Simplicity and complexity of microsporidian genomes.

The adaptation to a parasitic lifestyle typically precipitates a number of profound changes to various levels of biological organization, including cell structure, genetics and genomics, metabolism, and biochemistry. These changes can lead to a seemingly paradoxical mixture of characteristics: on one hand, parasites may evolve extremely complex and sophisticated mechanisms to invade their host and evade its defenses, while on the other hand, they may also appear more “simple” by dispensing with characteristics they no longer need as they increasingly depend on host metabolism for nutrients and energy. The reductive element of parasitic evolution has led several parasitic protozoa to be considered ancient or earlybranching eukaryotes because they lack characters common to all other eukaryotes, but in reality, these organisms are highly derived. No group represents this phenomenon better than the microsporidia, a group of obligate intracellular parasites that exemplify the contradictory forces of simplification and complex specialization.