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Ardhanarishvara
is just form of the god Shiva.
As
a male, he is considered the ultimate man,
James
Garner mixed with a little Steve McQueen.
Parvati,
his wife, wanted to share his experiences,
so
he became half her. That’s one
progressive
marriage.
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Shiva is male and celibate, although he has a female consort named Parvati (aka. Shakti,
Devi, or Uma). There is also a deity called Ardhanarishvara, which is a half male/half female
representation of Shiva + Parvati. The icon is found in most temples to
Shiva, but this deity rarely has temples dedicated to him/herself.
Evolution chose to
go the other way with nature. In more complex animals, the sexes are separated and join
energies to reproduce. In biological terms, it’s a matter of increasing genetic
diversity, the source of mutations and drift for natural selection.
However, like Ardhanarishvara, nature sometimes gives us a mixture; a
normally two sex species will produce an individual that is part male and part
female. And sometimes they're exactly half and half. This is going to take
some explaining.
Every once in a
while, some embryos have a mistake in mitosis. When the chromosomes line up for
random assortment and portioning into the daughter cells, things can go wrong.
Once in a while, two chromatids (the two copies of a replicated chromosome) may get
pulled into the same daughter cell instead pulled apart with one going to each
daughter (called a non-disjunction event).
This produces one
cell with too many copies of that chromosome, and one cell with too few. Both
outcomes can cause problems. Sometimes,
the problem is just cosmetic; sometimes it’s deadly.
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Gene
loss can come from losing a part of one
chromosome,
or you might lose the whole
chromosome
(monosomy). It could occur from a
non-disjunction
or from some toxic event. A 2015
study
shows that smoking can cause a loss of the Y
chromosome
in some cells. This makes men more
at
risk for some cancers due to smoking (those
outside
the lung). Still want a cigarette?
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A third possibility
exists, where a mutation occurs in one chromatid after replication, so that
even if the mitosis is normal (which it almost always is) one daughter will
have a mutation (one normal and one mutated gene on the two chromosomes of the
same type) and the other won’t (two normal genes on two normal chromosomes).
From then on, every
time the daughter cells divide they increase the number of mutated and normal
cells. The animal, if it survives to be born, will be a chimera (a mixture of two genotypes). The original chimera was a
Greek mythical figure made from the parts of many animals and which breathed
fire. It was a half-brother to the Hydra and Cerberus, the three-headed dog.
Here it means something less menacing, but just as interesting.
Special circumstances
can bring special kinds of chimeras. Which type is formed depends on when the
mutation, non-disjunction, or chromosome loss occurs. In some animals, the
first cell division after fertilization establishes right and left halves of
the animal. Every progeny cell from one of the first daughters will be one side
of the body, while every cell coming from the other original daughter will be
on the other half of the animal.
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The
lobster on the top is a mosaic, the mutation
which
changed the pigment occurred at a point when
some
mutated and some non-mutated cells were on
each
bilateral half of the embryo, so there are patches
of
each. The bottom version had a mutation that
occurred
precisely as the embryo was determining
right
and left sides.
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A 2013 review talks
about mutations in different populations of cells and the right left isolation
of some mutations. The authors point out that in bilateral chimeras, it is easy
to study subtle effects of the gene mutation – one half displays the mutation,
and the other half doesn’t. A single animal (could be a person) can serve as the experimental model AND the control.
For example, in
fruit flies (Drosophila melanogaster)
the males are XY and the females are XX. If there was chromosome loss early in
development, with a single X lost in one daughter cell, there will be XX
daughter cells and X (called X0) daughter cells. X0 cells are male because the primary sex
determining is located on the X chromosome. In this case just described, the XX cells are
female and the X0 cells are male, in the
same animal!
This animal would be
a gynandromorph chimera. The word is very telling, since gyno = female and andro =
male. This is different from a hermaphrodite.
The hermaphroditic animal has two sets of genitalia, one female and one male
(whether they work or not is another question). In a gynandromorph, the two
cell populations of the entire animal show different sex chromosomes.
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The
patterning on the thorax and abdomen is a bit
hard
to see, but the eyes are easily picked out on the
gynandromorphic
fruit fly. The pigment genes are
on
the sex chromosome.
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Birds and arthropods are the two animal groups where we have seen gynandromorphs.
We gave the example of fruit flies above. You can check out the picture of one
to the left. This is specific example of gynandromorph, a bilateral gynandromorph. The left side is female and the right half
is male.
In different systems of embryonic development, chimeras can
develop side to side (bilateral), front to back (polar), or corner to corner
(oblique). This is if the mutation or change in chromosome or gene number takes
place at exactly the right mitotic event that divides an animal. If it is any
of the other time, the animal will be a mosaic.
In the bilateral gynandromorphic fruit fly above, the color of the
eyes is different on each side, as is the body coloring and some other
characteristics. This is because the secondary sex characteristics that
determine sexual dimorphism are linked to the sex chromosomes.
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Spiders
have funky sex-determination systems, but
they
can still have gynandromorphs. The coloring is
different,
but there’s more. It is hard to see, but only
the
male side (purplish) has the palp organ growing
on
the second appendage for the transfer of
reproductive
cells. Image via: spider silk stockings
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Is a bilateral difference in coloration enough
to call an animal bilaterally asymmetric? They are phenotypically (how they
look outwardly) asymmetric, but you cut them in half the silhouettes would
be exactly the same (body plan is still symmetric). You can argue amongst yourselves as to what makes an animal bilaterally asymmetric.
Gynandromorphs in vertebrates are extremely rare. The reason
for this is that sex characteristics aren’t only controlled by genes on the sex
chromosomes. They are also under the control of hormones. But gynandromorphy
does occur in birds – they’re vertebrates, but different somehow. No one is
quite sure why gynandromorphs are possible for them.
It could be that the mistake comes when multiple male
reproductive cells are successful in fertilizing one egg cell. When the fertilized
egg cell divides, the daughters would probably be asymmetric with respect to
sex chromosomes. Since the sex chromosomes control the production of the
reproductive organs, and those organs then make the hormones, you can see how
the two are linked.
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The
gynandromorphic chicken on the left is more a
mosaic
than completely bilateral. The male side (your
right)
has bigger breast muscle, leg spur, bigger wattle
and
white feathers (see the sporadic darker feathers –
it’s
a mosaic). A 2010 study showed that if you
transplanted
male cells on to the female side, they
retained
their secondary sex characteristics –hormones
ain’t
everything. The cardinal on the right is striking –
the
perfect Ball State University mascot.
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A very rare gynandromorphic cardinal was spotted and subsequently
studied for 40 days from afar. The paper reporting this study stated that the
bird never sang, never drew the attention of other birds, and never mated. It
was a complete loner. But could he/she mate?
Most female birds have one horn of the uterus (left side)
that is functional while the other is small and nonfunctional (makes them
lighter for flight). Male birds usually have one long testis that is
functional, the right one. Since neither male or female birds (most of them)
have external reproductive organs, then a gynandromorph bird where the left
half is female and the right half is male might actually have a shot at being
fertile. It would all depend on how the hormone battle played out.
However, gynandromorphs in mammals don’t happen. The sex
hormones control too much of the systems and flow throughout entire body, so
you can’t really keep secondary sex characteristics limited to a geographically
determined set of cells, even if the sex chromosomes are different in the
cells.
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Butterflies
show sexual dichromatism (different colors
in
males and females) and well as different morphologies
of
wing (shapes), The gynandromorphs display both, so
they
are truly bilaterally asymmetric. Butterflies have an
XX
and XY (XO) sex determination system like fruitflies,
except
here the XX’s are male.
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The example I like to
give for the bilateral gynandromorphy that shows true bilateral asymmetry is
butterflies. The male and female often have different coloration and wing
shape. This makes them sexually dichromatic within one animal but also
bilaterally asymmetric.
Next week we’ll back
to the butterflies and asymmetry. There’s one butterfly who is attractive to
the girls precisely because he’s asymmetric.
For
more information or classroom activities, see:
Sex-determination
system –
Gynandromorphy
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