Field of Science

Fighting dengue with mosquito semen

Dengue virus, DENV - an important mosquito-borne virus
Arthropods are important vectors in the transmission of a number of animal and human pathogens. A major vector group are the mosquitoes of which there are over 3,000 species. However, during their life cycle some mosquitoes feed on the blood of other animals - creating an excellent chance for the direct transfer of manymicrobial species. From here the bacteria/viruses/parasites can initiate infection of the new host which then, following another blood feed, may transmit the pathogen to an uninfected insect. This particular lifestyle allows for the development of pathogen control strategies aimed at interfering with the vector species. If we remove or inhibit the vector, we may prevent the spread of the pathogens they carry.

 Why dengue?

Mosquito sex
Another group of mosquito-borne pathogens are the closely related - but distinct - dengue viruses (DENV). DENV is commonly responsible for a 'flu-like- illness' in humans but complications may include a potentially fatal hemorrhagic fever. The incidence is mainly constrained to tropical and sub-tropical areas although in recent decades it has spread to other areas where it may cause massive epidemics. The World Health Organisation states that, "Some 2.5 billion people – two fifths of the world's population – are now at risk from dengue. WHO currently estimates there may be 50 million dengue infections worldwide every year." There is currently no commercially licensed vaccine or antivirals available for the treatment of dengue leaving the only option to prevent transmission through control of mosquito populations.

The most important vector species is the predominantly urban species Aedes aegypti - control of which may aid DENV eradication. There are a number of potential strategies that could be employed to reduce the numbers of this species including chemical poisoning, genetic strategies and biological control. A major goal is therefore to inhibit  mosquito reproduction and feeding behaviour yet this requires intimate knowledge of mosquito reproductive biology. Insects communicate via a number of chemical signals, one mode of communication is via the males ejaculate -what happens to be a convenient opportunity to control a females behaviour. A mated female is behaviourally very different to an unmated one and there is evidence suggesting that this change is initiated by the transfer of male-derived signaling proteins during mating. It may then prove to be useful to identify some of these molecules so as to possibly control mosquito behaviour ourselves.

What is so good about male semen?

Sirot et al recently report, using proteomic analysis, the identificantion of a number of proteins, termed seminal fluid proteins (Spfs) transferred from males (with labelled proteins) to females (non-labelled proteins) during mating. Of which some may be responsible for the male control over female post-reproductive behaviour; this they say, lays the groundwork for future studies investigating the molecular mechanisms behind how they work and their potential use in controlling vector populations. However, care must be taken in interpreting these results as this study does not directly look at the biological effects of these proteins and does not prove that they do have any effect on female behaviour.

Labelled insect sperm

So, what do these semen proteins actually do?
Using this approach they identified 145 proteins transferred from males to females, 17 of which were previously unknown to science - 93, they say, could be assigned as potential biologically active proteins. What function do this proteins have? Well, based on the previous annotation in protein databases, they were able to assign each of their proteins a potential function indicating the potential important roles in female behaviour. These proteins are predicted to be involved in their reproductive biology, specifically protein degradation and hormonal signalling.

This work has identified a number of proteins present in the semen of the dengue vector, Aedes aegypti which are transferred from males to females during mating. This may mean they are involved in the control of female behaviour. Although this work did not look at the function of any proteins directly, it does lay the foundations for future studies. Researchers may now focus there investigations on a set of a now verified smaller set of proteins and genes. This  species is also the vector for a number of other viruses such as chikungunya and yellow fever and so any work on this may aid the control of these important diseases.
Gillott C (2003). Male accessory gland secretions: modulators of female reproductive physiology and behavior. Annual review of entomology, 48, 163-84 PMID: 12208817

Sirot, L., Hardstone, M., Helinski, M., Ribeiro, J., Kimura, M., Deewatthanawong, P., Wolfner, M., & Harrington, L. (2011). Towards a Semen Proteome of the Dengue Vector Mosquito: Protein Identification and Potential Functions PLoS Neglected Tropical Diseases, 5 (3) DOI: 10.1371/journal.pntd.0000989

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