Selection of Anopheles dirus for refractoriness and susceptibility to Plasmodium yoelii nigeriensis

Two lines of the Oriental malaria vector mosquito Anopheles dirus species A (Diptera: Culicidae), one fully refractory and one fully susceptible to Pl asmodium yoelii nigeriensis (an African rodent malaria parasite), were esta blished after 17 generations of mass selection, followed by single female s election for one or two generations. Prior to selection, the stock colony o f An. dirus was 17% refractory. Both lines of An. dirus produced abundant o okinetes that started to invade the midgut within 24h post-infection, as se en in histological sections. In most of the refractory mosquitoes, oocysts stopped development <12h post-invasion, indicating a rapid defence mechanis m. Dead P. y. nigeriensis parasites were apparently localized as small mela nized spots (2-5 mu m) seen in wet preparations of mosquito midguts dissect ed 5-7days post infective bloodmeal. In some refractory An. dirus females, apart from the spots, a small number of totally encapsulated oocysts (c. 10 mu m) were also present. These larger melanized parasites predominated in a few females: they appeared 2-3 days post-infection as a secondary delayed defence mechanism. The progeny of reciprocal matings between susceptible a nd refractory lines had similar to 50% susceptibility. Backcrosses of F-1 h ybrids with susceptible or refractory lines increased or decreased the susc eptibility of backcross progeny accordingly. Overall, these results suggest polygenic control of susceptibility to P. y, nigeriensis infection. The re fractory line of An dirus showed normal susceptibility to natural infection s of the human malarias P. falciparum and P. vivax from local patients.