Mendelian Genes and Rates of Development in Gammarus Chevreuxi
Authors: Ford E. B. , Huxley J. S. 
Published: April 2021

ABSTRACT
In the present paper the following facts and theoretical considerations have been brought forward:
(1) All the mutations so far examined which influence the colour of the facets (as opposed to total absences of black and red pigment) in the eyes of Gammarus chevreuxi appear to act by modifying the time relationships governing the deposition of melanin. (2) All coloured eyes are at first colourless, and then become scarlet owing to the formation of a red pigment soluble in alcohol; later they may darken by the deposition of melanin. (3) In the normal black eye (RR), melanin begins to be deposited about 2 days before hatching (at 23° C.); its deposition is very rapid, and complete saturation, giving a dense black colour, is reached at or about extrusion. (4) In red (rr) eyes the time of onset and the rate of melanin formation is always slower. The rate may be modified by a factor “s” slowing it down considerably, and a factor “m” slowing it down to a less degree. (5) At 23° C. rrSSMM eyes begin to darken at about 4 to 6 days after extrusion, and reach an equilibrium position (of a deep chocolate colour) in about 3 weeks. (6) Under the same conditions, rrssMM eyes begin to darken at about 4 to 8 days, and reach an equilibrium (of a lighter chocolate shade) in about 5 to 7 weeks. The degree of phenotypic divergence between the colours of rrSS and rrss is at first nil; it then increases to a maximum at about 212 to 3 weeks from extrusion, after which it diminishes once more. It is, however, not completely obliterated in the adult. (7) In addition a factor “d” has been found which delays the onset of rapid pigment development until sexual maturity. The rate of subsequent darkening is that characteristic of the other factors present. (8) Other agencies also influence melanin formation. In rr eyes melanin is formed rapidly at 23° C. (above which temperature the stocks are unhealthy), less rapidly at 20° C., and only to a slight extent, beginning after about four months, at or below 14° C. (9) In RR (wild type and black-no-white) eyes, however, low temperatures merely cause a slight delay in melanin formation. (10) The area of the eye also influences the colour. The eye facets continue to increase in size after the melanin has attained its equilibrium position. Thus the same amount of pigment is spread over a greater area, and the eye appears slightly paler. This again does not apply to RR eyes, presumably owing to the large quantity of black pigment formed. New facets are added so long as growth continues. Since they behave as distinct units in pigment formation, those which have appeared most recently will be bright red while the older ones are already blackish. (11) Other instances are given of genes which appear to control rates of development in various animals. (12) It is suggested that the effects of multiple allelomorph series (e.g. for eye-colour in Drosophila) may represent a cross-section through a series of developmental curves of one and the same substance, the curves differing as regards rates of formation of the substance, times of onset of deposition, and final level of the equilibrium position obtained.