|Wolbachia in Humans?
|What is Wolbachia?
|Wolbachia is an endosymbiont bacteria that infects the
gametes of up to 76% of the world's insect species, causing
such bizarre effects as cytoplasmic incompatibility,
male-to-female sex-change, parthenogenesis, and in some
cases outright killing off of all male off-spring.
|Cytoplasmic incompatibility can come in two forms:
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|Table 1 - Unidirectional cytoplasmic
|Note in the unidirectional case (Table 1, Row 2) how an infected female can still become
pregnant from an uninfected male. This is because of a peculiar feature of endosymbiont
bacteria in that they cannot infect sperm cells. Sperm cells do not need to live long and thus
have no need of cytoplasm, and as such, no capacity for a cargo of Wolbachia.
This does not mean that a male insect cannot be infected with Wolbachia. They can and
frequently are; they just cannot inherit the disease vertically the way their sisters do.
Instead, they must contract the disease through horizontal transfer, e.g., the way humans
might catch pneumonia.
In males, Wolbachia occupy the cells responsible for the manufacture of sperm cells where
they paint the maturing sperm cells with a toxin. On contact with the uninfected female
egg, this toxin prevents the proper formation of a zygote and the egg dies. However, if the
toxic sperm cell should encounter an infected female egg of the same strain of Wolbachia,
the egg produces an antidote to the toxin and the zygote is allowed to form.
|Table 2 - Bidirectional cytoplasmic
|In the bidirectional case (Table 2, Row 2) the differing strains of Wolbachia make
fertilization impossible since the female's antitoxin is not able to counter the male sperm's
toxin and no pregnancy occurs.
|Male-to-female sex-change is common among Wolbachia-infected pill bugs,
Armadillidium vulgare, aka the common wood louse. Male pill bugs are quite rare in the
wild. Wolbachia benefits from this by skewing the sex ratio in favor of females. (Recall
only females can pass on Wolbachia through inheritance. Males must pick it up through
infection, hence, when they reproduce, no Wolbachia are reproduced.) Pill bugs have a WZ
sex-determination system, in contrast to the more common XY system found in humans.
Male pill bugs have ZZ chromosomes, female WZ. Males have all the requisite genes to
become females, needing only a dose of male-inducing hormone to produce maleness.
Wolbachia can inhibit the production of this hormone, thereby controlling gender in pill
bugs. Hence, when an infected WZ female gives birth the offspring will be mostly WZ
females and ZZ male-to-female conversions. The few males that are born become much
sought after members of the colony.
|Parthenogenesis, according to John Werren of the University of Rochester, has been
identified in three dozen species of insects, mostly wasps. The bacteria accomplish this by
duplicating and rejoining the chromosomes in the female egg during meiosis, the net result
being a genetic duplicate, or clone, of the mother. Hence, only daughters are produced.
Males in such a society quickly become obsolete in the practical sense, and what is obsolete,
Nature quickly discards in the genetic sense. In some species when the Wolbachia are
eliminated with antibiotics, the insects can no longer reproduce sexually. For example,
when the wasp, Encarsia formosa, is treated for Wolbachia, males are produced in the
offspring but they are incapable of mating, having lost their genes for male courtship. In
other species, the females have lost the courtship response. In others the males have lost
the genes responsible for making sperm. Wolbachia has developed complete control over
the reproductive capabilities of these insects, giving some entomologists the idea that a way
to combat insect pests is to eliminate their Wolbachia.
|Killing off male offspring is common among a species of Ugandan butterfly, Acraea
encendana. [See the Francis Jiggins reference in Zimmer.] This practice may seem
counterproductive to Wolbachia, since killing off its male host amounts to suicide. However,
if one assumes a given ecosystem can only support a certain number of butterflies, then from
a Darwinian perspective, it makes sense for Wobachia to favor the sex that can support the
most Wolbachia. Normal reproductive behavior in insects calls for hoards of males to
congregate around choosy females with the females having the final say as to who gets to
father the next generation. In A. encendana these roles are reversed because Wolbachia has
reduced the male population from a norm of 50% to only 6%. The surviving males--now
surrounded by hoards of females--are selective about whom they mate with, preferring
non-infected females since they are the only ones that can produce males.
|In essence, Wolbachia is a parasitic bacterium
that controls its host's reproduction.
|How is a species' identity affected by Wolbachia?
|The litmus test for whether two animals are of the same species is whether a pair can mate
and produce fertile offspring. For example, a horse and donkey can breed but their
offspring, a mule or jenny, cannot. Hence, the horse and donkey are said to be of different
species. Had they not been domesticated, over time the horse and donkey would likely
become more genetically differentiated to the extent that they would be able to produce no
offspring at all. This phenomenon is known as genetic drift, a key mechanism for speciation
in the wild.
Genetic drift is the evolutionary tendency of all life forms to modify their DNA to meet the
changing demands of their environments. It applies to all life forms, even humans. For
example, as groups of humans became geographically isolated during the ice ages, they
began to adapt to their particular environments, producing what we think of today as racial
differences. Had the ice sheets not receded, thereby reuniting these disparate groups, the
'racial' differences would eventually have become so profound as to prevent interbreeding.
It can be said that it is the constant shuffling of sexual partners that causes a species to
retain its genetic identity.
However, it does not take an ice age to separate a group within a species. There are many
ways to achieve sexual separation besides geographical. Among humans cultural taboos
can be very effective. Insects have no culture to speak of, but they do have Wolbachia. The
presence of a particular strain of Wolbachia within an insect colony effectively causes that
colony to become genetically isolated from other colonies of the same species. In time
genetic drift will have its way and a new species will be born.
|Wolbachia could be a significant driver of speciation.
|Can a bacterium common to insects invade a human?
|In 2001, Wolbachia was the subject of a biology class project at the University of Wyoming.
Presented in PowerPoint as a CDC investigative report, complete with official looking logos
from the Center for Disease Control's Division of Parasitic Diseases, the whole thing was
entirely hypothetical, although the stir it caused was reminiscent of Orson Welles' famous
radio broadcast of 'War of the Worlds.'
The instructor was Dr. Merav Ben-David and the author of the presentation was one of her
students, Samin Dadelahi. The premise of the scenario was that the Wolbachia bacterium
had accidentally been incorporated into a bacterium that was virulent to humans, thereby
causing the local human population to display the some of the same symptoms as insects do
when similarly infected. In this scenario the symptom that attracted the attention of the
CDC was parthenogenesis, females spontaneously cloning daughters.
In the spring of 2000, the 'CDC' dispatched an investigative team to West Africa where the
accident occurred to determine if there was any truth to the rumors of 'virgin' births. The
team returned with the conclusion that the asexual pregnancies were real, the proof being
that only females resulted from these pregnancies and in each case they were genetic
duplicates of the their mothers. Further investigation revealed Wolbachia had infected the
gametes of the mothers.
Years ago, a laboratory experiment to infect mice with Wolbachia failed. Scott O'Neill of
Yale Medical School points out that the bacteria are intolerant of the basal body temperature
of mammals. However, Werren points out that it is too early to dismiss the possibility of
Wolbachia infecting vertebrates.
|At least some scientists believe the notion of
Wolbachia in humans is not entirely
|How could an insect bacterium invade a human?
|Although there have been no known infections
of Wolbachia occurring in the vertebrates to date,
there are biologically plausible means for such a
transfer to take place.
|Could Wolbachia be co-opted as a weapon of terrorism?
|The motive for applying Wolbachia to terrorist agendas already exists. Western
thinking received a serious wake-up call September 11, 2001. Before 9/11 who would have
believed that a group of people would deliberately commandeer an airliner, kill the crew,
and then crash the airplane into a skyscraper? And yet it happened, almost four times in a
single day. Modern 'anything-goes' terrorists don't think like the work-a-day folks of
western society. The only question that remains is what terrorist groups might have an
interest in controlling reproduction? Following are some examples:
That Wolbachia could theoretically find its way into humans has been
established, the major biological impediment being Wolbachia's disdain for the more
tropical climate found in mammalian gametes. Skeptics might also argue that, as with
insects in the laboratory, Wolbachia have proven easy to eliminate with antibiotics such as
tetracycline. (Set aside for the moment that overuse of antibiotics can render them
ineffective against microbes.) Moreover, with the development of the polymerase chain
reaction, endosymbiont microbes such as Wolbachia have become much easier to detect.
Finally, there is the problem of propagation. How would a terrorist group disseminate such
a weapon on a local, national, or international scale? So at first blush Wolbachia would only
make for a low-tech, easily-countered weapon.
Wolbachia could be made into a sophisticated and sinister weapon of
reproductive control. Consider the well-known organelle common to almost all
nucleated cells in nature, the mitochondrion. Normally responsible for providing for the
cell's internal energy requirements, the mitochrondrion shares its genetic lineage with
Wolbachia and the two organisms harbor many of the same behavioral characteristics, e.g.,
both are only inherited along maternal lines. Indeed, it has often been observed in such
plants as corn and rye that the mitochondrion is responsible for skewing the sex ratio in
favor of females, the better to propagate more mitochondria. This gives rise to two notions:
modified to behave as Wolbachia could be exceedingly difficult to detect. How does one
tell a benign mitochondrion from a modified one, given the only difference is in the internal
DNA? Admittedly, with modern gene sequencing equipment, this is not impossible, but
imagine trying to diagnose a local population of millions of people with such means! On the
other hand, destruction of such a mitochondrion through antibiotics is a far simpler
proposition until one has to address the host gametes' need for an alternate internal energy
supply. The cure is effective but lethal.
|Could Wolbachia be co-opted as a weapon of terror?
How do you know it hasn't already?
presentation done as a class project for Evolution 2001, Deleted from Site 2002.
Island Model, Integrative and Comparative Biology, April 2002.