The Rana pipiens Problem, a Proposed Solution

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Abstract
A hypothesis is given to explain the connection between (1) the temperature adaptation of Rana pipiens populations in localities having different climates, and (2) the abnormalities observed in hybrids between two differently adapted populations. The common cause is presumed to be some sort of adjustment of metabolic pathways in early development. The attainment of this adjustment by a population confers a better adapted embryo but also requires an adjustment of genes so that later development is not too strongly modified. The first adjustment is transmitted to the offspring as a "maternal effect" while the second adjustment is made by the zygote's genome itself. In hybrids the two systems-egg cytoplasm and (hybrid)-zygote genome-are not correctly balanced and abnormal development follows. Evidence is provided that abnormality can be measured by excess or deficiency in the number of notochord cells. A simple model based on this hypothesis correctly predicts the outcome of all the relevant experiments reported to date. Further experiments are proposed to test the hypothesis.

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