One of the things we have been asked several times at the Royal Society Summer Science Exhibit is whether butterflies always have symmetrical wing patterns. This is almost always the case, because the wing patterns are hard-wired in the genome, and all cells on both wings have the same genetic code. However there are rare exceptions, where a single egg gets fertilised by two sperm, or a single individual developed from two genetically distinct cell lineages. These rare specimens are known as gynandromorphs, and can tell us something about wing development. Here, Martin Thompson describes a few specimens of gynandromorphs in the genus Papilio.

This is a sex mosaic (‘mosaic gynandromorph’) Papilio dardanus. The left wing is mostly the pattern of the female form cenea, but the right wing shows lots of yellow patches of male pigment. There is even a segment at the bottom corner with male patterning and even a little bit of a tail.

It is thought that this kind of pattern arises from the loss of one of the sex chromosomes from one of the two daughters of a cell division, or possibly when an the egg contains an incorrect number of sex chromosomes.

In butterflies and moths, the sex determination system is different to humans – males have two identical sex chromosomes (ZZ rather than XY) and it is the females with two different sex chromosomes (ZW contrasting with XX in humans). If this loss happens very early in development whilst the embryo is only a few cells, the result can be a striking bilateral gynandromorphy, with one half of the body male and one half female:

However, if the error occurs later in development, the result will be a mosaic as in the first picture. Isn’t biology cool?!
Mosaics such as these can teach us about how wings are formed and patterned when the butterfly is still a pupae. By looking at a large number of mosaic butterflies, scientists have found that there are several ‘compartments’ to the wing: that there are boundaries which cells never cross during development. These boundaries are usually invisible, but in mosaic gynandromorphy wings, each differently-coloured patch arises as a handful of cells. When we see lots of mosaic patches with similar, sharply-defined and straight boundaries, this tells us about the position of the wing compartments.

Notice that the region where black meets yellow on the forewings is a straight and clear line. This marks a compartment boundary within the wing.