Enzyme analyses of natural populations ofSchistocephalus solidusandLigula intestinalis

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Abstract
Results are reported of enzyme analyses, following isoelectric focusing (IEF) in polyacrylamide gels, of Plerocercoids of the pseudophyllidean cestodesLigula intestinalisandSchistocephalus solidus. No polymorphic variants were detectable for the enzymes lactate dehydrogenase (LDH), malate dehydrogenase(MDH), glucose phosphate isomerase (GPI) or phosphoglucomutase (PGM) in 34 individuals ofS. solidus. Similarly, no variants were observed in LDH, MDH or GPI of 159 individuals ofL. intestinaliscollected from four cyprinid fish species from several locations in southern England. In contrast, PGM ofL. intestinalisis Polymorphic. The enzyme appears to be controlled by three loci and one of these loci, designated PGM-2, is Polymorphic with three recognizable phenotypes. The polymorphism is not related to the species orgeographical origin of infected fish and it does not reflect strain variation inL. intestinalis. Instead, it is typical of a genetic polymorphism under the control of two co-dominant alleles. Such a balanced polymorphism requires that cross-fertilization must occur, at least transiently, inL. intestinalis. The work indicates that enzymes can be used as markers in future genetic studies with cestodes, and that the combination of IEF and zymogram analysis represents an important method for the detection of cross-fertilization in these worms.