In situ observation of the Cardinium symbionts of Brevipalpus (Acari: Tenuipalpidae) by electron microscopy

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Abstract
Brevipalpus (Acari: Tenuipalpidae) mites are important pests on a variety of host plant species. The mites damage their hosts directly by feeding and some species also serve as vectors of plant viruses. Among more than 200 described Brevipalpus species, three are recognized as vectors of plant viruses: B. phoenicis, B. californicus and B. obovatus. These species occur worldwide in subtropical and tropical regions. Brevipalpus mites reproduce mostly by thelytokous parthenogenesis and this condition was attributed to a bacterial endosymbiont, recently characterized as a member of the genus Cardinium. The same symbiont infects many other arthropods and is capable of manipulating their host reproduction in various ways. Generally the presence of Cardinium is determined by molecular, PCR based, techniques. In the current work we present visual evidence for the presence of these bacteria by transmission electron microscopy as a complement of previous detection by PCR. Cardinium is easily identified by the presence of a unique array of microtubule-like structures (ML) in the cell. Symbionts have been observed in several organs and eggs from different populations of all three Brevipalpus species known as vector of plant viruses. Cardinium cells were always immersed directly within the cytoplasm of infected cells. Bacteria were observed in all females of all instars, but were absent from all males examined. Females from some Brevipalpus populations were observed to be uninfected by Cardinium. This observation confirmed previous PCR-based results that these populations were aposymbiotic. The observed distribution of the bacteria suggests that these bacteria could have other functions in the mite biology beside feminization.

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