Evolution under the microscope

Phenotypic variations caused by corrupting genes

Drosophila

Drosophila melanogaster[1]

Drosophila eye colours

Drosophila

Drosophila melanogaster[1]

The normal ‘wild type’ eye colour of Drosophila melanogaster is a reddish-brown. The overall colour is due to two principal pigments, ommochrome which is brown and drosopterin which is red, and various minor components related to drosopterin which are predominantly blue or yellow. The pigments are synthesized by the biochemical pathways shown below.

These pathways indicate the colours of some of the pigments and intermediates, and the reactions that are disrupted in various mutant strains. Note that when a reaction is blocked, not only are the pigments downstream not synthesized, but there can be a build-up of the intermediates immediately before the disruption which may also affect eye colour.

eye pigments

Biosynthetic pathways for the production of eye pigments in Drosophila. [2]

So, for example, if both synthetic pathways are disrupted early on, so that no pigments are produced, then the flies have white eyes. If only the ommochrome route is affected then the eyes are vermilion (due to drosopterin and related compounds such as xanthopterin); conversely if ommochrome is synthesized but none of the pteridine pigments, then the eyes are brown. If only drosopterin synthesis is prevented then the eyes are sepia (due primarily to ommochrome and xanthopterin) and become progressively darker as excess sepiapterin is produced; and if only xanthopterin is affected then the eyes are rosy due mainly to ommochrome and drosopterin. Many other eye colours arise depending on exactly which step(s) in the syntheses are affected.

Most of the mutations cause deactivation of enzymes converting one compound to another; but white and brown disrupt transport into the cell of the starting compounds (white affects the transport of both tryptophan and GTP, brown of only GTP).

The important point is, of course, that whereas early geneticists thought the mutants with different eye colours were examples of constructive mutations – involving the production of new pigments – we now know they are due solely to the disruption of the normal production of the usual pigments. That is, they are deleterious mutations, arising from the corruption of existing genes, not the production of new ones.

 


Notes

Notes display in the main text when the cursor is on the Note number.

1. Image by André Karwath aka Aka (Own work) [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons; https://commons.wikimedia.org/w/index.php?curid=227170.

2. The pathway to ommochrome is fairly well established. That to the pterins is less certain; the one shown is based on Kamleh et al., Towards a platform for the metabonomic profiling of different strains of Drosophila melanogaster ..., The FEBS Journal, (2009) doi:10.1111/j.1742-4658.2009.07397.x Image by David Swift.

Page created April 2017.