Possible Genetic Difference between the Head Louse and the Body Louse (Pediculus humanus L.)

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Abstract
Alpatov and his collaborators demonstrated that, if the progeny of head lice are kept under environmental conditions approximating those of body lice, they become gradually transformed in several generations, and become indistinguishable from normal body lice. A high mortality, especially of the immature stages, is observed during the early generations of the transformations. This mortality suggests that selection may be involved. The present paper reports a model of a genetic mechanism which might account for the available facts. This model assumes that the louse populations are polymorphic for several pairs of genes with additive phenotypic effects, which produce heterozygotes more highly fit (heterotic) than the corresponding homozygotes. Some of the homozygotes are, however, relatively more fit in the environments in which head lice normally live, and other homozygotes in the body lice environments. A population may, then, be transformed in the direction of either the head louse or of the body louse phenotype, depending upon the conditions of its existence. It is evident that such a genetic plasticity may be highly adaptive in a parasite, which is thus enabled to exploit difficult environmental opportunities. The genetic mechanism suggested is obviously not the only possible one. Another possibility would be possession by the lice of two kinds of cytoplasmic particles, or, symbionts, one of which was relatively more favorable in the head louse environments and the other in the body louse environments. The proportions of the two kinds of symbionts would then be changeable in accordance with the environments in which a population lived.

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