In Man, the destiny of the reproductive glands to develop into either male (testes) or female (ovaries) is determined by the sex chromosomes. The sex of all other parts of the body is controlled indirectly by hormones produced by these glands. These hormones circulate through the body via the bloodstream. The sex chromosomes that are present in all the cells play no part in this process. In insects, however, the situation is very different.
In the early cellular developmental stages of butterflies the sex chromosomes present in every cell decide and control the sex of that particular part of the body. The sex of a butterfly is initially determined by the number of Z chromosomes present in the zygote, the fertilized ovum: if two Z chromosomes are present it will be a male; if only one it will be a female. The zygote subsequently divides and each of these cells will go on to develop into right and left sides of the body. At every cell division there is always a very small possibility of a Z chromosome being lost in the process, causing an imbalance between the autosomes (any chromosome that is not a sex chromosome) and remaining sex chromosome. The resulting cell left with just one Z chromosome changes to being female in character and all further development is along those lines. An individual organism that contains both male and female tissue is termed a gynandromorph. This is the most commonly accepted method by which gynandromorphs arise.
Gynandromorphism in the Lepidoptera is most often associated with wing coloration. Should these features appear combined on the same wing as a mosaic it is referred to as a mixed gynandromorph. But sometimes an insect is bisected, such as the individual below, with one side of the body including the wings being either entirely male or entirely female. These remarkable and rare individuals are termed bilateral gynandromorphs. In many species of butterfly where extremes of sexual dimorphism occur, spectacular gynandromorphs can sometimes arise, particularly where one half is male and the other female.
My thanks to Alec Harmer for his assistance with this article ...
Brimstone, Gonepteryx rhamni (bilateral gynandromorph). Image copyright OUMNH.