Posted by: arrogantscientist | January 29, 2009

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.

eyes1

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.

eyes2eyes3

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.

eyes4

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.

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Responses

  1. can you tell me about starlet on drosophila’s eye?
    what gen control this alel?
    and what will happen if we cross this with normal type (red collor)?
    thaks for help

  2. Do you mean scarlet (st)? A mutation in scarlet prevents the production of brown eye pigment, giving resulting in bright red eyes.

    Crossing in a wild-type copy of scarlet will result in wild-type colour eyes, as it’s recessive, assuming all the other genes for eye colour are all also wild-type.

    In a white background though, altering or compensating for a mutation in scarlet will have no effect, as the eyes can only be white.

    I should point out that this article only concerns mutations to the white gene, which affect levels of brown and red pigments in the eye. Production of red and brown pigments are separate, and mutations in either pathway produce different colours. I’ll probably do an article on these colour at some point….

  3. Hi. Can you tell me the difference between scarlet, vermillion, and rosy eyes? I have a mutation that is the same shade of red as the wild type but it is slightly duller and I can’t figure out what it is. Thanks.

  4. Vermillion and scarlet mutations give bright red eyes, so it wont be one of those. I’ve not seen a rosy mutation in the flesh, but it’s described as “pinkish”. Mutations like carnation do look duller than wt, but are more brown. Maybe its a mutant causing rough eyes? That would make them look less shiny.

  5. can u tel me who discovered / isolated white apricot mutation, vestigial gene mutation and curly gene mutation in drosophila

  6. white apricot – R. R. Huestis (1923)

    vestigial – not sure, earliest paper I could find: Williams, J.A., Pappu, S.S., Bell, J.B. (1988). Suppressible P-element alleles at the vestigial locus in Drosophila melanogaster. Mol. Gen. Genet. 212: 370–374

    Curly – Lenore Ward (1923) – http://www.genetics.org/cgi/reprint/8/3/276.pdf

  7. […] 3. Variation in eye colour – the eye at the bottom is a complete knockout of the white gene. By ArrogantScientist […]

  8. Hello everyone
    in my project i had to cross white male with scarlet female
    can any one give me the phenotype of f1 and f2
    i notice in my result that f1 was wild type
    and after crossing them i got white male and scarelt and wild male and female

    thnx alot

  9. F1 will produce only flies with wild-type eyes as males will be heterozygoous for scarlet. Females will be hertrozygous for white and scarlet.

    F2 will produce a mix of genotypes with the following visible phenotypes:
    male, white
    male, wt
    male, scarlet
    female, scarlet
    female, wt

    This would be much easier to understand with a punnet square 😉

  10. i see thanks alot 🙂

  11. what would be the molecular basis for these different eye colors?

  12. Hello, can you tell me the reason of mosaic eye in a drosophila?

  13. Then, w1118;201y-Gal4. Contain white or red eyes?

  14. This is amazing!!!
    I’m a graduated student from Taiwan and use Dmel as my model as well.
    Recently I’ve been struggling with eye disc immunostaining of mcd8RFP.
    My professor told me that I can distinguish those got the mdc8RFP ; driver(derived form mcd8rfp/cyo ; driver/TM6B line and DsRed was used) from those don not via their eye color (’cause I balancer this line on my own)
    But after the staining I couldn’t read any RFP signal. I’ve checked my flies and their all are red-eyed.
    So they do get mcd8RFP right?
    And do you think there is any other factor that can affect immunostaining? (I’ve checked the first and the second antibody already)

    THX a lot!!!

  15. i have a query that i crossed a second balancer with a third balancer fly which was the first step of my crossing scheme to create a double balancer fly. i get few flies which have strange eye color phenotype. few fles are have varrigated blackish red eye color. although both balancer parent flies were white eyes i.e. they contained a w1118 mutation then how is it possible to get a varigated red eye color in the few progeny flies.

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  18. I am performing an experiment on Drosophila and a few weeks ago, they were true breeding for very dark brown-black eyes but now their eyes are brownish-red(like the eye color on the left of the wildtype on your first picture. Do you know how could this have happened? Thanks

  19. I am performing an experiment on Drosophila strain Pm, they have combination of red and purple eyes. My seniors say that Pm carries lethal gen. Can you tell me why they carrier lethal gen? Thanks


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