Archive for the ‘parasite’ category

The bacteria that kills only male butterflies!

23 October 2009

If you think the swine flu virus was scary, this one makes it look like a Fairy Godmother! Fortunately it is restricted to arthropods and as such is harmless to humans.

But introductions first.

You, Wolbachia does not recognise as you are NOT an arthropod. For Wolbachia, we’ll let Wikipedia do the talking :-

Wolbachia is a genus of inherited bacteria which infects arthropod species, including a high proportion of insects. It is one of the world’s most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere. One study concludes that more than 16% of neotropical insect species carry this bacterium and as many as 25-70% of all insect species are estimated to be potential hosts.”

Wolbachia inside an insect cell.

Wolbachia inside an insect cell.

Now this is really mindboggling. When you say 25 to 70% of all insect species we mean between 1 million to four million ”species” as per the conservative estimate of Gaston (1991) That means Wolbachia is a potential infectant of literally trillions of organisms.

Wolbachia is everywhere, you can’t get away from it.

But so what, we did not know anything about Wolbachia till right now. Why bother?

Because Wolbachia is an organism which detests males, and, by a variety of means, systematically deprives a population of males.

Now, like all males, I am sensitive about my virility, gonads, maleness or what you will. So I instantly recoiled with horror to learn about this organism which specifically targeted the male sex for elimination, albeit in arthropods.

It all started harmlessly when I chose a post from “Not exactly rocket science” in the Science Blogs RSS feed in my Google Reader.

The post states that Wolbachia has seriously affected populations of Hypolimnas bolina, a Nymphalid butterfly called as the Great Eggfly in India or the Blue Moon elsewhere.

The male Blue Moon or Great Eggfly

In grave danger – the male Blue Moon or Great Eggfly.

Commonly found in India, the Great Eggfly is extant from Madagascar in the west across South and Southeast Asia to parts of Australia, Japan and New Zealand and even remote South Pacific islands such as French Polynesia, Tonga, Samoa and Vanuatu.

The female Great Eggfly, once infected, cannot mate successfully with uninfected males.

The female Great Eggfly, once infected, cannot mate successfully with uninfected males.

It appears that this butterfly has been affected severely by Wolbachia. In some islands, notably the Polynesian islands of Ua Huka and Tahiti, Wolbachia, which was virtually absent in the past, is common now and consequently, male butterflies are extremely rare here.

Wolbachia are present in mature eggs, but not in mature sperm. Hence it can propogate only thrugh females and not males; hence the reason for the bacteria developing violently anti-male attitudes!

Only infected females pass the infection on to their offspring. Wollbachia acts in four different ways –

* Firstly, it kills infected males.
* Secondly, it causes infected males to develop into females or infertile pseudo-females.
* Thirdly, it permits reproduction of infected females without males.
* Lastly, the bacterium causes cytoplasmic incompatibility between infected and non-infected insects. Wolbachia-infected males are unable to successfully reproduce with uninfected females or females infected with another Wolbachia strain.

Of this, I find the fact that enables female butterflies to mate without males extremely interesting! Some scientists have suggested that parthenogenesis may always be attributable to the effects of Wolbachia. Parasitic bacteria like Wolbachia have been noted to induce automictic thelytoky in many insect species with haplodiploid systems. They also cause gamete duplication in unfertilized eggs causing them to develop into female offspring.

An example of a parthenogenic species would be the Trichogramma wasp. This wasp has evolved to procreate without males with the help of Wolbachia. Males are rare in this tiny species of insect, possibly because many have been killed by that very same strain of Wolbachia.

All this has been proven thanks to the fact that hundreds of pinned, preserved and catalogued specimens of Great Eggflies can be found in today’s museums. These are from all over and from the heyday of collecting (before Independence).

Emily Hornett, a biologist at UCLA, asked the question –

“How have the ratios of male butterflies to female ones changed over time?”

Concentrating on specimens from the Pacific islands, she observed a number of cases where males reduced in numbers as time passed. Since the DNA of these specimens was still viable, she invented a test which detected the gene sequence of Wolbachia in the genome of the dead insects. She found a very high increase in Wolbachia infection with corresponding effect on the ratio of males to females. From her research, it was clear that Wolbachia steadily went from island to island decimating the males.

Surprisingly, she also detected Wolbachia-resistance in today’s individuals and also in the past in the Phillipines and Samoa. There was evidence of a full scale drawn-out battle beteen Wolbachia and its hosts. In one case, resistance to Wolbachia developed over a very short period of time. This is of great import to us.

The infection mechanism of Wolachia is being extensively studied with the hope of employing this mechanism to halt another killer disease in India, this time of humans, namely malaria.

It is also used to kill the worms which cause filiarisis (elephantiasis). The worms require highly toxic chemicals to kill but a simple antibiotic will kill the Wolbachia in them and they’ll just die because of that!

So, in every cloud, there’s a silver lining. Wolbachia’s hosts have very short lifespans – so it is possible to study evolution in action by researching Wolbachia. And it may possibly help us solve the malaria problem.

Read more about Wolbachia on cheshire’s and hotbacteria’s blog!

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