The ecology of microsporidia - a balancing act between minimizing host damage and maximizing pathogen reproduction and transmission

The Phylum Microsporidia, a monophyletic taxon of eukaryotic unicellular pathogens that appear to be ubiquitous in invertebrate populations, possess a suite of unique genetic and biological characteristics that determine interactions with their hosts. Similar to pathogens in other taxonomic groups, interactions between each microsporidian species and its host(s) can vary considerably from those of other microsporidian-host relationships, but the overall trend is a general chronicity that maximizes pathogen production, vertical transmission or both. Typically, microsporidia do not cause detectable harm to the cells of their hosts until sporulation begins. When sporulation is completed, horizontally transmitted species often cause high mortality in their hosts. Vertically transmitted species tend to be less virulent. Characteristics that best describe microsporidia include an uncertain taxonomic placement related to Fungi; obligate intercellular development; absence of many eukaryotic features, notably functional mitochondria; interactions with host mitochondria in infected cells that suggest direct energy (ATP) uptake; lack of plant and fungal hosts; and possession of a unique invasion apparatus consisting of a polar filament through which the infective form is injected from the spore into a host cell. Microsporidian life cycles range from simple to highly complex and, in some cases, include obligate intermediate hosts. As well, pathogen-host population dynamics and impacts on host populations are highly diverse. Here, we highlight some of the unique characteristics of microsporidia that govern the ecology of these primary pathogens of invertebrate animals.