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Strangers in Parasites
It's impressive enough that many bugs, including aphids and mealybugs, carry their own special bugs—typically, bacteria belonging to the gamma-subdivision of Proteobacteria—to keep themselves (the insects) properly fed. But some of those insect-borne bacteria are carrying their own endosymbiotic bugs, namely other bacteria, according to Carol von Dohlen and her collaborators at Utah State University (USU) in Logan, Utah. For what purpose those secondary endosymbionts are maintained she cannot say for sure, except to note that these two distinctive types of bacteria within citrus mealybugs apparently are "symbiotic" and to speculate that the innermost of those bacterial types could be providing replacement genes, or gene products, to their immediate host if some of its genes have drifted into dysfunction over millions of years of intracellular existence.
Mealybugs feed on plant sap, a resource that, although plentiful, is lacking in essential nutrients. Endosymbiotic bacteria presumably bridge this nutritional gap for mealybugs and many other sap-sucking insects. Thus, for example, after being treated with antibiotics, mealybugs soon lose their endosymbionts and then starve to death. Indeed, maintenance of this endosymbiosis is so important for mealybug survival that the developing embryo at an early stage produces specialized cells that re-engulf the endosymbionts that were released from maternal cells and penetrated oocytes when they developed during an earlier phase in maturing females, thus ensuring the safe passage of the microbes into subsequent mealybug generations.
Although researchers in this field for many years accepted that general picture outlining the biological perpetuation of mealybug endosymbiosis, some of them more recently reported somewhat inconsistent findings over the precise type of bacteria occupying the endosymbiotic niche within the citrus mealybug, Planococcus citri, and its close relatives. Some investigators identified the particular endosymbionts to be gamma-type Proteobacteria, others reported finding beta-type Proteobacteria, and still others said that both types are present within these or closely related mealybugs. One particularly odd aspect of mealybug endosymbionts is that they are packaged within "symbiotic spheres," whose origins have never been explained but were reported to have three bilayer membranes surrounding them.
Now, by means of electron microscopy and fluorescent in situ hybridization techniques, von Dohlen and her colleagues have resolved several of these seemingly inconsistent observations—and determined that the symbiotic spheres within specialized cells of the citrus mealybugs are actually beta-subdivision Proteobacteria that, in turn, contain their own, gamma-subdivision proteobacterial endosymbionts. This ordering of the endosymbionts thus helps to explain, for instance, that the unusual-appearing, seemingly unprecedented membranes around the spheres are not a peculiar structure of purely insect design. Instead, they result from the juxtaposition of two bacterial-type membranes and another of insect origin, which forms a vesicle containing both those endosymbionts. The USU researchers report their findings in the 26 July 2001 issue of Nature (412:433-436).
The specialized polyploid insect cells group to form an oval organ, which "is bright orange-yellow and shimmery, occupying about one-third of the insect's abdomen" and harbor both types of bacteria, von Dohlen says. Typically, specialized insect cells contain about 5 to 8 beta-type endosymbionts per cell, while each of those bacterial endosymbionts, in turn, carries about "5 to 10 or more, but not hundreds, of gamma-cells. The gammas can take on a sausage shape, whereas the betas are spherical or globular." Because these ratios appear to be relatively stable during the insect life cycle, "there's got to be some host control of cell division [in the endosymbionts]," she points out. But how it might be controlled is not known.
"These betas are thought to be the original symbionts," von Dohlen continues. For example, on the basis of ribosomal RNA gene-sequence comparisons, the beta-type endosymbionts of several mealybug species form a unique lineage, whereas the gamma-types belong to different lineages, suggesting more recent, and likely independent, acquisitions of these different types of endosymbionts among mealybugs.
"These bacteria are in very small populations within host cells and likely cannot exchange genes between populations, and there is a bottleneck for each small population across each mealybug generation," she says. "Apparently, there are long-term genetic effects associated with being an endosymbiont," she adds, pointing to a gradual accumulation of deleterious mutations and genetic drift. However, previously published data indicated that the ribosomal genes within the beta-types are relatively free of nucleotide biases that tend to accumulate with such genetic drift, leading her to speculate that the secondary, or gamma-type, endosymbionts might be serving the beta-types as a source of replacement genes. "Maybe they help slow down genetic degradation and drift," she says. "There's absolutely nothing [else] known about what these gamma-types do, but it's also quite possible they produce something used by the mealybugs...or serve metabolic needs of the beta-types."
Figuring out how this intricate, three-part biological equilibrium is maintained, how it was established, or what service it performs will not be easy, according to von Dohlen. "Other mealybugs in other genera have only one bacterial endosymbiont, and I'd like to know if it's a member of the same beta lineage, or something else," she says. "Did the beta develop the ability to internalize gamma only in one branch of mealybugs, or have these endosymbionts been acquired several times? I'm interested in insect evolution...and I'd love to compare the bacterial patterns with an evolutionary tree of mealybugs."
Jeffrey L. Fox
Jeffrey L. Fox is the ASM News Current Topics and Features Editor.
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