Hans Rudolf Kobel

Genetics of polyploid Xenopus

Abstract Polyploidy, in providing duplicate genetic information, might have been an important factor in vertebrate evolution. The African Clawed Frogs of the genus Xenopus , which comprises bisexual species of several different ploidy levels, represent a promising model for an experimental approach to the many problems associated with a polyploid condition.

Chapter 2 Genetics of Xenopus laevis

Publisher Summary This chapter summarizes various approaches to the nuclear genome of X. laevis and discusses the methods and results that represent selected examples. A peculiarity of the genus Xenopus is that, all but 1 of its 16 to 20 species is of polyploid origin, forming a polyploid series in the proportions 2 : 4 : 8 : 12. Xenopus Laevis belongs to the tetraploid class. Evidence for tetraploidy comes from comparisons of DNA content in various species of the family Pipidae and from the fact that, in X. Laevis, a number of genes are represented by two copies, generally of less than 10% sequence divergence. On the other hand, X. Laevis shows several characteristics of a diploid species. Thus, X. Laevis displays all features of an ancient tetraploid species that is completely diploidized. If polyploidy resulted from interspecific hybridization (allopolyploidy), which presumably is the case in Xenopus , diploidization (that is, disomic inheritance) would probably have occurred at once, because interspecific divergence in genome structure could by itself prevent homoeologous chromosome pairing in newly created tetraploids. The architecture and composition of the nuclear genome as a whole, and of particular genes and gene clusters, are analyzed in detail.

The genetic control of histocompatibility reactions in natural and laboratory-made polyploid individuals of the clawed toadXenopus

Family studies inXenopus laevis (2n=36 chromosomes) demonstrate the expression of a single major histocompatibility complex in this species. Mixed leukocyte studies in two families ofXenopus vestitus (2n=72 chromosomes) indicated that this reaction was also under the control of a single genetic region. These studies suggest that, in this polyploid species, the switch from tetrasomic to disomic inheritance has already been accomplished for this locus. In contrast, segregation of mixed leukocyte reaction determinants and patterns of graft rejection in two families ofXenopus ruwenzoriensis (2n=108 chromosomes) were incompatible with the expression of a single major histocompatibility complex, and suggest that polysomic inheritance of this locus is maintained in this species. This interpretation was confirmed by the finding in a sibship of hybrids betweenXenopus ruwenzoriensis andXenopus laevis (2n=54+18) of more than four classes of mixed leukocyte reaction-identical sibs. In laboratory-created triploid animals (trispecies hybrid amongXenopus laevis, Xenopus gilli, andXenopus clivii), mixed leukocyte reaction and grafting experiments demonstrated that the major histocompatibility complex of each constituting species was codominantly expressed.

Hyperdiploid species hybrids for gene mapping in Xenopus.

SPONTANEOUS endo-reduplication in the female germ line of hybrids has made it possible to produce several strains of isogenic Xenopus, providing one of the many features required of a laboratory animal1,2. The usefulness of Xenopus in the laboratory, however, would be improved if a gene map were available. We report here that hybrids can also be used to define linkage groups and their assignment to chromosomes. Our method is based on the possibility of creating aneuploid Xenopus which are diploid for the chromosomes of one species but have supernumerary chromosomes of a second species3. This method complements those already available4–8, and has the advantage that because traits can be monitored in the whole animal, a large repertory of markers will be made available.

A Xenopus laevis creatine kinase isozyme (CK-III/III) expressed preferentially in larval striated muscle : cDNA sequence, developmental expression and subcellular immunolocalization

A cDNA containing the nearly complete coding sequence of CK-III subunit of X. laevis was isolated, sequenced and further identified by comparing the tissue distribution of CK-III/III isozyme with that of its messenger. Comparison of CK-III deduced amino acid sequence with other CK sequences published reveals its close homology to M-CK subunits. Results using both cDNA probes and monoclonal antibodies specific for CK-III subunits indicate that the appearance and the accumulation of CK-III occur in parallel with myoblast differentiation. Moreover, subcellular immuno-histolocalization shows that CK-III/III isozyme is especially concentrated on larval myofibres at the level of A-bands.

Purification and characterization of cytoplasmic creatine kinase isozymes of Xenopus laevis

The soluble creatine kinase isozymes CK-II, CK-III, and CK-IV fromXenopus laevis have been purified to apparent homogeneity and their subunits characterized by means of molecular weight, peptide pattern, and dissociation-reassociation experiments. CK-III and CK-IV are homodimeric isozymes whose subunits are distinct in both molecular weight (42,000 and 41,000, respectively) andStaphylococcus aureus V8 peptide pattern. In dissociation-reassociation experiments, those two subunits do form active heterodimeric isozymes with one another or with rabbit M-CK subunits. Hybrid CK-III/IV isozymes occur also during embryonic differentiation and in adult heart muscle, whereas most other adult tissues contain only homodimeric CK-III or CK-IV isozymes. The CK-II isozyme is a heterodimer composed of one CK-III subunit and another subunit specific to CK-II (Mr=41,000). Neitherin vivo norin vitro does this subunit seem able to form homodimers or heterodimers with CK-IV and rabbit M-CK subunits. If we take into account the apparent association of CK-I isozyme with cellular organelles, these results corroborate earlier statements and suggest that the CK isozyme system ofX. laevis is encoded by at least four differentially regulated genomic loci.