Four more ants deciphered
Using the tools of ultra-fast DNA sequencing, scientists have recently reported four ant genomes.
Before you doze off, take a moment to appreciate ants. These social insects are some of the most successful critters on the planet: Ants are invaders. Armies. Pests. Even farmers.
Ants live in colonies with dozens to millions of members, and whether judged by weight or impact, they can dominate ecosystems.
Explanations for the ants’ extraordinary success lie in their genomes – their entire catalog of genes. In the last month, scientists have published four ant genomes, adding to two published last year.
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One of the new genomes covered the highly invasive Argentine ant, which has spread from its native South America to Europe, California and Japan. The ant “is a species of special concern because of its enormous ecological impact,” said Neil Tsutsui, associate professor at the Department of Environmental Science, Policy and Management at the University of California at Berkeley. “When the Argentine ants invade, they devastate the native insect communities while promoting the population growth of agricultural pests.”
Like all social insects, Argentine ants communicate via chemical signals, and in 2009 Tsutsui ignited an ant war among friendly ants by doping them with chemicals that trigger aggressive behavior. Similar endeavors could be aided by the new genome map, which detected 367 genes for odor and 116 for taste.
Although the human genome project has yet to deliver its promised cargo of health benefits, Tsutsui said the new genome for the pesky ant “will provide a huge resource for people interested in finding effective, targeted ways of controlling the Argentine ant” by manipulating genes to interfere with mating, sparking inter-colony wars, developing repellants or luring ants into traps.
Ants practice agro-forestry
Another new ant genome covers the industrious leaf-cutter ant, which lives in giant colonies and farms fungus for a living. (Seen the ant-cam?)
Leaf-cutters nibble tree leaves into pieces, then haul them, porter style, to underground “gardens” where the leaves are devoured by a fungus.
This is not ornamental gardening. The fungus is the only thing these ants eat.
After million of years, the ants and fungi have evolved together, developing a serious case of co-dependency. “The ants need the fungus, and if they lose it, they die,” says Garret Suen, an assistant professor of bacteriology at the University of Wisconsin-Madison.
Suen, a key author of a recent genome of the leaf-cutter, adds that the reverse may also be true, since fungus has never been found outside ant gardens, and “it has co-evolved in tight association with the ants.”
This is some crazy co-evolution! The subterranean garden of one ant colony can reach a volume of 600 cubic meters. To feed the fungus, leaf-cutters ants can harvest as much as 17 percent of the leaves in a forest –- making this tiny critter the biggest herbivore in many new-world tropical forests. (Leaf-cutters don’t live in Asia, Africa or Europe.)
Other insects, like termites, house symbiotic microbes that “eat” biomass for them, but with the leaf-cutters, the symbiotic microbes live externally.
The new genome emphasizes the fundamental nature of the symbiosis, says Suen, because it showed that the ants lack a gene for synthesizing the amino acid arginine. Amino acids are building blocks of proteins, and all ants require arginine, but the pathway to make arginine in the leaf-cutter is broken, Suen says. “Presumably it used to be complete, and it’s complete in all other ants and many other social insects, except for our ant.”
When Suen and his colleagues finish analyzing the genetic sequence of the fungus, he suspects it may well show an enhanced ability to make arginine.
Losing a gene
Evolution selects for genes that are needed for survival and reproduction, but since it’s wasteful to make things that have no purpose, genes that are no longer needed tend to break down or disappear over time.
Because leaf-cutter ants descended from ants that grew fungus with less sophistication, in much smaller gardens, the gene may have disappeared even before the leaf-cutters evolved between 8 and 12 million years ago, says Cameron Currie, associate professor of bacteriology at UW-Madison and study co-author. “They could have lost the genes 30 million years ago. Other symbiotic systems that are dependent on each other for nutrition have evidenced a similar loss of genes.”
The leaf-cutter also has a deficit in genes for making trypsin, an enzyme that breaks down proteins in food to make amino acids. “They are feeding on the fungus and it provides them with free amino acids, so it does not need these enzymes,” Suen says.
Another gene that evolution has shortchanged – but not totally eliminated – makes the protein hexamerin, which stores amino acids until they are needed during development. ”We think the developing brood has a constant source of amino acids from the fungus,” Suen says, “so it does not have to store them.”
Beyond confirming predictions of evolutionary theory, these genetic deletions could explain the long-lasting mutualism between ant and fungus, says Currie. If the ants would die without their fungus crop, they have a survivalist interest in blocking the entry of other fungi.
Although such a symbiosis looks like a great deal for both parties, cheaters can sabotage symbioses. “Evolution predicts that there should be instability, or cheating, in these cooperative relationships,” Currie says. “If I am giving a benefit to you at a cost to me, you can just take your benefit and not provide anything in return, which means you would be more successful compared to someone who cooperates and pays the cost.”
If you take your ailing auto to the car-fix, you could save money – once — by writing a rubber check. But repeated interaction is conducive to cooperation, Currie says. “If you write a bad check, next time you will not get your car fixed, and this applies to mutualism as well.”
In the ant-fungus relationship, Currie says, “The partners just can’t go and find new partners; they are locked together.”
In other words, the deletion of the ant’s arginine gene could explain why the co-dependency has lasted upwards of 8 million years.
Leave the leaves alone!
As ants don’t do a lot of reading and talking, chemical communication will likely be a focus for further genomic analysis. Plants dislike being eaten by any herbivore, so they produce toxic compounds to deter would-be browsers. Although the ant-fed fungus can eat more than 10 percent of the species of tree in the forest, some leaves are toxic to the fungus.
How do the ants know which type of leaves will kill it’s sole food, and how do they “talk about it”? It’s clear that the ants keep an eye on their crops, Suen says. When, as an experiment, scientists treated leaves with fungicide, the ants quit collecting that species, Suen says. “The ants remember and won’t touch those trees for two weeks because they are killing the fungus. How they do this, we have no idea, but now we can do an experiment to see what genes are being turned on or off” under those circumstances, and therefore must be involved in recognizing the death, and warning the colony about it. “We are pretty sure there is some communication between the fungus and the ant,” Suen says.
More broadly, information about the genes of a highly successful organism with millions of cooperating individuals ought to be intriguing to another highly successful, but sometimes less cooperative, organism that has more brains, fewer legs, and equally large cities.
Leaf-cutter ants live a complicated life, and the identical set of genes allows them to become queens, soldiers, or several types of worker. “They do this with a brain that is incredibly small, but it’s collective, hard-wired behavior,” Currie says. “It’s amazing; there are 5 to 10 million ants with many different tasks that are done by different workers of different sizes,” and it all starts from the same genes.
The genome has yet to reveal a “farmer gene,” Currie says. He expects that candidate farmer genes will emerge when the leaf-cutter’s genome is compared to close relatives that do not farm. These may explain the leaf-cutter’s curious capacity for growing its food with the help of fungi, “But we are a long way from that.”