The role of tryptophan and its derivatives for development of malaria parasite in vector mosquito

Xanthurenic acid (XA), a lateral reaction product of tryptophan metabolism in the omochrome pathway of eye pigment synthesis in insects, induces gametogenesis of malaria parasites. We have succeeded in measuring XA contents in the mosquito tissues using a high performance liquid chromatography with electrochemical detection system. XA content is not enough for activating gametocytes in the midgut where blood meal is taken. We have found that the salivary gland of mosquito contains a sufficient amount of XA for activating gametocytes and mosquito ingests saliva into the midgut during blood feeding. Taken together, it is likely that XA is discharged from salivary gland during blood feeding and is swallowed to the midgut where it affects malaria gametocytes. In the present study, we compared young mosquitoes (2–3 day-old after emergence) with old mosquitoes (12–14 day-old after emergence) in terms of transmission efficacy. XA contents in whole body and the salivary gland were larger in the young group. Numbers of oocyst developed on the midgut after taking blood from the same mouse with malaria were also higher in the young group. When both groups fed a blood meal with cultured ookinetes of malaria parasites, the numbers of oocyst were similar in both groups, suggesting that conditions for development from ookinetes to oocysts were similar in young and old mosquitoes. Taking these results together, we conclude that transmission efficacy is controlled by the amount of XA in the salivary gland.