Selection of allelic isozyme polymorphisms in marine organisms: pattern, theory, and application

The evolutionary significance of allelic isozyme polymorphisms in several Mediterranean marine organisms was tested initially by post-hoc gene frequency analyses at 11-15 gene loci in natural populations of barnacles, Balanus amphitrite, under thermal [Nevo et al, 1977] and chemical [Nevo et al, 1978] pollutions. We next carried out pre-hoc controlled laboratory experiments to test the effects of heavy metal pollution (Hg, Zn, Cd) on genotypic frequencies of 15 phosphoglucomutase (PGM) genotypes in thousands of individuals of the shrimp Palaemon elegans [Nevo et al, 1980, 1981a, and the present study]. Similarly, we tested the effects of Hg, Zn, Cd, Pb, Cu pollutions on the genotypic and allelic frequencies of five phosphoglucose isomerase (PGI) genotypes in the two close species of marine gastropods, Monodonta turbinata and M turbiformis [Lavie and Nevo, 1982, and the present study]. In both the thermal and chemical pollution studies, we established in repeated experiments statistically significant differences of allele frequencies at 8 out of 11 (73%) and 10 out of 15 (67%) gene loci, respectively, between the contrasting environments in each. While no specific function could be singled out in the post-hoc chemical study due to the complex nature of polluted marine water, temperature could be specified as the primary selective agent in the thermal study. The strongest direct and specific evidence for significant differential survivorship among allelic isozyme genotypes was obtained in the pre-hoc studies in Palaemon and Monodonta. Their differential viability was probably associated with the different degree of heavy metal inhibition uniquely related to each specific pollutant. Furthermore, we demonstrated in the two closely related Monodonta species parallel genotypic differentiation as a response to pollution. Our results are inconsistent with the neutral theory of allelic isozyme polymorphisms and appear to reflect the adaptive nature of the allelic isozyme polymorphisms studied. Allelic isozyme genotypes are sensitive to and vary with the quality and quantity of specific pollutants. Therefore, they can provide precise genetic indicators of the effects of pollution on the short- and long-term genetic changes of populations. Ideally, in different marine species specific genetic loci, either singly or in combination, may prove sensitive markers to different pollutants and could easily be assayed by quick electrophoretic tests and be used as genetic monitors. An extensive search for the appropriate enzymatic systems in various relatively sedentary marine species exposed to pollutants is therefore urgent.