Chapter 21, Problem 1DQ

Summary Introduction

To review:

The control of diseases such as malaria by altering insect metamorphosis.

Introduction:

Malaria is one of the most widespread vector-borne diseases common in tropical and subtropical areas of the world, including regions in Africa, Asia, and America. Malaria is caused by the protozoan parasites, belonging to the genus Plasmodium, residing in some female mosquitoes of the genus Anopheles.

Explanation of Solution

Hormonal control is essential for coordination and regulation of many aspects of the developmental process in many organisms. Insect molting and metamorphosis are controlled by two effector hormones: the steroid 20-hydroxyecdysone (20E) and the lipid juvenile hormone (JH). 20E has been characterized as the principal molting hormone in insects (Gilbert et al., 2002), which initiates and coordinates each molt (larva-to-larva, larva-to-pupa, pupa-to-adult) and regulates the gene expression changes that occur during metamorphosis. Molting and metamorphosis are initiated by a rise in the titer of the ecdysteroids. Upon initiation of the endocrine signaling, 20E binds to the ecdysone receptor (EcR), a member of the nuclear hormone receptor superfamily, which heterodimerizes with another nuclear receptor. The regulation of insect metamorphosis is controlled by the juvenile hormone (JH), ecdysone, and active hormone 20E. 20E and JH together cause molts forming next larval instar. The low concentration of JH, the 20E-induced molt produces pupa instead of larva. When 20E acts in the absence of JH, the imaginal discs differentiate and the molt gives rise to an adult (imago).

In Lepidoptera, there is one or more small peaks of ecdysteroids in the absence of JH following attainment of a critical weight that initiate metamorphic events such as wandering behavior. Then a high peak of ecdysteroids in the presence of JH initiates pupation. In numerous studies, precise fluctuations of ecdysteroid titers in last instar larval and in pupal stages of Ae. Aegypti have been determined. Many studies attempted to correlate ecdysteroid levels to gene expression profiles to measure ecdysteroid fluctuations in our system. Expression of ecdysone receptors, transcription factors and other ecdysone-regulated genes was compared with the timing of ecdysteroid peaks.

Conclusion

The future research focuses on the specific function(s) of ecdysone receptors and ecdysone regulated transcription factors during mosquito development to control malaria.