The eye-pigmentary system ofdrosophila - V. the pigments of the light and dark groups of mutants
The second and third groups of eye-colour mutants ofDrosophila melanogaster, studied histologically and photometrically, are the light group with the mutants light (lt), lightoid(ltd) and persimmon(pers), and the dark group with the mutants sepia(se), clot (cl), Henna-recessive (Hnr) and Plum2 (Pm2).
Inlt andpers a small percentage of granules is larger than normal and the type of pigmentation histologically is a light brownish; inltd the general colour is reddish brown, most granules are much larger than normal but less in number, and while the primary and basal pigment cells are not clearly delineated by their pigment granules, a clumping of granules occurs in the proximal parts of the secondary pigment cells. Incl and Hnr the histological picture is like that of the wild-type, while inse andPm2 it resembles the brown mutant type in general colour.
In all the mutants, excepting two, the normal types of red and brown pigments occur. Inse a modified red pigment is found, with a greenish yellow colour in solution and a changed spectrophotometric curve with a peak of absorption at 415 mµ instead of 480 mµ; incl a mixture of this modified pigment and the normal red pigment occurs.
Inltd, lt andpers a great reduction in the amounts of both pigments as compared with the wild-type was determined; inPm2 the brown pigment content is equivalent to that of the mutant brown, but the red pigment content is appreciably higher; inHnr,se andcl the content of red pigment, or its modified form, shows a reduction as compared with the wild-type, but the amount of brown pigment is significantly higher. It is calculated that the eyes ofcl contain about 36% of the red pigment found in the wild-type and 84% of the yellow pigment found in sepia.
An attempt is made to indicate the possible modes of action of these genes in eye pigmentation. It is postulated that the normal alleles oflt andpers, like those of the ruby and carnation mutants, are eye-colour genes indirectly only, their main function being the directing of enzyme specificities, especially for the breakdown of proteins during metamorphosis, some of the breakdown residues being utilized in the pigmentary process; the normal allele ofltd seems to play some part during pigment granule production. The action ofPm2 seems to be clue to a position effect of thebw+ locus, its prohibitive effect on red pigment production being less efficient than that of the mutant brown. The normal alleles of Hnr,se andcl seem to fit into the reaction chain for the production of a common basic precursor or constituent which is later differentiated by thew+ gene into specific substrates for the formation of the red and brown chromophore groups. Hnr seems to divert some of this constituent from the red pigment to the brown pigment pathway, whilese andcl seem seem to stand in a sequential relation to each other, their action (in the case of the latter only partly) being to modify the basic precursor in such a way that a modified red pigment results, and in addition a part of this substance destined for the red pigment substrate is diverted to the brown pigment production chain.
Volume 100, 2021
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