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parasites of parasites

Erich Schwarz emsch at its.caltech.edu
Wed Jul 25 22:28:28 PDT 2001


    After our discussion of this topic, note that apparently there
*do* exist cellular organisms that parasitize hosts, which
themselves parasitize larger hosts...


--Erich Schwarz

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Von Dohlen et al. (2001), Nature vol. 412, pp. 433-436.

Mealybug [beta]-proteobacterial endosymbionts contain [gamma]-
proteobacterial symbionts

Some insects have cultivated intimate relationships with mutualistic
bacteria since their early evolutionary history. Most ancient
'primary' endosymbionts live within the cytoplasm of large,
polyploid host cells of a specialized organ (bacteriome). Within
their large, ovoid bacteriomes, mealybugs (Pseudococcidae) package
the intracellular endosymbionts into 'mucus-filled' spheres, which
surround the host cell nucleus and occupy most of the cytoplasm. The
genesis of symbiotic spheres has not been determined, and they are
structurally unlike eukaryotic cell vesicles. Recent molecular
phylogenetic and fluorescent in situ hybridization (FISH) studies
suggested that two unrelated bacterial species may share individual
host cells, and that bacteria within spheres comprise these two
species. Here we show that mealybug host cells do indeed harbour
both [beta]- and [gamma]-subdivision Proteobacteria, but they are
not co-inhabitants of the spheres.  Rather, we show that the
symbiotic spheres themselves are [beta]-proteobacterial cells. Thus,
[gamma]-Proteobacteria live symbiotically inside
[beta]-Proteobacteria. This is the first report, to our knowledge,
of an intracellular symbiosis involving two species of bacteria.

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