Different truffles all smell delicious

An international effort to understand the world of fungi has resulted in the sequenced genomes of some of the world’s most prized delicacies: truffles. Published in the journal Nature Ecology and Evolution, four new genomes help to understand the evolution of these scrumptious subterranean snacks.

Fungi, a kingdom of life all its own, separate from plants and animals, is a polarised one: species are either deadly or delicious, but when they are the latter, they are spectacular. And truffles surely fit the bill.

These earthy little nuggets are actually the fruiting bodies of fungus species that live symbiotically on the roots of certain trees. Filled with spores, they emit alluring cocktails of volatile organic compounds (VOCs) that produce the distinctive truffle aromas in the hope of enticing animals to dig them up and spread the spores around, thus facilitating their reproductive cycle.

In 2010 Francis Martin and a team of researchers sequenced the genome of the most highly prized truffle of them all, the Périgord black truffle (Tuber melanosporum). 

Martin, director of the Laboratory of Excellence for Advanced Research on the Biology of Tee and Forest Ecosystems (ARBRE) at the French National Institute for Agricultural Research, in Nancy, France, is also one of the founding members of the 1000 Fungal Genomes Project run by the US Department of Energy. The five-year project seeks to start making inroads into the genomes of the 1.5 million fungal species that make up a substantial proportion of the living world.{%recommended 6411%}

Now Martin, alongside PhD student Thibaut Payen and a large international team, has published the genomes of four other delectable truffles: the Piedmont white truffle (Tuber magnatum), the Burgundy truffle (Tuber aestivum), the pig’s truffle (Choiromyces venosus) and the desert truffle (Terfezia boudieri) found in the Middle East and northern Africa.

Of these T. magnatum and T. aestivum are the most closely related, having split from the black truffle 100 million years ago. C. venosus split from these Tuberaceae, or Tuber species, some 150 million years ago, and Te. boudieri is the most distantly related, having forged its own evolutionary pathway roughly 400 million years ago. 

Despite having different evolutionary lineages, the Tuberaceae have unexpectedly high genetic similarity and show some interesting differences from other non-truffle species. They share genes that allow them to live harmoniously with their symbiotic partner-trees and all share a large array of genes that seem to be involved in VOC production.

While the distinctive aroma of truffles appears to be controlled by the selective expression of these shared genes, rather than by the presence or absence of the genes themselves, the researchers could not pin the differing smells on specific ones. 

Given the underlying genetic similarity, they conclude that the unique combination of volatile organic compounds may might well be partly caused by species-specific colonies of bacteria, yeasts and moulds.

The Tuberaceae are different from other fungi, too. Most fungi are saprotrophic, meaning they live by breaking down the cell walls of dead or decaying plant matter. Tuber species, however, live symbiotically and thus display a much-reduced set of genes involved with digesting dead things. Indeed, they share interesting gene networks that help them derive nutrients from the soil.

Although many of the genes uncovered by the research have unknown functions, the work helps to piece together the evolution of these terrifically tasty troglodytes.