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Abstract
The sudden searing pain shocked me, a fiery sensation that made my eyes roll back in their sockets - so unexpected and so concentrated at a single point on the side of my little toe.
| Widely feared, bullants are gaining more positive respect. |
I was in my own Sydney backyard and that was my first unforgettable childhood sting by an Australian bullant, arguably the world's largest and certainly one of the meanest and most primitive ants. This pugnacious social insect chose my bare foot to demonstrate its most unsociable weapon, which has won the wary respect of other creatures back into the mists of time.
That potent sting means they are widely disliked. They are feared, too, because they are potentially fatal to some unlucky people who suffer anaphylactic shock when stung - an estimated 50,000 Australians are at risk from such a reaction.
But a new and more positive kind of respect for bullants is gradually emerging from varied research revealing that these hardy insects have been harboring some surprising scientific secrets.
| "They were probably stinging the toes of dinosaurs 80 million years ago." |
Bob Taylor, an honorary research fellow at CSIRO Entomology, Canberra, Australia, and an international authority on ants, notes that bullants are the largest of an ancient and primitive ant group, the Myrmecia, which includes its smaller leaping cousins, the jack-jumper ants. Fossils and specimens preserved in Baltic amber reveal that ants looking almost identical to Myrmecia walked much of the Earth when our own ancestors were little more than small furry animals. "They were probably stinging the toes of dinosaurs 80 million years ago," Taylor quips. His ongoing research into bullants and jack-jumpers has left Taylor in awe of their extraordinary genetic flexibility.
The 200-odd species in this genus are found alive nowhere else but in Australia (one species has been reported from New Caledonia in modern times but is presumed extinct). Why such ants disappeared elsewhere is unknown, but it's no surprise they're still thriving in Australia. Taylor says, "They've got a ground plan that is very, very successful. They're sort of unrefined and not very sophisticated socially. They don't have complex social castes like other ants do. But like sharks, they work so well in the job they do, and Australia is the continent of ants with probably over 4,000 species."
| Some may be almost the length of your big toe. |
The bullant's popular reputation as the world's largest ant is disputed with alternate claimants in South America and South Africa. But Taylor says the big-headed workers of the bullant species Myrmecia brevinoda probably take the prize. Other species may be bulkier, but these individuals are certainly the longest, sometimes exceeding 40 mm - almost the length of your big toe.
Far more interesting to Taylor and his colleague H.T. Imai of the National Institute of Genetics in Mishima, Japan, is the great chromosomal variety now being discovered among the Myrmecia. Their biological versatility is apparent in the fact that different species thrive in environments as distinct as the snowy chill of the Australian Alps and the continent's scorching central deserts.
| The range of chromosome numbers is huge. |
Molecular studies have shown that their chromosome numbers vary accordingly. Indeed, the variety is huge - one of the greatest known. One Myrmecia species has been found to have 84 pairs of chromosomes, for example, while others have just four pairs or even fewer.
"Excluding polyploidy - where the same chromosome can be doubled or trebled, as in some butterflies - the range of chromosome numbers in the Myrmecia and closely related Nothomyrmecia ants is as great as the range in all insects," Taylor says. "It's really quite remarkable." Molecular studies have shown that most of this genetic diversity seems to have evolved over the past 50,000 years alone: "Yet all of them seem to have approximately the same amount of DNA. It's just packaged differently."
| Some species have just one pair of chromosomes. |
While the basic number of chromosomes in the genus seems to be four pairs, the highly unusual Myrmecia croslandi normally has all of its genes packaged in just one large pair of chromosomes - the theoretical minimum. Individuals within the species are polymorphic and some workers have one, two, three, or four chromosome pairs. "It's the only known organism higher than a nematode worm that has just one pair of chromosomes, so that's quite exceptional," Taylor says.
Equally strange is an as yet unnamed bullant species with 14 chromosome pairs in which the two halves of each pair do not match and so, with a few minor exceptions, do not cross over during meiosis. Somehow the mismatched pairs are able to work together in the cells of individuals despite the genetic odds being heavily stacked against them.
| One species has dispensed with males altogether. |
As a consequence, however, reproduction is bizarre in this species. It has completely dispensed with males and reproduces by parthenogenesis - one of only a few such cases reported in ants. This odd arrangement is thought to have resulted from a freakish hybridization. Taylor believes he has identified one of the original two species that provided half of the 14 chromosome pairs, but the source of the other half remains a mystery.
Few such hybrids are likely to survive, but the wide geographical distribution of this species in southeastern Australia suggests it has managed to prosper for a long time. All this genetic variety among the Myrmecia and some other ants has led Imai to propose that, contrary to accepted wisdom, evolution tends to foster an increase in chromosome numbers over time.
| Chromosomes swing "like hammocks in a breeze." |
The explanation is complex but, to use a simile, the central idea is that during meiosis chromosomes are strung across the nuclear sac and mechanically behave a bit like hammocks in a breeze. Larger ones "swing" more freely and are, thus, more likely to bump into their neighbors, swapping pieces as they do. In turn, that tends to promote repackaging of groups of genes into new chromosomes that are smaller and more tightly strung.
At the Centre for Biodiversity and Bioresources at Sydney's Macquarie University, a research team led by Andrew Beattie has had bullants under scrutiny for very different reasons for more than a decade. The team has been seeking answers to the worldwide concern about the increasing resistance of many infectious microbes to antibiotic medicines. Their approach has been to study how living things have evolved their own defenses against harmful bacteria.
| What allows them to resist disease? |
"It's called bioprospecting," says team member Michael Gillings. "The idea is that you can predict the likely occurrence of active compounds by knowing the biology of species that live in particular environments. If you take carrion beetles or maggots as examples, they must have potent defenses against disease to be able to survive so much close contact with rotting flesh. In the case of bullants, they live in dirt and they are constantly dragging dead things into their nest. If you look at it like that, you have to ask yourself what is it that allows them to resist multiple forces likely to infect them with disease."
The first surprise was the discovery that bullants have an immune system, which came as something of a shock in such a small and primitive insect. Unexpected proteins appeared in a bullant's blood after it was infected by invading bacteria. The proteins - now subject to patent - proved to be new to science and to have strong antibacterial activity.
| The purpose of the gland has long been a mystery. |
The second surprise was that bullants have an external defense system as well, in the form of a chemical secretion from their tiny metapleural glands. The glands lie midway along the ant's body; their purpose and that of their secretions (known as metapleurins) have long been a mystery. Only the longer-lived queens and workers have this first line of defense - the more expendable males do not.
Electron microscopy clearly shows that the gland is shaped like a tiny pair of saddlebags straddling the ant's body on the thorax between the second and third pairs of legs with an obvious hole on each side.
| "You would have to milk at least 5,000 ants to get a single teaspoonful." |
Noticing that a liquid oozes from the holes and that the ants wipe their bodies with it, the researchers decided to collect some and test it. In the laboratory, each ant is gently but firmly pinned down onto a glass slide with an elastic hair band. Using a tiny glass pipette, the scientists have managed to extract minute droplets of the viscous, creamy-yellow liquid. "You would have to milk at least 5,000 ants to get a single teaspoonful," Gillings estimates.
"Milking" bullants may seem preposterous, yet testing of the secretion against some common infectious bacteria has shown it to be a highly effective antibiotic against even organisms such as Staphylococcus aureus, better known as the infamous "golden staph" that is the bane of hospitals.
Promising studies are still in progress to synthesize this new "bullantibiotic," although work is slow due to a shortage of funds and the many complexities of developing, patenting, and testing such a potentially lucrative new medicine.
| "Bullantibiotic" has been in the works for 60 million years. |
In the new book he has coauthored with Paul Ehrlich, Wild Solutions (Yale University Press/Melbourne University Press), Beattie contrasts the evolution of this antibiotic with modern pharmaceutical production: "The research and development time has been approximately 60 million years, millions of prototypes have been tested, and the entire research and development program was free - courtesy of evolution by natural selection. This is a modern industrialist's dream come true."
Blessed with such potential, the humble bullant may yet win a better reputation. It's easy to imagine, though, that Australians will still quickly stomp on any that crawl too close for comfort.
Bob Beale is a freelance science and environment writer based in Sydney, Australia. He has been a journalist for 20 years.
Susan Wolsborn is Web designer of HMS Beagle.
Australian Ants Online - offers a guide to the Australian ant fauna including the genus Myrmecia.
Alternative Reproductive Strategies: A Queen Perspective in Ants - focuses on the genetic, behavioral, and evolutionary consequences of queen turnover within colonies. From Trends in Ecology and Evolution, 2000, 15:12:508-512.
Cooperation Among Unrelated Individuals: The Ant Foundress Case - a review of recent work on the cooperation and competition among unrelated foundresses. From Trends in Ecology & Evolution, 1999, 14:477-482.
Antbase - provides a wealth of information including databases, images, projects, and news. Maintained by the American Museum of Natural History and Ohio State University.
Myrmecology - provides some general information on ants and an extensive collection of links.
AntCast - view a live-leaf cutter ant colony. From London's Natural History Museum.
Japanese Ant Color Image Database - an extensive database of ants. Includes Gakken's Photo Encyclopedia: Ants.
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