Host-parasite co-selection (and cross-infection) design. Solid lines represent host and parasite co-selection over generations. Only one replicate illustrated, for simplicity, from: three lines of S. mansoni co-selection passage through either R (artificially-selected resistant snails); R:U (equal mix of resistant-selected and unselected snails) or U (unselected snails), of four replicates per co-selection line per P1-F3generation (s for snail; p for parasite line) mass exposed to a matched dose of S. mansoni, at n = 20 s snails per 'population' per line replicate per generation. Broken lines represent parasite (p) exposure to novel (n) non-coselected snail (cross-infections) at n = 10 n snails per 'population' per line replicate per generation). Given the generation times of the parasite for harvesting in each of its obligatory host stages (7 weeks in the snail; 8 weeks in the mouse), each line/replicate here represents a minimum of 62 weeks (approx 1.2 years) continuous co-selection data collection (in addition to the preceding approx 2.4 years continuous snail host only P1-F6 artificial selection for resistance).