The Genetics of Eye Colour

One of the most striking and functionally useful properties of Drosophila is the ease of manipulation of their eye colour.

Various examples of eye colour. The fly on the far right is wild-type and the one to it's left is w1118/w1118. The others have eye colours produced by the P-elements they contain.

There are two major pathways that give rise to the distinctive, wild-type red colour of Dmel eyes. One gene, known as white is the linchpin of the system and variations of this gene itself, and it’s location, can give any colour from white to red (passing through yellow and orange). (Note that a fully functional white gene leads to red eyes).

The simplest white mutation is one that disables it’s function entirely (w-), such as w1118. Flies homozygous or hemizygous for w1118 have white eyes (picture above, bottom fly), whereas those heterozygous for w1118 and wild-type have the normal red eyes.

The white gene is normally located on the X chromosome, but it doesn’t have to be to produce an eye colour. This means the gene can be used as a ‘marker’ to show the presence of transposeable elements in the genome. Transposeable elements, or P-elements, are able to hop around the genome and land on any chromosome, when a suitable transposease enzyme is present. They occur naturally, but are something extensively exploited for genetic manipulation. It is a relatively simple process to create an artificial P-element and insert it into a genome. But how could you tell if a P-element has become established in a population of flies? Simple, if the P-element contains a white gene to affect the fly’s eye colour, then it’s presence is obvious in a w- background. It is also simple to find out to which chromosome the P-element has inserted itself on by performing crosses and watching where the eye colour goes.

Two factors can affect the eye colour produced by P-element-borne white genes – the allele of white and the position in the genome. The same P-element containing a white gene capable of producing a red colour could produce anything on the white-red spectrum depending on where it lands. This is because the dosage of the white gene product affects the colour – less = lighter eyes. If the P-element lands somewhere where it is poorly transcribed, less protein will be produced, and the fly’s eyes will be lighter. Dosage also allows you to determine if a fly is homozygous for the chromosome carrying the P-element, or heterozygous for another chromosome. Unless the colour is stuck at red or white, flies with two copies should be distinct, with a darker colour, than flies with only one copy.

P-elements have many uses and in my work I have used them to generate mutants – they can cause deletions when they “hop out”. This involves taking a strain of flies with a single P-element near your gene of interest, introducing a transposease on another chromosome, and the removing the transposease in a later generation. With an eye colour gene on the P-element, you can tell if the transposease has done it’s job. If it has, the fly’s eye colour will change – usually from, say, yellow to white, occasionally getting darker if the P-element is still present, but in a different position. While the transposease is present, the P-elements in every cell of the fly’s body have the potential to move. This is of little consequence in most somatic cells, and only useful in germ-line cells, but if it happens in some of the cells that go on to develop into the eyes, it can have some noticeable effects, such as the two flies above above.

Eye colour is one of the most versatile genetic markers in Dmel, thanks to the range of colours, non-restrictive localisation of extra white genes and the lack of any negative affects on the fly.