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Structural and regulatory variation of phosphoglucomutase in rainbow trout
Abstract:
The results of inheritance experiments with allozymic variants indicate that two pairs of duplicate loci encode PGM in rainbow trout. Two of the loci (Pgm-3,4) share five electromorphs and are highly polymorphic. The large number of phenotypes and instability of these isozymes make them difficult to score in population studies. The other pair of duplicate loci (Pgm1 and Pgm2) have diverged both structurally and in their patterns of tissue-specific expression. We have detected four electromorphs at Pgm2; this locus is expressed approximately equally in all tissues examined. Two electromorphs and a null allele have been detected at Pgm1. PGM1 activity is greatest in skeletal muscle, heart, and brain; only weak activity, if any, is detectable in liver, eye, stomach, and kidney. Ten percent of the trout from the Arlee strain have a greater than 100-fold increase in the expression of Pgm1 in the liver but have normal expression of this locus in other tissues. Results of genetic crosses are consistent with a single regulatory gene (Pgm1-t) with additive inheritance being responsible for the differences in liver PGM1 activity. The allele responsible for the expression PGM1 in the liver is rare in rainbow trout and is apparently a recent mutation. The presence of PGM1 liver activity has a variety of phenotypic effects that are likely to be of adaptive significance. Embryos with liver PGM1 activity develop more quickly than their full-sibs lacking activity. This difference apparently results from increased flux through glycolysis in embryos with liver PGM1 activity while they are dependent on the yolk for energy. The more rapidly developing individuals begin exogenous feeding earlier and obtain a size advantage that is maintained until sexual maturity. This size advantage also produces a tendency for earlier age of first sexual maturity. Fish with liver PGM1 activity are also more developmentally buffered, as indicated by less fluctuating asymmetry of five meristic traits.