Pigment Pathways of Drosophila melanogaster

 

The eye colors seen in D. melanogaster result from the interaction between two pigment pathways: the drosopterin pathway which produces pigments with red tones and the ommochrome pathway which produces brown shades of pigments. If an eye color deviates from the wildtype brick-red color, it is the result of a deficiency of some pigments coupled with excesses of others. A pigment deficiency may result from a mutation in a pigment synthesis gene or a mutation in a gene that codes for pigment transport. If the ommochrome pathway is interrupted, the percentage of brown pigments in the eye will decrease, and so the eye will be a redder color. If a gene in the drosopterin pathway is mutant, like sepia, the eye will be more brown than the wildtype eye. Genes involved in the drosopterin and ommochrome pathways are located throughout the genome of D. melanogaster.

 

Table1: Examples of Pigment Pathway Genes

 

Name

Symbol

Color

Map Location

brown

bw

light brown

2

-

104.5

cinnabar

cn

bright red

2

-

57.5

maroon-like

mal

deep reddish

1

-

64.8

Plum

Pm

red-brown

2

-

104.5

purple

pr

purplish

2

-

54.5

rosy

ry

pinkish

3

-

52.4

scarlet

st

bright red

3

-

44.0

sepia

se

dark brown

3

-

26.0

vermilion

v

bright orange-red

1

-

33.0

whiteapricot

wa

light orange

1

-

1.5

whiteeosin

we

pink-orange

1

-

1.5

white

w

white

1

-

1.5

 

 

 

Many of these genes encode enzymes that control one step in the biosynthetic pathway. Each step of the pathway is catalyzed by a separate enzyme, so if an enzyme is not produced or is not functional because of a mutation, the pathway is halted.

 

In the ommochrome pathway, the brown pigment xanthommatin is produced from the amino acid tryptophan by the following biosynthetic pathway:

 

 

 

The cinnabar gene codes for kynurenine-3-hydroxylase. When a fly is homozygous mutant for cinnabar, functional kynurenine-3-hydroxylase is not produced so kynurenine is not converted to 3-hydroxykynurenine. The pathway is halted, xanthommatin is not produced, and the fly has a bright red eye.

 

 

Drosopterin Pathway

A model pathway for the production of drospterins is shown below.

 

 

This model provides a framework for the analysis of today's chromatography results.

 

The eye color of a fly is the sum of varying concentrations of different pigments. When dissolved in 1:1 n-propanol and ammonium hydroxide, these pigments will separate during chromatography based on differences in molecular size and solubility. The solvent is absorbed by the silica gel on a thin-layer chromatography plate and moves towards the top of the TLC plate. Depending on how well each pigment dissolves into the solvent, the individual pigments will travel along with the solvent, being deposited on the plate at the point where it is no longer soluble in the solvent. Because drospterins fluoresce, the drospterin pigments present in a fly eye can be seen under ultraviolet light on a chromatogram. Below is an example of the chromatogram of a wildtype sample.

 

 

 

With the completed chromatogram results, calculate the Ratio-to-front, or Rf, value for each pigment in each lane. The Rf value is a ratio of the distance a pigment traveled to the distance the solvent traveled.

Measure the distance that each pigment traveled from its starting spot to the center of the pigment spot. Measure the distance from the starting line of the samples to the solvent front. 

Divide the pigment distance by the solvent distance for each pigment.