During my Ph.D. defense, Cliff Gilman asked "Why do flies have red eyes." My mentor, Jerry Wasserman, started to scold him for asking a silly question, when I interrupted and offered an evolutionary justification.

In my dissertation's Introduction, I had this picture to explain how eye color pigments point each ommatidium in a different direction for the sake of acuity.

Modeled after a 1965 paper on houseflies by Timothy Goldsmith at Yale, my Drosophila study showed that eye color pigments lower ERG (electroretinogram) sensitivity by preventing the short wavelengths they absorb from spreading to the whole eye. The evolutionary explanation (at the time) was that rhodopsin is sensitive at short wavelengths, and thus the fly can afford to sacrifice sensitivity for acuity in that spectral range (but not at long wavelengths). The evolutionary argument could be amended later with the discovery (see, e.g. this page) that long wavelength light regenerates rhodopsin from metarhodopsin.

Eye color pigments in Drosophila have served for about a century as a genetic model system. I did some crosses in Gateway middle school projects (1998-2001) and posted This picture of white and red eyes.

This classic genetics book:
A.H. Sturtevant and G.W. Beadle, An Introduction to Genetics, New York, Dover Publications, Inc., 1962 (except note this from Preface, " This book represents the way genetics looked to us in 1939. In the past twenty-two years there have been far reaching changes in the subject, but these have not been incorporated."

Has a section on:
Eye color hormones in Drosophila
where it is stated:
"...eye-pigmentation in Drosophila must be dependent on a diffusible substance..."

To understand that odd use of the word "hormone," look at this catalogue:
D.L. Lindsley and E. H. Grell, Genetic variations of Drosophila melanogaster, Carnegie Institution of Washington, 1967
a book that cost me $3.00

Here is the entry for:
cn: cinnabar
nonautonomous in ...transplanted eye disks
3-hydroxykynurenine,,, the cn+ hormone

The transplantation of imaginal disks to establish autonomy (vs nonautonomy) is very fundamental.

From mutants that block the brown pigment (xanthmmatin), here is the biosynthesis which I found in this paper.

tryptophan -> formylkynurenine
(tryptophan pyrrolase) [vermillion lacks]
formylkynurenine -> kynurenine
(kynurenine formamidase)
kynurenine -> 3-hydroxykynurenine
(kynurenine 3-hydroxylase) [cinnabar lacks]
3-hydroxykynurenine -> xanthommatin {interconverts oxidized and reduced}
(phenoxazinone synthetase) [low in cardinal]

Here is the entry (Lindsley and Grell) for:
bw: brown
Nolte
autonomous when transplanted into wild-type host

Brown flies lack drosopterin which looks like this, (from this paper by Ziegler, an authority on pterines which are important for many reasons)

There were frequent references to Nolte, and I was delighted when he sent me a stack of reprints including this one in response to my reprint request. Incidentally, Nolte was also my wife's maiden name, and a different scientist named Nolte had worked on a study I knew about Limulus vision.

Here is the entry for:
w: white
autonomous in larval optic disk transplanted into wild-type host
Optomotor response absent (ref) but fly phototactic.

I also had studies of my own on phototaxis eventually, most of them on white eyes. The lack of optomotor behavior is obviously explained by the lack of optic isolation (because eye color pigments are lacking) between ommatidia.

There had been several excellent papers on structure of eye color pigment granules including this paper by Fuge in 1967. (He even speculated on the cell biology of granule formation.) Perhaps because there had been so many papers, it was hard to get mine accepted, but I do feel proud of some of the 1988 vistas I obtained (see this page). Also, this paper has been cited which helped me to find some interesting new developments, e.g.:

V Lloyd et al., Not just pretty eyes: Drosophila eye color mutations and lysosomal delivery, Trends in Cell Biology 8, 257-259, 1998. (in which it is stated that pigment granules are specialized lysosomes)

and

GDEwart and AJHowells, ABC transporters involved in transport of eye pigment precursors in Drosophila melanogaster, chapter 15 in Methods in enzymology 292, 213-224, 1998 (addressing how the autonomy is explained since it is not the synthesis but the incorporation that is blocked)

August 2008, Ms. Christine Simmons assisted in this demo of eye color extracts: (from left to right) (1) cn basic, (2) cn acidic, (3) bw basic, (4) bw acidic. (A few days later, the basic cn tube had bleached.) On the basis of this pilot experiment, we suggest the following recipes for the optimum demonstration:
Use10 flies per tube (for cn).
Use 20 flies per tube (for bw) (since considerably less pigment per head is extracted).
Start with 10 drops per tube of 30% ethanol brought to pH=2.0 (using HCl).
To change pH, 10 drops of 1.0 N NaOH.
To compare acidic and basic tubes, add 10 more drops of the acidic ethanol (to bring tubes to equal volume).

This page was last revised on 8/13/08

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