Temperature-dependent kinetic variation among phosphoglucose isomerase allozymes from the wing-polymorphic water strider, Limnoporus canaliculatus |
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Authors: | Zera AJ |
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Institution: | Department of Ecology and Evolution, State University of New York, Stony Brook. |
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Abstract: | Phosphoglucose isomerase (PGI) allozymes were isolated from the wing-
polymorphic water strider, Limnoporus canaliculatus, and were characterized
biochemically with respect to temperature-dependent kinetic and
thermostability properties. At higher temperatures, the allozymes exhibited
significant differences in Michaelis constant (Km) values for substrates of
both the forward and reverse reaction directions. Results were consistent
with expectations of adaptive kinetic differentiation based on the
latitudinal variation of PGI allele frequencies. PGI genotypes also
differed with regard to maximal velocity (Vmax)/Km ratios at higher
temperatures. These differences were due primarily, if not exclusively, to
allozyme-dependent variation in Km values. The allozymes also exhibited
dramatic differences in thermostability. However, no thermostability
differences were observed when the substrate analogue 6-phosphogluconate
was present in the incubation medium. The data from this study, together
with data from Mytilus edulis and Metridium senile on temperature-dependent
kinetic variation among PGI allozymes, form a consistent picture of natural
selection influencing the clinal variation of alleles at this locus in
these three phylogenetically distant organisms. More definitive support of
this hypothesis, however, must await additional studies on the
physiological effects of the allozymic variation as well as direct
measurements of fitness differences among the enzyme genotypes.
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