GENETIC MODELS FOR DEVELOPMENTAL HOMEOSTASIS : HISTORICAL PERSPECTIVES

Three major models have appeared in the literature for the genetic mechanisms giving developmental homeostasis. A model based on the contributions of James F. Crow and Herman J. Muller states that Darwinian fitness (which includes developmental homeostasis) results primarily from the additive action of dominant alleles at various chromosomal loci. According to the Crow-Muller model overdominance plays a minor role, if any, for Darwinian fitness. I. Michael Lerner proposed a more elaborate genetic mechanism for developmental homeostasis consisting of (1) coadapted heterozygosity in complex polygenic systems and at a limited number of other loci, (2) coadapted homozygosity, and (3) coadapted interlocus interactions of alleles at loci in homologous and non-homologous chromosomes. A uniqueness of Lerner's model is his proposal that segregants of some of the coadapted highly heterozygous polygenic systems are phenodeviants. Although emphasizing the role of heterozygosity in some Mendelian populations, Lerner stated that no population can afford to have too many loci manifesting overdominance simultaneously. Unfortunately, Lerner's views on the importance of heterozygosity for developmental homeostasis are often represented incorrectly in the literature. While supporting the neoDarwinian view that homozygosity for specific alleles, combinations of alleles, and interlocus interactions of alleles are the essence of Darwinian fitness, Theodosius Dobzhansky in 1950 became a strong proponent of the additional importance of coadapted heterozygosity in the evolutionary process. A few years later, however, he became a spokesperson for the hypothesis that heterozygosity for many genes and gene complexes may produce higher fitness even without prior coadaptation. There is little evidence at present to support Dobzhansky's model for the importance of generalized overdominance in Mendelian populations, and there is no unequivocal evidence to rule against the Crow-Muller model. Lerner's model has not been fully tested. Answers are needed to the following questions to help decide between the Crow-Muller model and Lerner's model: (1) How often does overdominance occur in diploid species? (2) Do complex polygenic systems occur in Mendelian populations, maintained by heterozygote advantage, that have phenodeviants as segregants? (3) What is the true relationship between homozygosity in Mendelian populations and the presence of developmental instability? Creative research is needed to find answers 3 3 to the questions.

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