Figure 6. Tracheal Melanization can be Induced by Microorganisms.

(A) Larvae in which Spn77Ba RNAi was activated with btl-Gal4 were raised under 3 different conditions: sterile, normal, or + bacteria. The percentage of larvae and pupae showing no, mild, or severe melanization as seen in (B) was visually recorded. At least 200 larvae and pupae were counted for each condition.

(B) Representative examples of pupae with no melanization, mild melanization, and severe melanization. Arrow indicates melanization.

(C) Quantitative RT-PCR was used to monitor Drs expression level of prepupae showing either mild or severe melanization when cultured under sterile or non-sterile conditions. Data represent the average of at least three independent experiments ± SDV.

(D-G) Tracheal PO activity was assayed ex vivo in the presence or absence of bacteria (see Experimental Procedures). A mixture of E. coli and M. luteus induced in wild-type trachea a high level of PO activity, which was suppressed by either Spn77Ba over-expression or MP1 RNAi.

(H and H’) Exposure of wild-type larvae to B. bassiana spores induced melanization on cuticle (arrows) and Drs expression in both trachea and fat body.

(I) Cuticle melanization induced by B. bassiana was partially suppressed by Spn77Ba over-expression or MP1 RNAi. The percentage of larvae that showed melanization 24 hours after infection was counted.

(J) Drs expression induced by B. bassiana was partially suppressed by Spn77Ba over-expression or MP1 RNAi. The percentage of larvae that showed Drs-GFP expression 24 hours after infection was counted. *: p<0.05 (paired T-test).