Induced melanism in lepidoptera

Abstract

In two previous papers (Harrison and Garrett, 1926; and Harrison, 1928) the experimental production of melanic varieties of the moth Selenia bilunaria , Esper, was reported, following upon the feeding of the larvæ upon hawthorn leaves which had absorbed small quantities of lead and manganese salts respectively. These induced melanic forms were inherited in subsequent generations as recessives on ordinary mendelian lines. The production of these melanics was associated by Harrison with the occurrence of melanics in various species of Lepidoptera in industrial areas both in England and Germany where it might be expected that the food plants would become contaminated with metallic fumes. The production of a variation so striking and heritable in what had hitherto appeared to be a fairly stable species seemed to call for further experimental investigation, especially as a natural melanic in the species had been reported (Mansbridge, 1927-28). While it is known that certain violent disturbances of the organism, as by treatment by X-rays, may induce “mutations” which are heritable, it is always difficult to be sure that the species in nature does not contain an occasional recessive gene which escapes observation because of the extreme rarity of the matings which would result in the emergence of the recessive phenotype. A fresh series of experiments was therefore initiated at the John Innes Horticultural Institution in order to ascertain if melanic variations could be induced with any regularity by the method indicated by Harrison. In any case it was difficult to understand that manganese should be a casual factor, in view of the invariable presence of manganese in plant tissues and the absence of evidence that the proportion is significantly greater in leaves in industrial areas.