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STABILIZING AND DISRUPTIVE SELECTION ON A MUTANT CHARACTER IN DROSOPHILA. II. POLYMORPHISM CAUSED BY A GENETICAL SWITCH MECHANISM

1970; Oxford University Press; Volume: 65; Issue: 4 Linguagem: Inglês

10.1093/genetics/65.4.681

1943-2631

W. Scharloo,

Neurobiology and Insect Physiology Research

T H E occurrence of two or more distinct phenotypes within a population is an important device for adaptation to a heterogeneous environment.These differences can be found on different phenotypic levels, e.g., enzymes or morphological characters.Morphological characters are under the control of many genes.Discontinuity in the variability of a morphological character can only be brought about by some kind of switch mechanism that steers development into one of alternative pathways.Much attention has been given to genetical switch mechanisms (FORD 1965) involving genes (or supergenes) with large effects on some character.In addition, the possibility exists that a switch mechanism is a property of the developmental system of the character.A threshold mechanism can determine that minor genetical or environmental differences are sufficient to cause the appearance of alternative phenotypes.MATHER ( 1955) suggested that disruptive selection could generate polymorphism.In the first paper of this series (SCHARLOO, HOOGMOED and TER KUILE 1967) it was shown that artificial disruptive selection causes polymorphism.Under this type of selection the initially approximately normal distribution of the length of the fourth longitudinal wing vein of the mutant cubitus interruptus dominant of GLOOR became changed into a distribution with two modes which were clearly separated.But the mechanisms by which this happened were different under the two systems of disruptive selection, viz., disruptive selection with random mating of the selected flies (DR) versus disruptive selection with compulsory mating of opposite extremes (D-) .In the D-line the increase in phenotypic variance was caused by an increase of all variance components, viz., genetic variance, environmental variance, and within-fly variance.In the accompanying paper (SCHARLOO 1970) it is shown that the polymorphism in the Dline was produced by a developmental switch mechanism.In contrast, the increase in phenotypic variance in the DR selection line was caused by an increase in genetic variance only.The difference between the extreme flies of the DK line must therefore be a genetic difference.The genetic analysis presented here shows that the polymorphism was governed by a genetical switch mechanism located on the second chromosome.