A factor preventing melanization of sephadex CM C-25 beads in Plasmodium-susceptible and refractory Anopheles gambiae

One major quantitative trait locus controls melanization of both malaria ookinetes and Sephadex CM beads in a refractory strain of the mosquito, Anopheles gambiae. Hemolymph transferred from a nonmelanizing, Plasmodium- susceptible strain (4arr) to a melanizing, Plasmodium-refractory strain (L35) caused a reduction in the melanization of CM beads. In addition, when beads were first incubated in vivo in susceptible mosquitoes and then recovered, washed, and transferred to refractory mosquitoes, a strong reduction in melanization was observed. No changes in melanization were observed when beads or hemolymph were transferred in the opposite direction or within a strain. Incubation of beads in vitro in refractory or susceptible hemolymph resulted in a reduction of melanization when these beads were subsequently transferred to refractory mosquitoes. This reduction was significantly stronger when susceptible hemolymph was used as the incubating medium. Protection from melanization was observed after 3-, 6-, and 24-h incubations of beads in susceptible mosquitoes with longer incubations resulting in greater protection. Treatment of protected beads with 1 M NaOH resulted in the loss of the protection but treatment with 1% sodium dodecyl sulfate (SDS), 1% SDS/DTr/boiling, or 1 M NaOAc (pH 8.9) did not. These results show that a melanization-preventing factor covalently binds to the surface of CM beads in susceptible mosquitoes and can subsequently prevent melanization in refractory mosquitoes.