We have explored the gene expression and localization of an aquaporin (AQP) in the silk gland of Bombyx mori. Whole mount in situ hybridization studies with digoxigenin-labelled RNA probes derived from an open reading frame of B. mori AQP (Bommo AQP) showed its mRNA distribution strictly at the anterior silk gland (ASG) from vigorously feeding larva. Its expression disappeared from actively spinning larva. Northern hybridization analysis of Bommo AQP in the ASG demonstrated that a single transcript of 2.3kb was abundantly present during the feeding phase of fifth instar stadium and its expression decreased after spinning. Immunocytochemical studies using an antipeptide antibody against the Bommo AQP molecules revealed that the positive reaction was localized at the apical surface of ASG cells from feeding larva and disappeared from spinning larva. Further the apical surface localization of Bommo AQP was found in the posterior division of the middle silk gland (posterior MSG) and the strong immunoreaction was also observed at the apically-opened vacuoles, cavities and pits predominantly existing in the most posterior MSG. The immunoreaction at the posterior MSG almost disappeared in the spinning phase. The occurrence and disappearance of AQP at the limited region of the silk gland was evidently coincided with that found in the H+-translocating vacuolar-type ATPase. An osmoregulatory work along the length of the silk gland enables a silkworm larva to stabilize the liquid silk in a native state with entrained water during growth and development.
