Electrophoretic variation in allelozymes related to function or structure?

Sources of variance in protein heterozygosity: the importance of the species-protein interaction

We report on a detailed survey of protein heterozygosity in Canadian freshwater fish and mammals. A simple one-way analysis showed substantial variance among species. However, a two-way analysis of species and proteins showed that there was little if any variance among species or among proteins, but a very large species-protein interaction. We could not remove this interaction by analysing taxa separately by constructing completely balanced datasets, by eliminating study bias or by excluding monomorphic proteins, and we could not decompose the interaction by classifying enzymes according to their form and function. We conclude that most of the variance in protein heterozygosity is attributable to species-protein interaction. This casts some doubt on the interpretation of comparative studies of mean heterozygosity among species or among proteins. Our result seems inconsistent with the neutral theory of protein variation but not with the differential action of natural selection.

Biochemical correlates of genetic variation in marine lower invertebrates

In this paper extensive data on enzyme variation in 23 species of coelenterates and sponges were used to investigate the possible correlation of levels of genetic variation with various parameters of enzyme molecular structure and function. The data provide an opportunity not only to look for such correlations for the first time in lower invertebrates, but also to study organisms with far higher average levels of genetic variability than those used in any previous work. A clear inverse relationship was found between enzyme subunit number and levels of polymorphism, with monomers being more variable than dimers or tetramers. No significant difference in polymorphism could be found in enzymes of the functional groups I and II of Gillespie and Langley (1974). Regulatory enzymes appeared to be significant relationship was observed between regulatory power and subunit structure which could bias this result. The results suggest that both neutralist and selectionist ideas may have a useful role to play in the understanding of the factors which can influence or limit levels of genetic variation.

Interpreting the behavior of enzymes: purpose or pedigree?

To interpret the growing body of data describing the structural, physical, and chemical behaviors of biological macromolecules, some understanding must be developed to relate these behaviors to the evolutionary processes that created them. Behaviors that are the products of natural selection reflect biological function and offer clues to the underlying chemical principles. Nonselected behaviors reflect historical accident and random drift. This review considers experimental data relevant to distinguishing between nonfunctional and functional behaviors in biological macromolecules. In the first segment, tools are developed for building functional and historical models to explain macromolecular behavior. These tools are then used with recent experimental data to develop a general outline of the relationship between structure, behavior, and natural selection in proteins and nucleic acids. In segments published elsewhere, specific functional and historical models for three properties of enzymes--kinetics, stereospecificity, and specificity for cofactor structures--are examined. Functional models appear most suitable for explaining the kinetic behavior of proteins. A mixture of functional and historical models appears necessary to understand the stereospecificity of enzyme reactions. Specificity for cofactor structures appears best understood in light of purely historical models based on a hypothesis of an early form of life exclusively using RNA catalysis.

Correlative genetic variation in natural populations of cats, mice and men

The study of the extent and basis of gene-enzyme variation has long been a principal concern of population genetics. Numerous surveys have indicated considerable amounts of genetic variation detectable in natural populations, with few exceptions. The variances of average heterozygosities (H) between species and among populations within species are large, prompting Lewontin to emphasize the importance of large gene sample sizes and Selander to encourage analysis of variation of homologous gene-enzyme systems when making species comparisons. We present here a comparative genetic analysis of electrophoretic variation at 57 homologous biochemical loci of cats, mice and men. The distribution of polymorphism among the sampled loci in the three species was nonrandom. A large group of sampled loci (60%) were monomorphic in all three species, whereas a second group (30%) of the loci were polymorphic in two or more species. This conservation of the tolerance of genetic polymorphism is apparently more a characteristic of a particular locus than of the vertebrate species or of the genome. The current hypotheses for classifying polymorphic and monomorphic loci in terms of physiological and physical enzyme characteristics have been re-examined.

[Electrophoretic polymorphism of seric and erythrocytes proteins and enzymes in the chimpanzee (Pan troglodytes) (author's transl)]

The electrophoretic mobilities of serum enzymes and other proteins were investigated at 35 loci in 25 unrelated individuals of the chimpanzee Pan troglodytes. In this population the mean average of individual polymorphism is P = 0.181, and the average level of heterozygoty H = 0.022. The same pattern of individual variability is found in Pan troglodytes and Homo sapiens.

Isozymes and allozymes: alternate forms of protein adaptation?

The hypothesis is put forward that constant or predictable environments will select for multiple locus enzymes (isozymic variation) while variable or nonpredictable environments will select for multiple alleles at single loci (allozymic variation). If this hypothesis is valid, isozymes and allozymes should represent alternative forms of enzyme adaptation and the levels of the two types of variation should be negatively correlated. This prediction was tested from data reported in the literature for five groups of organisms: mammals, other vertebrates, insects, other invertebrates and plants. Single substrate enzymes showed the predicted negative relationship between allozymic and isozymic variation. Multiple substrate enzymes did not show a significant relationship. Isozymic variation was greater for insects, other invertebrates and plants, in multiple substrate enzymes compared to single substrate enzymes. For mammals and vertebrates, allozymic variation for multiple substrate enzymes was approximately double that of single substrate enzymes, while isozymic variation remained constant.