The size of a genome tells us nothing about the comprehensiveness of the genetic information it contains. The genome of powdery mildew, which can destroy entire harvests with its fine fungal threads, is a good example of this. Although the pathogen has almost 120 million base pairs, and therefore one of the largest genomes of the sac fungi, at barely 6,000, its gene count is far lower than that of comparable species. It has lost many of the genes required for separate metabolism found in other fungi. Thus, from a genetic perspective, powdery mildew is stuck in an evolutionary dead end from which it is unable to liberate itself. (Science, December 10, 2010)
Based on the comparison of fungal genomes, Ralph Panstruga from the Max Planck Institute for Plant Breeding Research in Cologne and his colleagues from an international research consortium discovered that powdery mildew forfeited a large part of its genetic complexity in the course of evolution. The considerable size of the mildew genome is largely due to so-called "jumping genes". These genes introduce new sequences into the genome and repeatedly mix up the genetic material by inserting and deleting themselves, causing errors as a result. Due to these changes, the powdery mildew fungus gained a considerable number of new base pairs, but it also lost a lot of genes as their reading frames were interrupted by the insertion of the jumping genes.
As the international consortium of scientists succeeded in demonstrating, the plant pathogen lacks 99 genes that enable independent life, yet are still found in baker's yeast, another sac fungus. Therefore, powdery mildew cannot fix nitrogen, harness energy from alcoholic fermentation or produce certain metabolic products from inorganic compounds. As a parasite, powdery mildew does not require these synthesis processes; it obtains everything it needs from the host plant. Panstruga explains: "It can do without these genes. However, the price
|Contact: Ralph Panstruga|