My idea in
writing about evolution is to arouse the curiosity of the readers and
inform them that truth is often stranger than fiction. The premises of
Darwin’s theory are that the innate nature (genes, not acquired
characteristics) of an organism are faithfully passed on to the progeny
via chromosomes; there are random mutations that occur in genes during
the process of inheritance and the environment (natural selection) leads
to differential survival and more importantly differential reproduction
(survival of the fittest) of those organisms whose genes are better
adapted to the current environment. In part one examples were given of
parasites that modify the behavior of intermediate hosts to make them
more susceptible to predation by the final hosts of the parasite to
enhance the survival and reproduction of the parasite.
Other
interesting examples are certain flukes which infect snail shells and
secrete substances that make the snail grow a thicker shell than normal,
thus protecting the snail from easier predation and prolonging the
parasite’s survival. Mice infected with the tapeworm Spirometra
mansonides grow faster and more than uninfected mice and thus provide a
more robust home for the parasite. Trilobium beetle larvae when infected
by the sporozoan Nosema don’t mature by molting into adult beetles and
repeatedly molt into bigger larvae up to six times instead of the normal
just once. They then become a super-sized larvae. The Nosema
micro-parasites who are all sibs, achieve this by collectively secreting
the beetle juvenile hormone to keep the larva repeating its molt without
progress.
Some parasites like the barnacle Sacculina castrate the host, a crab.
When it comes to sexual behavior there are other fascinating examples.
For this it is important to understand that most plants and animals we
know are Eukaryotes formed by the merger of two separate creatures
nearly three billion years ago. Before that the earth had very little
oxygen and bacteria the only life forms, lived anaerobically without
oxygen. Then Cyanobacteria evolved and with their chlorophyll they could
use sunlight, carbon dioxide and water to survive and breathed out
oxygen. This is the opposite of what we do which is breathe in oxygen
and breathe out carbon dioxide. Slowly the atmospheric oxygen level
increased. A new bacterium evolved that could live on oxygen. The merger
of an old bacterium with a new mitochondrion (oxygen consuming) became a
eukaryotic cell from which we evolved. A merger of an old bacterium cell
and a cyanobacterium (sunlight using by chlorophyll) gave rise to a
eukaryotic cell from which plants evolved.
Bacteria reproduce asexually by simple division. They don’t need a date
on Saturday night. Now and then they do have sex, but it is purely
recreational. They don’t need it for reproduction. The donor bacterium
donates a copy of some or all of its genes and the recipient
incorporates the genes in its gene pool and both go their merry way with
no further contact just like a one night stand. Each of them divides
into two later on to reproduce. The recipient has an enhanced gene pool
that it passes on to its progeny by bifurcation. Whenever a bacterium is
infected by a plasmid (smaller extrachromosomal ring of gene bits,
sometimes a virus) it really gets an itch to have sex, an unnatural
behavior for it but very necessary for the plasmid or viral
bacteriophage to spread. This is why indiscriminate misuse of
antibiotics leads to the development of resistant bacterial strains
which cause an itch for sex in the resistant bacterium with a plasmid
and thus spread of the antibiotic resistance both by sexual and normal
propagation.
A bacterium called Agrobacterium often gets infected by a plasmid named
Ti. The infected bacterium then infects a plant and causes cancer in the
plant. The indiscriminately growing cancer is called a crown gall and is
of no benefit to the plant. The Ti plasmid has a gene which is turned on
in the plant and induces it to make a substance called opine for which
the plant has no use. Amongst the genes of the Ti plasmid is another
gene that allows infected bacteria to use the opine as food. This leads
to a profuse growth of the Ti infected bacteria which ensure survival of
the Ti gene by simple division of the infected bacteria and increase in
their penchant to have sex with uninfected bacteria to spread the Ti
plasmid.
Another fascinating case is of a tiny parasitic wasps. Such wasps lay
their egg in the tail end of the larvae or pupae of other insects after
paralyzing them. The wasp larva slowly eats its way through the body of
the paralyzed caterpillar and emerges as an adult wasp after devouring
the caterpillar’s head after a few days. This is what made Darwin lose
faith in a god. William Hamilton discovered that the parasitic wasp
Trichogramma lays its eggs within the egg of a butterfly or moth. The
wasp larvae eat the host egg, then pupate and emerge as adult wasps.
Hamilton found that all the progeny were female. This means that the
wasps were all females with no males and gave birth to only females
without having sex. This is called parthenogenesis, literally virgin
birth as in the case of Jesus Christ (except he was a male).
On further study it was found that the cytoplasm of the wasp egg had
bacteria in it. When Hamilton fed the adult female wasps with a course
of honey laced with the antibiotic tetracycline, the wasps resumed
having sex with other males and produced a mixed litter of males and
females. It is important to understand that most animal sperms have
hardly any cytoplasm and few mitochondria (our ancestral merger partner)
and they dump them before penetrating the ovum which is loaded with
mitochondria. All children male or female get their mitochondria from
the egg and hence the mother. For mitochondria, being in a male body is
a dead end and being in a female body is the passport to immortality.
Mitochondria would love it if all progeny are female. Thus it was a
conspiracy between the cytoplasm based (not in the cell nucleus)
mitochondria and bacteria that subverted the development of wasp eggs in
this particular case to all females. Some of the eggs were male but the
bacteria directly or at the prompting of egg mitochondria, both being
cytoplasm based and not wanting to end up dead in the future in a male
body sabotaged the male egg to develop as a female.
Host behavior modification is seen in rabies. The rabies virus infected
dog has a noticeable change in temper because of brain infection. For a
day or two after infection, it is more affectionate, salivates more and
has increased licking of human or canine contacts. Then it wanders
restlessly and bites aggressively. Even fruit eating bats when infected
with rabies will bite humans. All these help the rabies virus to spread,
infect and propagate.
Human beings infected with cold, influenza and respiratory viruses have
increased sneezing and coughing which help the virus spread. An
experiment done by channeling mosquitoes in a controlled environment has
shown that the female Anopheles mosquito is preferentially attracted by
the body odor of a malaria infected person. The periodic rigors and
breaking of fever by sweats tends to make the malaria infected person
throw off body covers and increase exposure to mosquito bites.
Small roundworms like pin-worms that parasitize the human intestine are
passed on by consuming food contaminated with their eggs. They utilize a
fiendish strategy. The pin-worms crawl out to just outside the anus and
lay their eggs. These eggs secrete a highly irritant chemical that makes
the infected person feel a strong peri-anal itch. The natural tendency
is to scratch. The eggs stick to the fingers or nails and lead to
self-infection or infection of others when food is handled without
proper hand washing.
Self-destructive behavior induced by parasites reminds me of old
Sanskrit and English adages surprisingly independently and identically
coined. In Sanskrit it is “Vinaash kaale viparit buddhi”. In
English it is “Those whom the gods wish to destroy they first make mad”.
Apparently history also repeats itself, first as a tragedy and then as a
farce. Mad King George caused the loss of American colonies to the
British Empire and now Mad Boy King George is going to lose the Middle
East for the American Empire. Unfortunately we cannot do a brain MRI on
the current George to diagnose the illness and cure it because he is so
afraid that the brain MRI may show absence of a brain and then some
child may cry out “ Look, The King has no brain”.
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