Antibody diversity, as measured by isoelectric focusing of dinitrophenol-specific antibodies, was compared in different polyploid species of the clawed toad Xenopus. Antibody heterogeneity increased with chromosome number and DNA content from Xenopus tropicalis (2n=20 chromosomes) to Xenopus ruwenzoriensis (2n=108 chromosomes). Laboratory allopolyploids made by hybridization between two species showing different antibody diversities and different chromosome numbers gave antibody patterns intermediate between the two parents. On the other hand, autopolyploid individuals showed no increase in antibody diversity, showing that increased polyploidy alone cannot be responsible for increased heterogeneity. In contrast to the increase in antibody diversity following polyploidization, the number of expressed major histocompatibility complex alleles, as measured by a mixed lymphocyte reaction, did not increase. This locus appeared to be diploid or in the process of rediploidization in all the Xenopus species studied. Selection has thus operated differentially on the polyploid immunoglobulin and major histocompatibility loci. It apparently preserved the additional heterogeneity acquired for immunoglobulins favoring the expression of an expanded antibody repertoire in polyploid species.
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