Chromosome Constitution and Behavior in Eversporting and Mottling in Drosophila Melanogaster.

Eversporting of somatic characters is found in nature in many plants and animals, in the form of variegated leaf, flower, or fruit color in plants, piebald coat color in animals, et cetera. Since every cell in an adult plant or animal is derived from a single germ cell by a process of cell division in which equal distribution of chromatin is supposed to take place, it is difficult to see how two adjacent cells in the petal of a flower, for example, can assume different colors. Two possibilities exist :-either (1) the genetic constitution is stable and differences are due to environmental variations; or (2) genetic constitution is unstable. Unstable genetic constitution has been suggested as responsible for several cases of eversporting. Variegated pericarp in Zea mays, (EMERSON 1917), variegated flower color in Delphinium ajacis, (DEMEREC 1931), variable wing shape and eye color in Drosophila virilis (DEMEREC 1926, 1927), have all been attributed to unstable genes, genes which mutate with a definite frequency. Eversporting eye color in Drosophila melanogaster is a common result of X-ray treatment. One case (PATTERSON 1932a) has been attributed to unstable genetic constitution, not an unstable gene in this case, however, but an unstable translocation of several genes to an abnormal position. Many other cases remain unexplained. Most of them involve chromosome translocations near one of two loci,-either the white region of the X chromosome, or the plexus to speck region of the second chromosome (MULLER 1930, VAN ATTA 1932). Factors of the latter type are dominant to normal red and produce a recessive lethal effect. Factors of theother type are recessive and require the presence of heterozygous white for somatic expression in females. These factors are usually lethal to males and when males do survive they are usually sterile. PATTERSON’S case mentioned above belongs in this class of sex-linked mottled-eye colors. A third