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).