Abstract.-Northeastern North American populations of the sea anemone Metridium senile show marked differences in levels of genotypic diversity. Comparisons with expectations generated by computer simulation show that some populations are genotypically as diverse as expected for sexually reproducing populations with free recombination, whereas others are significantly less diverse than expected, despite efforts to avoid collecting clonemates. These reductions in diversity are not attributable to the Wahlund effect; they probably result from extensive clonal reproduction. Reduced genotypic diversity may be produced by low rates of recruitment of planktonic larvae, followed by asexual proliferation. The resulting founder effect may account for previously documented random allele-frequency variation between adjacent populations. It is presently uncertain whether the few genotypes found in some populations are particularly well-adapted to local conditions.
We tested the hypothesis that kinetic differences among allelic variants of glucose--phosphate isomerase (GPI; D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9) from the sea anemone Metridium senile differentially modulate glucose metabolism at the glycolysis,-pentose-shunt branch point. Fractional contribution of pentose shunt and absolute flux of glucose in glycolysis were measured in fasted or fed anemones acclimated to 50C or 15°C. When fed, anemones of genotype Gpiss routed a greater fraction of glucose through the shunt than did Gpifanemones; the effect was more pronounced at 5°C than at 15TC. This confirms predictions from kinetic and population data and is consistent with thermal selection maintaining the variation. Relative levels of shunt metabolism increased at 5°C, compared with 15°C, in fed anemones regardless of genotype, but the proportion of glucose metabolized by the pentose shunt was unchanged by temperature in fasted anemones. Glucose flux through the shunt was constant at approximately 5 pmol mg-4ihr'1 in fed anemones at 50C and 15°C and in fasted anemones at 15°C, indicating apparently near-perfect thermal acclimation of the absolute flux of glucose through the shunt in fed, but not in fasted, anemones. Rates of glucose oxidation and flux through the shunt in freshly collected anemones were similar to those ofanemones fed and acclimated at 150C in the laboratory. If these differences affect organismallevel processes, Gpi variation could contribute to Darwinian fitness in thermally varying environments.A central issue in evolutionary biology is the elucidation of mechanisms by which genetic variation might lead to differences in fitness among individuals in a population. Many studies have shown on the basis of in vitro kinetics that functional differences may exist among allelic enzyme variants (allozymes) (1)(2)(3)(4)(5)(6)(7)(8). In several important cases, this biochemical approach has been extended to include organismal-level correlates of genetic variation. Physiological and behavioral differences among individuals have been accurately predicted from kinetic differences among enzyme variants, and persuasive arguments have been made that natural selection acts ultimately upon organismal-level differences deriving from kinetic differences among allozymes (9-15).Recent theoretical considerations suggest that variation in properties of any single enzyme in a pathway may have little effect on pathway flux (16, 17), a view supported by work with alcohol dehydrogenase mutants in Drosophila melanogaster (18). Nonetheless, a few studies show that allozyme variation may influence metabolism. For example, variants of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase alter pentose-shunt metabolism in Drosophila (19), and rates of alanine accumulation during osmotic stress are influenced by activity variants ofglutamicpyruvic transaminase (alanine aminotransferase) in a copepod, Tigriopus californicus (20). Preliminary data suggest that allozymes of glucose-6-phosphate i...
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