Monday, March 11, 2013

Tick, Mite, Mosquito, WOLBACHIA?!

by Matteo Vaiente

"So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite ’em;
And so proceed ad infinitum." - Johnathan Swift

In an earlier blog, I discussed some current research in the field of microbial symbiosis within a human host. However, bacterial symbiosis is known to be present in a variety of different animals.  While attending a physiology lecture by Professor Emily Taylor at Cal Poly SLO, the role of bacterial symbiosis in a species of mites was presented, with striking results of this symbiotic association. The bacterium, in this case of the genus Wolbachia, has been shown to increase the proportion of females in a population, among other things, due to direct effects on the reproductive physiology of the mite itself.  Upon hearing that presence of a bacterial species can drastically affect the physiology of arthropods, I became interested in the role of this bacterium in arthropod species, and decided to share some current research aiming to characterize the symbiotic relationship between Wolbachia and its arthropod hosts.

In a recent paper by Zug and Hammerstein, it is estimated that the bacterium is present in approximately 40% of terrestrial arthropod species1.  This alarmingly high proportion suggests that there may be some conferred fitness advantages due to this symbiotic association. Wolbachia has been studied extensively for its ability to modulate host reproductive physiology, inducing large proportions of female progeny in infected individuals through mechanisms such as cytoplasmic incompatibility (CI). Interestingly, in a review by Crespi and Schwander, it is suggested that asexuality induced by Wolbachia infections may contribute to the accumulation of deleterious genetic elements that are normally eliminated via recombination and other mechanisms exclusive to sexual reproduction2. Why then would there be such an alarmingly high proportion of arthropod species infected if this symbiosis is genomically disadvantageous?

A recent study by Xue et al examined the physiological roles of Wolbachia symbiosis in Bernisia tabaci, otherwise known as the common whitefly. The role of Wolbachia symbiosis in whitefly biology is poorly defined, so this study represents the first of its kind to asses various reproductive and non-reproductive roles of this particular relationship3. To assess the role of Wolbachia symbiosis, targeted removal of this organism was achieved using rifampicin. The authors were able to show, using PCR and other molecular techniques, that this antibiotic could be used to remove Wolbachia populations without affecting the presence or abundance of other microbial symbionts of the whitefly. Whiteflies were separated into two groups, treatment and control. Within the treatment groups, whiteflies were exposed to rifampicin for 12, 24, 36, or 48 hours.  The development time of F1 progeny hatched from eggs in which the mother was exposed to rifampicin increased significantly in all time groups. Additionally, the percent viability of F1 progeny was compared between treatment and control groups.In the flies where Wolbachia was removed the viability of their F1 progeny decreased, and tended to decrease more with longer rifamipicin exposures. These results suggest that Wolbachia presence in the mother before reproduction confers a strong fitness advantage to those progeny, while the absence of Wolbachia is deleterious to development time and also survival of new progeny. Also shown previously, this study reports that there is an increase in the proportion of male progeny as exposure time to rifamipicin increased.  It seems as though symbiosis is crucial for whitefly fitness, perhaps by providing the host with some essential non-dietary nutrient, which explains the harsh reduction in percent survival and growth suppression present in Wolbachia negative flies.

The physiological effects of Wolbachia colonization are exerted through maternal, vertical transmission which may also be why the organism has evolved to increased female progeny in hosts. There is also recent evidence that this bacterium may be spread horizontally among species. The study published by Zhang et al suggests that this may be the case. Using a variety of molecular techniques to assess diversity (including PCR and multi-locus sequence typing), the authors report that immensely similar Wolbachia strains have been observed in divergent species, suggesting that there may be horizontal transmission of this bacterium within species4.

Recently, the biology of this organism has been studied for use as a control strategy for diseases that are commonly transmitted by arthropods, such as malaria and West Nile Virus. Husain et al describe a decreased viral secretion in cultured cells from Aedes aegypti which are infected with Wolbachia5.

The possibility of horizontal transmission between other arthropods makes Wolbachia an attractive candidate for a biocontrol agent that can be used to decrease the transmission of diseases in areas where other interventions are too costly to implement. However, the varied effects on the physiology of different arthropods species needs to be rigorously tested before full scale implementation. In at least one study, mosquitoes that were artificially infected with Wolbachia were shown to be invasive over uninfected species.
While there is a paucity of evidence describing the exact mechanisms in which Wolbachia exerts this modulatory effect on its arthropod hosts, there is compelling evidence that this symbiosis is not only necessary, but beneficial to may organisms, including ourselves.

1.            Zug, R. & Hammerstein, P. Still a host of hosts for Wolbachia: analysis of recent data suggests that 40% of terrestrial arthropod species are infected. PLoS ONE 7, e38544 (2012).
2.            Crespi, B. & Schwander, T. Asexual evolution: do intragenomic parasites maintain sex? Mol. Ecol. 21, 3893–3895 (2012).
3.            Xue, X. et al. Inactivation of Wolbachia reveals its biological roles in whitefly host. PLoS ONE 7, e48148 (2012).
4.            Zhang, Y.-K. et al. Diversity of Wolbachia in Natural Populations of Spider Mites (genus Tetranychus): Evidence for Complex Infection History and Disequilibrium Distribution. Microb. Ecol. (2013). doi:10.1007/s00248-013-0198-z
5.            Hussain, M. et al. Effect of Wolbachia on replication of West Nile virus in a mosquito cell line and adult mosquitoes. J. Virol. 87, 851–858 (2013).

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