Comprehensive immunogenetics / W.H. Hildemann, E.A. Clark, R.L. Raison.

  • Hildemann, W. H., 1927-
Date:
[1981], ©1981
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Comprehensive immunogenetics / W.H. Hildemann, E.A. Clark, R.L. Raison. Source: Wellcome Collection.

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    72 Microbial Antigens and Serotype Patterns of chromosomes. Although the phenotype is determined by the genes in the macronucleus, macronuclei in successive sexual generations are derived anew from germinal micronuclei. There is reciprocal fertilization during sexual con¬ jugation which yields exconjugants (i.e., a pair of organisms after conjugation) containing identical diploid nuclei derived by fusion of haploid nuclei from each mate. Thus, the genotypes of any two exconjugant clones are identical, although such cells may be heterozygous at many loci. A variable exchange of cytoplasmic constituents also occurs during conjugation. The essential features of conjugation are illustrated in Figure 3-7. A self-fertilization process, known as autogamy, involving meiosis usually occurs at intervals after many asexual generations. Autogamy is the internal fusion of two genetically identical hap¬ loid nuclei and each exautogamous clone is found to be completely homozy¬ gous. Although ciliates possess unusual life cycles from a cytogenetic standpoint, nearly all of their traits appear to be inherited in conventional Mendelian fashion. It should be noted that even dominant mutations occur¬ ring in the polyploid macronucleus may not be revealed amid the mass of un¬ changed genomes. On the other hand, mutations in micronuclear genes may be detected after one or two sexual generations. Perhaps most remarkably, the breeding relations in ciliates involve systems of mating types or multiple sexes. Different stocks or clones, i.e., progenies of single individuals, may be assigned to groups such that all stocks of the same group interbreed freely while different groups of stocks are incompatible and do not mate successfully. Each variety or syngen is represented by diverse clones and contains one or usually two mating types that will agglutinate and conjugate when mixed. Thus, conjugation requires contact between two indi¬ viduals of complementary mating types, but the mode of recognition of com¬ plementarity is not known. In recent studies, the terms clone and syngen have been used in preference to stock and variety, respectively. The differentiation of mating type systems shows remarkable similarities to the serotype systems of ciliary antigens, which may now be considered in some detail. The extensive polymorphism of ciliary antigens in P. aurelia and Tetrahymena pyriformis provides scope for increased understanding of gene-antigen relation¬ ships as well as nuclear-cytoplasmic interactions. Each individual of any one clone has the macronuclear genetic potential for the expression of a series of alternative ciliary antigens or serotypes. Ciliates of each serotype may be im¬ mobilized by dilute homologous antiserum, but not by antiserum against other serotypes. The antiserum is usually prepared by repeated immunization of a rabbit. The serotype specificity resides in a single, water soluble protein lo¬ calized primarily in the cilia and body wall. Molecular weight determinations of immobilization antigens from several different serotypes indicate they are proteins of 310,000 to 320,000 daltons. After appropriate isolation, immobi¬ lization antigens may also be compared by immunodiffusion tests in agar gels. Although multiple alleles at many loci are associated with diverse immobi¬ lization antigens in P. aurelia, each antigen is usually referable to a single gene or cistron. Normally only one of these antigens is detectably present at any