Stock populations and experimental conditions
The experimental populations were established from a long-term laboratory population called Ives (IV) that was initiated from about 200 wild D. melanogaster of each sex collected in Massachusetts in 1975 (Charlesworth & Charlesworth, 1985). This population has been maintained in the lab at high density, with a census size in thousands, for more than 30 years and is adapted to the laboratory environment (Houle & Rowe, 2003). In the sexual competition experiment, we also used a reference population homozygous for a recessive ebonymutation previously backcrossed into the IV stock. To estimate pathogen virulence during experimental evolution, at each generation we ran a control using a line homozygous for a recessive relish mutation. The relish mutation blocks the Imd pathway that plays an important role in defense against gram negative bacterial pathogens (Hedengren et al. , 1999)); relish mutants are therefore highly susceptible to P. entomophila (Vallet-Gely et al. , 2010).
All flies in the experiment were maintained on fly media composed of (for 1L water): 6.2g Agar powder (ACROS N. 400400050), 58.8g Farigel wheat (Westhove N. FMZH1), 58.8g yeast (Springaline BA10), 100ml grape juice; 4.9ml Propionic acid (Sigma N. P1386), 26.5 ml of Methyl 4-hydroxybenzoate (Nipagin M, VWR N. ALFAA14289.0) solution (400g/l) in 95% ethanol. Populations were kept at 25⁰C with a 12L:12D cycle.