Transferrin Polymorphism in Pigeons
<p>Transferrin polymorphism, detected by differences in electrophoretic mobility among allelic forms, is widespread in vertebrates. In pigeons, two alleles have been reported. In a developmental study, I have shown that for a period following hatching the young bird's transferrin phenotyp...
| Summary: | <p>Transferrin polymorphism, detected by differences in electrophoretic mobility among allelic forms, is widespread in vertebrates. In pigeons, two alleles have been reported. In a developmental study, I have shown that for a period following hatching the young bird's transferrin phenotype reflects the maternal rather than its own genotype, although the squab is actively synthesizing transferrin in its liver. This probably reflects transfer of maternally derived protein through the egg. As the period of maternal transfer corresponds to the period of immunoincompetence on the part of the squab, the transferrin is known to be bacterio-and fungistatic, I investigated the possibility of differences in the funistatic effects of the three transferrin phenotypes, using yeast as an assay organism. I found that transferrin derived from heterozygotes is much more effective in the inhibition of yeast growth ·than that from homozygotes. Under the breeding conditions in our flock, embryonic mortality was significantly lower among the progeny of heterozygous females. This suggests that transferrin polymorphism is maintained by selection against the progeny of homozygous females. An algebraic consideration of this form of selection leads to the prediction that a population at equilibrium for allele frequencies would be in Hardy-Weinberg equilibrium for phenotype frequencies. The published data of others, in addition to my own, show that real pigeon populations are in Hardy-Weinberg equilibrium.</p>
<p>Transferrin was purified (> 95% pure) from all three phenotypes; all had similar amino acid compositions and molecular weights. Peptide mapping of the allele products revealed a difference in one peptide, which on analysis was consistent with a single amino acid substitution (Ser → Asp). Mixtures of homozygous type transferrins, while appearing electrophoretically identical to the heterozygous type, do not equal its behavior in yeast inhibition. This suggests the presence of "hybrid molecules" synthesized in heterozygotes. I found that pigeon transferrins can be dissociated to 40,000 M.W. fragments by heating in sodium dodecyl sulfate (SDS). Removal of SDS results in reassociation into 80,000 M.W. dimers. All preparations show only a single amino terminal (alanine). CNBr cleavage produces only four fragments in spite of the fact that there are eight methionines. Sequence data may indicate multiple amino acid sequences. Quantitative hydrozanalysis yields only a single carboxyl terminal (serine). These data suggest that pigeon transferrin may be a dimer.</p> |
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