a1 Department of Genetics, University of Edinburgh
The relations between the quantity of red eye pigment and related pteridine compounds of Drosophila melanogaster have been studied in a variety of genotypes, which include strains selected for high or low pigment content, various derivatives of these lines and also lines in which one or other of the major autosome pairs were represented by homozygous chromosome pairs, derived by random sampling from the base population and also inbred lines. The quantity of red pigment was defined by the optical density when whole heads were extracted in a suitable solvent, while the pteridines were separated by chromatography and their amounts estimated by means of their characteristic fluorescence.
The evidence from selection, inbreeding and chromosome sampling from the base population demonstrated the presence of substantial genetic variation for pigment content and amounts of related pteridines.
The genetic and biochemical properties of the selected lines differed according to the direction of selection. High lines remained heterozygous after many generations of selection and displayed dominance and epistasis in favour of higher pigment content in crosses to the unselected stock. Selection for low pigment content led to fixation of recessive effects, attributable to particular chromosomes. The dominance-recessive relationship in red pigment differences was also applicable to the associated pteridines.
The metabolic pattern in all lines with reduced pigment content is compatible with the assumption of reduced enzyme activity at particular steps of the pathway leading to the drosopterins (red eye pigments). The two steps accessible to study are subject to genetic variation in the base population, while inbreeding or selection for low pigment content leads to genetically fixed alterations at one or other of these steps. The genetic analysis was consistent with the biochemical evidence.
Increase in pigment content above the normal level, either by selection or chance fixation, is accompanied by correlated increase in all the precursors. Several alternatives are possible but it is suggested that this may be due to an increase in early precursors, before the stages which have been altered in the low pigment lines.
Attention is drawn to the similarity in genetic behaviour between pigment content and body size. Particular emphasis is laid on the value of selection as a means of creating biochemical differences which offer a basis for relating biochemical function and genetic behaviour.
(Received July 16 1969)
* Member of the Agricultural Research Council Unit of Animal Genetics.