Gynandromorphs – breaking all the rules

I was scrolling through my tumblr feed a couple of weeks ago and came across this weird picture of an asymmetrical butterfly and wondered, ‘What on earth?!’. It’s a gynandromorph – an organism made up of half male and half female tissue (not to be confused with a hermaphrodite… read on). The word gynandromorph also gives us a clue, gyn comes from the Greek for female and andro comes from the Greek prefix meaning male.

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A gynandromorph butterfly – Lepidoptera love being gynandromorphs

 So, how do gynandromorphs arise?

In order for a fertilised egg (a zygote) to become a fully formed organism, it must divide many times. For it to divide and produce two daughter cells, the zygote must undergo mitosis, which is the process by which chromosomes inside a cell’s nucleus are divided into two identical sets, and split between two new separate nuclei.

Sometimes this mitosis goes wrong and mistakes in the chromosome division occur. Two processes where mitosis can go wrong are nondisjunction and chromosome loss, which lead to an aneuploid organism.

Nondisjunction is where more chromosomes than usual end up in one daughter cell and less chromosomes than usual end up in the other. E.g.

Mitotic nondisjunction - the daughter cells end up with too many chromosomes (left) or too few (right).

Mitotic nondisjunction – the daughter cells end up with too many chromosomes (left) or too few (right).

Chromosome loss is exactly what it sounds like – when the chromosomes are separating, a chromosome gets lost in the process and one daughter cell ends up with the normal amount of chromosomes and the other ends up with one less than usual. E.g.

Chromosome Loss -

Chromosome Loss – the daughter cells end up missing a chromosome (left) or with the normal amount (right).

An aneuploid organism is an organism with an abnormal number of chromosomes (which can arise from the two aforementioned processes).

If the mistake occurs early on in development, as in straight after the zygote is formed, these different daughter cells will continue to divide and produce different types of tissue. When the two tissues are produced side by side in the same organism, it’s called a mosaic organism.

When the mitotic mistake occurs with the chromosomes that determine an organism’s sex, a gynandromorph may arise.

A gynandromorph lobster

A gynandromorph lobster

Are there different types of gynandromorphs?

Not all gynandromorphs are obviously split down the middle male/female. Bilateral gynandromorphs are most commonly seen in nature, whereas others are harder to spot. Using computer models of cell development, basic and more complicated forms of gynandromorph can be simulated. The three main types of gynandromorphism are illustrated below.

Bilateral (left), Polar (centre) and Oblique (right) cell balls from computer simulations

Bilateral (left), Polar (centre) and Oblique (right) cell balls from computer simulations

– Bilateral:            left/right body halves of the opposite sex

– Polar: back/front halves of the opposite sex

– Oblique:            left/right/front/back halves of the opposite sex

What makes a gynandromorph different from a hermaphrodite?

Well, a gynandromorph actually looks like it’s half male and female, whereas a hermaphrodite, while possessing both male and female sex organs, can have the outward appearance of being one sex or the other.

A gynandromorph chicken - its left is a female and its right is a male

A gynandromorph chicken – its left is female tissue and its right is male tissue

Why don’t we see this crazy phenomenon in humans?

Gynandromorphs are found in insects, crustaceans and even birds. However, they don’t occur in organisms higher than this, like mammals. This is thought be because sex determination is more complex. In vertebrate development, developing embryos are ‘sexless’ until a sex-determining gene is ‘switched on’ later in development and allows the development of characteristics unique to each sex. Hormone effects also come into play.

FUN FACT: this is the reason why male mammals still have nipples. Nipples develop before sex determination takes place, therefore they persist in males but their main function in feeding offspring is only needed in females.

Birds seem to be the exception to this vertebrate rule (so far) but as to a reason why, that’s a whole other developmental kettle of fish (read more here and here…).

A striking example of gynandromorphism in a cardinal

A striking example of gynandromorphism in a cardinal.

Gynandromorphs are really fascinating phenomena that naturally occur in the animal world and in some cases are mind-bogglingly beautiful (yet a bit strange). If you’re interested in finding out more, have a click on the links below for a few interesting posts.

News article on the gynandromorph butterfly of the Natural History Museum

Blog post about gynandromorphs

Sciencemag article about gynandromorph chickens

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2 responses to “Gynandromorphs – breaking all the rules

  1. Pingback: Fluttering Both Ways: Thanks to a Genetic Anomaly These Butterflies Have Male and Female Wings | DARK IN THE BOY·

  2. Pingback: Symmetry in biology | Unascertainable·

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