Miniscule monsters

by Tyler Shendruk

Published: Feb 9

The problem
Genomicists have a serious bias toward “model” organisms. Model organisms are species that have historically been well studied. Fruit flies, yeast, zebrafish, and mice are examples of model organisms. So are humans.
But these model organisms are each just some leaf on a random twig of the tree of life. Scientists are only beginning to realize the true extent of biodiversity and the staggering variety of differing genes and structures that make up genomes.

The researcher
Nicolas Corradi studies comparative genomics, which means that he sequences organisms’ genomes and then compares their genes and structure to those of other species. Corradi’s lab in the biology department at the University of Ottawa focuses on unicellular eukaryotes, single-celled micro-organisms that harbour curious genomes in their nuclei.

The project
Corradi’s favourite eukaryotes are microsporidia, parasitic unicellular fungi. These little monsters are highly adapted for infecting host cells. They are opportunistic bugs that steal everything they need to survive from their host. In fact, the only time microsporidia spend outside of a host cell is as spores, scouring to invade other cells.

The key
Corradi sequenced the genome of the microsporidia Encephalitozoon intestinalis. This particular microsporidia has the smallest nuclear genome of any known organism. It is made of only 1,800 genes (1,500 times smaller than the human genome and 20 per cent smaller than the next smallest genome ever sequenced).
Why do they have such small genomes? Because these microsporidia are marauding picaroons. They don’t do anything they don’t have to. They steal so much from their hosts that they have shed every gene but the bare minimum needed to function.
Evolutionarily speaking, it is easier to lose genes than to gain them, so these microsporidia are extremely adapted for their parasitic lifestyle. Their genome is so compact that Corradi believes it may represent the limit for a fully functional genome.