Symbionts, including pathogens, of the predatory mite Metaseiulus occidentalis: current and future analysis methods

Metaseiulus (= Typhlodromus or Galendromus) occidentalis (Nesbitt) (Acari: Phytoseiidae) is an effective natural enemy of pest mites in a variety of crops around the world, although it is considered to be endemic in the western USA. A broad understanding of much of its biology, ecology, behavior, and genetics has been obtained over the past 60 years, but the role(s) symbionts play, which includes pathogens and other microorganisms, remains to be resolved fully. Until molecular tools became available, analysis methods were limited primarily to microscopic observations; some viruses and rickettsia-like organisms were observed infecting 'diseased' M. occidentalis, but it is not clear which one(s) was the causal agent(s) of the disease(s). Subsequent to the development of the polymerase chain reaction (PCR) and genome sequencing, we identified putative gut symbionts and reproductive tract symbionts in M. occidentalis, as well as a microsporidian pathogen. A new phylogenetic analysis of the Bacteroidetes-Flavobacterium group suggests the unnamed Bacteroidetes in M. occidentalis is associated with the digestive tract. However, much of our current information about the role these microorganisms play in the biology of M. occidentalis is based on correlation, lacking the strength of fulfilling Koch's postulates. We also currently lack any knowledge of the importance of these microorganisms under field conditions. In the future, it should be possible to learn what role(s) these organisms play in the biology of this important predator using metagenomics approaches to analyze the transcriptome and to determine their relative abundance within their hosts with the quantitative PCR. We have just begun to resolve these relationships.