Species identification in protozoa: glucosephosphate isomerase variation in the Paramecium aurelia group

Results are presented for intra- and interspecies variation in electrophoretic mobility of the enzyme glucosephosphate isomerase in the Paramecium aurelia species complex. Three new observations have been made: (1) the hitherto indistinguishable species 1 and 5 can be distinguished on the basis of GPI electrophoretic mobility, (2) the degree of intraspecies variation is much higher for GPI than for the previously studied mitochondrial dehydrogenases and esterases, and (3) several of the enzymatic variants observed in one species are apparently indistinguishable from some found in other species. The intraspecies variants found have been shown to be allelic, and, on the basis of the enzyme patterns of the heterozygotes, it is proposed that GPI is a dimeric enzyme determined by two loci. In view of the use of enzyme variation as a means of species identification in protozoa, these results suggest that the use of such methods can lead to underestimating the number of species and possibly to misclassification. The implications of these findings together with the results obtained with Tetrahymena are discussed.