Rare dual sex moth emerges at Museum

10 May 2008

A rare half male and half female moth has emerged today in the pupae nursery at the Natural History Museum's Amazing Butterflies exhibition.

The left wings of the Antheraea frithi moth look female and the right wings look male, with a definite dividing line running along the moth's body.

Moths and butterflies have a short life span, so visitors who are quick should be able to see it in the exhibition's butterfly house on the Museum front lawns for a short time only.

Not a hermaphrodite

The moth is a gynandromorph - gyn is Greek for female and andro is Greek for male. It is not a biological hermaphrodite, they have a fully functional set of both male and female sexual organs.

In this moth, the female half has only a partial set of female organs and the male half has only part of a male set, neither part being functional.

Discovering the moth

Upon discovering the moth Amazing Butterflies butterfly house manager, Luke Brown, said 'This is phenomenal and I am amazed to discover this moth.'

'This is only the second gynandromorph moth or butterfly I've seen in ten years. And I've seen over a hundred thousand pupae hatch.'

'We can only spot the amazingly rare condition in moths and butterflies because the males and females look very different from each other.'

Only 200 specimens

Once this moth dies, it will join the Museum's world-class Lepidoptera collection. Dr Ian Kitching, moth researcher at the Museum, said 'Gynandromorphs are incredibly rare. We only have 200 such specimens in our collection of some 9 million butterflies and moths. That's equivalent to far less than one percent of our collection.'

Genetic errors

The condition seen in this moth is called bilateral gynandromorphy and it can also be seen in some crabs and lobsters. Each half of the animal has all the characteristics of its gender, right down to divided reproductive organs.

'The bilateral gynandromorphy that this moth shows is the result of an error involving the sex chromosomes at the first cell division,' says Kitching.

'Sometimes, such errors occur later in development, whence the gynandromorphy is mosaic and the separation into the two sexes isn't so clearly defined.'

Cell division

When a zygote (a fused sperm and egg cell) divides, eventually making all the different cells in the body, some cells become muscle cells, some become nerve cells, some intestine cells and so on.

In some animals, including mammals and birds, cell division is initially indeterminate, meaning that the fate of the cells is not decided straight away.

However, in moths and butterflies, like all insects, each cell division from the very first is completely determinate, which means that decisions about what a cell will become are made straight away, not at a later stage in development. Most importantly, the first division into two cells separates left from right.

The sex of an animal is determined by X and Y chromosomes. But, unlike humans, where the male is XY and the female XX, in butterflies and moths XX is male and XY is female.

Unusual cell divisions

Normally, when a cell divides, the DNA is first copied and then a complete copy of the DNA is passed to each of the daughter cells.

However, if the two DNA copies of the X chromosome do not detach in an XX male during the first zygotic division, then an individual can develop that is male on one side and female on the other side. This is what has happened to the moth at the Amazing Butterflies exhibition.

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