ALLOZYMIC VARIABILITY IN TOXICITY-TESTING STRAINS OF CERIODAPHNIA-DUBIA AND IN NATURAL-POPULATIONS OF CERIODAPHNIA

Ceriodaphnia dubia Richard used in 7-d chronic toxicity testing from six environmental testing centers were assayed for allozymic variability at 11 enzymatic loci. Ten of the eleven scored loci were monomorphic and homozygous in all six stocks. One stock was homozygous for an alternative electromorph, as compared to the other five, at the glucose-6-phosphate dehydrogenase (G6PDH) locus. Laboratory stocks comprise two morphologically distinguishable forms of Ceriodaphnia dubia that may be environmentally determined. Ceriodaphnia lacustris and Ceriodaphnia reticulata from natural populations showed within- or between-population allozymic variability at 8 of the 11 loci, suggesting Ceriodaphnia possesses genetic variability equivalent to that of other cladocerans. The three species of Ceriodaphnia are closely related, having only one diagnostic electromorph that separates Ceriodaphnia dubia from the other two species. Possible causes of the monomorphism of the laboratory stocks are discussed.