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
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.
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?
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.
If the chromosome change or gene mutation occurs at this point, then exactly one half of the animal will have the change and the other half won’t. This is a bilateral chimera. On the other hand, of the mutation/change occurs at some other point, the there will be patches of one type of cell and patches of the other. This is called a mosaic (see picture to the right).
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.
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.
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.
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.
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.
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.
reviewed here). The sex characteristic is a default, and therefore is seen both males and females despite later hormone differences. That’s why men have nipples. Nippled is the default state, no nipples isn’t possible. And three nipples is just weird.
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 –