|Thermal adaptation and the properties of phosphoglucose isomerase allozymes from a sea anemone|
Hoffmann, R.J. (1985). Thermal adaptation and the properties of phosphoglucose isomerase allozymes from a sea anemone, in: Gibbs, P.E. (Ed.) Proceedings of the 19th European Marine Biology Symposium, Plymouth, Devon, UK, 16-21 September 1984. pp. 505-514
In: Gibbs, P.E. (Ed.) (1985). Proceedings of the 19th European Marine Biology Symposium, Plymouth, Devon, UK, 16-21 September 1984. Cambridge University Press: Cambridge. ISBN 0-521-30294-3. 541 pp., more
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Populations of the sea anemone Metridium senile from the northeast coast of North America have two alleles that code for the glycolytic enzyme phosphoglucose isomerase (E.C. 126.96.36.199). Allozymes produced by alternate homozygotes differ in their heat stabilities. The locus exhibits clinal variation, with the allele encoding the electrophoretically fast allozyme (Pgif) being nearly fixed in populations south of Cape Cod. The allele for the slow form (Pgi8) increases in frequency as populations are sampled farther to the north, and its frequency reaches an observed maximum of approximately 0.5 in northern Maine and southern Canada, the limits of sampling to date. Paralleling this cline in allele frequency is a thermal gradient in the summertime mean temperature. These patterns suggest the hypothesis that the fast allozyme is adapted to warm temperatures and the slow allele to colder temperatures. Partial support for a thermal selection hypothesis comes from examination of the kinetics of partially purified preparations of the allozymes from the genotype. There are significant differences in activity between forms, with the allozyme from Pgiff homozygotes being about 2.5- to 3-fold higher in specific activity than that from Pgi88 homozygotes. Heterozygotes produce a mixture of enzymes with intermediate activity. In addition, there are significant differences in Km for substrates in both reaction directions. In the gluconeogenic reaction direction, the fast form has slightly higher Km of fructose- 6-phosphate than the slow form at all temperatures from 5-25°c. In the glycolytic direction, the ordering of Km of glucose-6-phosphate is reversed, with the slow form exhibiting the higher values at all temperatures. Vmax/ Km ratios are also significantly different, with Pgiff homozygotes showing higher values at all tested temperatures. The greatest absolute difference in this ratio falls at 25°c, near the maximum temperature experienced in nature. These data suggest a biochemical reason for the predominance of the fast allozyme in warm waters resulting from its superior activity at high environmental temperatures, but they leave unexplained the reason for the rise in frequency of the slow form in colder waters.