgin males were orally infected with P. entomophila (protocol
described in the following section) in +P treatments and sham-infected
in –P treatments. Males were mated with virgin females for 72 hours
after being exposed to infection for 24 hours (see below). Under
the +SS experimental regimes, groups of 5 males and 5 age-matched virgin
females were placed in interaction vials. Under the –SS regimes, groups
of 1 male and 1 age-matched virgin female were placed in interaction
vials. Flies were left in these interaction vials for 72 hours, after
which mated females from each population were pooled and re-distributed
in groups of 20 to new vials for egg laying. Females were allowed to lay
eggs for 72 hours, after which they were discarded from the vials while
larvae developed. On emergence, virgins were collected and housed in
groups of 20 in single-sex vials until they were 5-6 days old, at which
point the experimental protocol was repeated. Populations were
maintained under the experimental regimes for 14 generations at a
population size of 160 individuals (80 males + 80 females).