Giant cell reveals its secrets The genome of a giant amoeba provides clues to the early evolution of contemporary genes
The “slime mold” Physarum polycephalum is an amazing creature. As a giant single cell it is visible with the naked eye. The yellow, slimy mass of protoplasm, which crawls over rotting logs engulfing its prey, seems as unusual as an alien from outer space. One specimen, which entered the Guinness Book of Records in 1989, covered a surface area of 5.5 square meters and weighed approximately 3 kgs-- obviously deserving its title as the largest single cell ever grown. This enigmatic organism has also attracted the interest of physicists, engineers, and artists who have taken inspiration from Physarum for developing computer algorithms, or directly used giant Physarum cells for steering robots, creating art projects, or electronic music. However, for biologists Physarum has been a sleeping beauty during the past decades, because almost nothing was known about its genome and genes. This shortcoming has now changed fundamentally.
In an upcoming issue of Genome Biology and Evolution, an international group of scientists reports on the sequence of the 188 million nucleotides that make up the Physarum genome. These nucleotides encode 34,000 genes, over 50% more than the human genome. Few other organisms have thus far posed such difficulties in deciphering their genome, which called for improvements of existing sequencing technologies, according to Pat Minx from the McDonnell Genome Institute at Washington University.
Comparison of its genes with those of other species proved what biologists already suspected. The so-called slime mold is in reality not a mold (fungus) but a giant amoeba, belonging to the amoebozoa group of organisms. In light of our contemporary understanding of molecular evolution, Physarum appears to be an ancient relic with similarity to the last common ancestor of Amoebozoa, fungi and animals (including humans): a prototypical cell from the era of early eukaryote evolution with some molecular features that were thought to be specific for either animals or plants.