Archive | July 12, 2011

The Symbiotic Way of Life Leaves Traces in the Genome of the Leaf-Cutter Ant

Photographer: Grey Wulf from Sheffield, UK

The improvements in read quality, data output and read length of current next generation sequencing technologies results in more and more complex eukaryotic genomes to be sequenced and published. In the following I will further illustrate one particularly interesting example, the genome of the leaf-cutter ant:

Next to humans, leaf-cutter ants are believed to form the largest and most complex animal societies on Earth. The impressive insects are found to be endemic to South and Central America and parts of the United States (Schultz and Brady, 2008).

Their way of life is based on an ant-fungus mutualism that is thought to have evolved approx. 50 million years ago (Schultz and Brady, 2008). The ants are cutting leaves from trees and carry the leaves in subterranean gardens (see figure). On the freshly cut plant material they are actively farming a specialized fungus which serves as their primary food source.

An international team of researchers published recently the approx. 290 Mbp genome of the leaf-cutter ant Atta cephalotes (Suen et al., 2011). The genome was sequenced using the 454 GS FLX Titanium platform only to an 18-20-fold coverage. The resulting 42’000 contigs could be further assembled into approx. 2’800 scaffolds by sequencing of 8 kbp and 20 kbp paired-end libraries.

Fascinatingly, the genome sequence analysis confirmed genetic modifications that reflect the ant’s obligate dependence on the fungus in nutrient acquisition. For example genes encoding serine proteases are extensively reduced. It seems that these types of enzymes are not necessary to the leaf-cutter ants any longer as the fungus primarily provides carbohydrates and free amino acids. According to the authors these and other gene losses have contributed to the stabilization of the mutualisms over its coevolutionary history.