1. 1. Among the 27 species of Amazon fish belonging to the orders Rajiformes, Clupeiformes, Osteoglossiformes, Characiformes, Siluriformes and Perciformes here analyzed, 56% showed an electrophoretic pattern of five, 7% of four, 30% of three, and 7% of two LDH isozymes, suggesting the presence of both LDH-A* and LDH-B* loci. In addition to these loci, the third gene LDH-C* was detected only in the Osteoglossiform species O. bicirrhosum and in the perciform species P. squamosissimus, with a generalized expression in the first and a restricted in the second. 2. 2. Only P. squamosissimus (Perciformes) showed a LDH reversed pattern, in which the A4 is more anodic than the B4. 3. 3. Like other vertebrates, in most (93%) of the species here analyzed, a direct correlation between electrophoretic mobility and thermostability was observed. The inactivation temperatures varied from 55°C in the Rajiformes species of 70°C in the Perciformes species. 4. 4. Polymorphism in at least one of the LDH loci was detected in 22% of the species studied here: P. castelnaena (Clupeiformes) and B.cf. cephalus (Characiformes) at the LDH-A* locus, R. myersi and H. unitaeniatus (both Characiformes) at the LDH-B* and L. agassizi (Characiformes) at both loci. 5. 5. No modifications of the classic LDH pattern found by other authors in organisms routinely subjected to hypoxic stress were observed in these Amazon species. In 93% of the species screened here, subjected to considerable hypoxic stress, large daily oscillations in temperature, O2 and CO2 levels, pH, low ionic content, and seasonal drought, a bidirectional pattern of expression of the LDH loci was observed. © 1991.
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