Leafcutter ants have a smaller genome than other species of ant with a more varied diet. They have lost unnecessary genes.
Leafcutter Ant Genome
On 11 February 2011 Katherine Harmon (Scientific American) reported on the work done by Suen et al. (PLoS GENETICS) with the Leafcutter Ant (Atta cephalotes). They studied the DNA of these ants and found that they lack "genes associated with nutrient acquisition and amino acid biosynthesis". They suggest that: "These genes appear to be no longer required because the fungus may provide these nutrients."
Harmon points out how important these ants are in 'New World' tropical ecosystems "responsible for shearing off at least 12 percent of tropical rainforests' foliage each year".
Leaves, Fungus, and Ants
Worker leafcutter ants (there are over 40 species) climb into trees and bushes to bite off leaf fragments. They then carry this fresh plant material to their underground nest.
Special chambers in the nest house fungi that the ants cultivate. They keep the fungus free of pests and moulds and keep it well supplied with fresh leaf fragments. In return the ants feed their young on the fungus.
The ant/fungus relationship is of benefit to both organisms – the fungus needs the ants, and the larvae need the fungus.
Proteins, DNA, and Amino Acids
Proteins are the stuff of life, many of them act as the enzymes that control biochemical pathways, and each protein has its own DNA code for the sequence of amino acids needed for its manufacture.
There are twenty or so amino acids needed to make proteins, and the DNA (genes) control the order in which the amino acids are lined up to make each different protein. The precise sequence of amino acids determines the structure and function of each enzyme.
Some amino acids cannot be made in animal cells, so these 'essential' amino acids must come from an animal's diet. In other words animals have to have a suitable diet if they are to function properly.
If an animal has a regular and reliable diet that provides most of its requirements it can 'slim down' its genome (all its DNA) by not keeping unwanted genes. Over millions of years leafcutter ants have lost many of the genes that their superior diet has made redundant.
Human Vegetarians
Different populations of human beings carry different genes, and some have lost a few biochemical pathways. Those people who have traditionally had a diet rich in animal protein (and therefore with good balance of the amino acids necessary for health) have lost the ability to make some of these amino acids for themselves – they have been able to rely on a steady supply from their food.
In contrast, populations who have always been vegetarian have needed to keep many more of their enzymes in order to make up for shortages (in some of the ‘essential’ amino acids). Their bodies can survive well on a fully vegetarian diet.
This difference between the genomes of different human populations explains how some people can thrive on a plant diet while others seem to need some meat.
These human differences are mirrored by the genetic differences between leafcutter ants and their relatives with less nutritious diets.
References:
'Fungus-farming leaf-cutter ant's genome sequenced', Katherine Harmon, Scientific American 11 February 2011
'The Genome Sequence of the Leaf-Cutter Ant', Suen et al., PLoS GENETICS 10 February 2011.
