The eggs of Asian tiger mosquito, Aedes albopictus, possess high desiccation resistance, which contribute the rapid spread of this mosquito across the world. Melanization of eggshell appear to play a role in the resistance to desiccation. Dopachrome-conversion enzyme (DCE, Yellow) significantly accelerates the melanization of the eggshell. In this study, we demonstrated functional importance of two yellow genes, AalY-g and AalY-g2, in the chorion formation. Both genes were highly induced in the ovary at 48 h after blood meal. Injection of dsRNA for AalY-g or AalY-g2 into adult females had no effect on fecundity. However, the outermost colorless exochorion of the eggs obtained from both dsRNA-treated females was fragile and peeled off in places, and melanization of the endochorion was obviously delayed by several hours. In addition, unlike eggs from control females which acquired high desiccation resistance between 18 and 24 h after oviposition (HAO), 60-70% 24 HAO eggs from either AalY-g- or AalY-g2-deficient females were collapsed when they were moved to an air-dry condition, and the desiccation resistance was not increased in later stages of embryonic development analyzed. TEM analysis revealed that abnormal morphology and ultrastructure of the endochorion, particularly outer-endochorion, in the 24 HAO and older eggs from either AalY-g-and AalY-g2-deficient females. These results indicate that AalY-g and AalY-g2 are required for morphology and formation of the endochorion (outer-endochorion), a structure that appears to be critical for desiccation resistance of the Ae. albopictus eggs. This work was supported by NRFs (NRF-2015R1A6A3A04060323 and NRF-2018R1A2B6005106)

Eggs from the mosquito genus Aedes exhibit high desiccation resistance that likely facilitate spreading some of them as a vector of human disease throughout the world. However, molecular mechanism underlying the embryonic resistant to desiccation has not well understood. In this study, we performed functional study of two ovary-specific yellow genes, AalY-g and AalY-g2, in the Asian tiger mosquito, Aedes albopictus. The eggs obtained from AalY-g or AalY-g2 RNAi females showed poor desiccation resistance. TEM analysis revealed that, unlike that seen in the dsEGFP-control eggs, no high electron-dense outer-endochorion was evident in the eggs from AalY-g- or AalY-g2-deficient females. These results suggest that both yellow genes at least play roles in the chorion formation, which appears to be critical for integrity and desiccation resistance in Ae. albopictus eggs.

The eggs of Asian tiger mosquito, Aedes albopictus, possess high desiccation resistance, which could lead rapidly spreading this mosquito across the world. Melanization and/or hardening of mosquito eggshell are critical for desiccation resistance to protect embryo from adverse environment. Dopachrome-conversion enzymes (Yellows) catalyze melanin synthesis in insects. Here we report functional importance of two ovary-specific yellow genes, AalY-g and AalY-g2, in desiccation resistance of Ae. albopictus egg. Loss of function of these genes by RNAi causes the fragile exochorion and delay initial melanization of the endochorion. Furthermore, eggs from control females exhibit high desiccation resistance under the air-dry condition, whereas those from AalY-g- or AalY-g2-deficient females are collapsed. TEM analysis reveals ultrastructural defects in the endochorion and serosal cuticle in these eggs. The results indicate that AalY-g and AalY-g2 are critical for integrity and desiccation resistance of the Ae. albopictus egg.

Pigmentation or melanization is an important physiological event in insects and is involved in cuticle tanning, wound healing and encapsulation as a defensive response. Dopachrome-conversion enzyme (DCE, Yellow) significantly accelerates the melanization of the chorion in mosquito eggs. In this study, we demonstrated functional importance of two ovary-specific yellow genes, AalY-g and AalY-g2, in the Asian tiger mosquito, Aedes albopictus. The transcripts of both genes were detected in the ovary of adult females only 48-72 h after blood feeding. RNAi for AalY-g or AalY-g2 had no effect on fecundity. However, the outermost colorless exochorion of the eggs was fragile and partially peeled off, and initial melanization of the endochorion was significantly delayed. Eggs from control females exhibited high desiccation resistance, whereas those from dsAalY-g- or dsAalY-g2-treated females were collapsed (50-80%) under the air-dry condition. Ultrastructural analysis revealed abnormal morphology of the endochorion and vitelline membrane in the AalY-g and AalY-g2-deficient eggshell. These results indicate that AalY-g and AalY-g2 are critical for integrity and desiccation resistance of the Ae. albopictus eggs. This work was supported by NRFs (NRF-2015R1A6A3A04060323).