THE GENIC CONTROL OF MATING TYPES IN PARAMECIUM BURSARIA

Time-course analysis of nuclear events during conjugation in the marine ciliate Euplotes vannus and comparison with other ciliates (Protozoa, Ciliophora)

Ciliates represent a morphologically and genetically distinct group of single-celled eukaryotes that segregate germline and somatic functions into two types of nuclei and exhibit complex cytogenetic events during the sexual process of conjugation, which is under the control of the so-called "mating type systems". Studying conjugation in ciliates may provide insight into our understanding of the origins and evolution of sex and fertilization. In the present work, we studied in detail the sexual process of conjugation using the model species Euplotes vannus, and compared these nuclear events with those occurring in other ciliates. Our results indicate that in E. vannus: 1) conjugation requires about 75 hours to complete: the longest step is the development of the new macronucleus (ca. 64h), followed by the nuclear division of meiosis I (5h); the mitotic divisions usually take only 2h; 2) there are three prezygotic divisions (mitosis and meiosis I and II), and two of the eight resulting nuclei become pronuclei; 3) after the exchange and fusion of the pronuclei, two postzygotic divisions occur; two of the four products differentiate into the new micronucleus and macronucleus, respectively, and the parental macronucleus degenerates completely; 4) comparison of the nuclear events during conjugation in different ciliates reveals that there are generally three prezygotic divisions while the number of postzygotic divisions is highly variable. These results can serve as reference to investigate the mating type system operating in this species and to analyze genes involved in the different steps of the sexual process.

Identification of Paramecium bursaria syngens through molecular markers--comparative analysis of three loci in the nuclear and mitochondrial DNA

This is the first attempt to resolve the phylogenetic relationship between different syngens of Paramecium bursaria and to investigate at a molecular level the intraspecific differentiation of strains originating from very distant geographical locations. Herein we introduce a new collection of five P. bursaria syngens maintained at St Petersburg State University, as the international collection of syngens was lost in the 1960s. To analyze the degree of speciation within Paramecium bursaria, we examined 26 strains belonging to five different syngens from distant and geographically isolated localities using rDNA (ITS1-5.8S-ITS2-5'LSU) fragments, mitochondrial cytochrome c oxidase subunit I (COI), and H4 gene fragments. It was shown that P. bursaria strains of the same syngens cluster together in all three inferred molecular phylogenies. The genetic diversity among the studied P. bursaria strains based on rDNA sequences was rather low. The COI divergence of Paramecium bursaria was also definitely lower than that observed in the Paramecium aurelia complex. The nucleotide sequences of the H4 gene analyzed in the present study indicate the extent of genetic differences between the syngens of Paramecium bursaria. Our study demonstrates the diagnostic value of molecular markers, which are important tools in the identification of Paramecium bursaria syngens.

New results on the genetics of mating types in Paramecium bursaria

1. The four mating-types I, II, III, IV known for P. bursaria, syngen 1 are determined by specific combinations of dominant and recessive alleles at two unlinked loci. Mating-type I is formed by cells with the genotypes AABB, AABb, AaBB and AaBb; type II is controlled by the genotypes aaBB and aaBb; the double recessive, aabb is type III, genotypes AAbb and Aabb bring about type IV.

2. Clones which are exceptions to these rules for mating-type determination occur in low frequency (2–3%) in one line of descent. All can be explained by assuming either a mutation of dominant gene B to its recessive allele b or to a loss of the B locus. The data suggest that in four clones, the macronuclear but not the micronuclear genotype is affected; in two clones both nuclei are aberrant.

3. The cytogenetic events of conjugation were verified by the use of the new genetic markers. No evidence for uniparental nuclear reorganization, autogamy, was found.

The mating-type substances of Paramecium bursaria

The detached cilia from sexually reactive cells of Paramecium bursaria will agglutinate with the cilia of intact sexually reactive animals of a complementary mating type. No such reaction will occur if incompetent cells are used or if the cells are of the same mating type. Particulates other than cilia do not adhere to tester cells; the cilia which carry the specific mating-type substances are restricted to the ventro-lateral surface of the animal.

Studies of the heat inactivation of the ability of detached cilia to agglutinate confirm in detail the hypothesis of Metz which holds that cell unions leading to conjugation are brought about by the interaction of two pairs of complementary substances, A and a and B and b; the former pair of substances is more heat labile than the latter. The data suggest that animals of each of the four mating types carry a unique combination of two substances, namely, AB, aB, ab, and Ab.

A protein called immaturin controlling sexual immaturity in Paramecium

As in many metazoans, clones of some species of Paramecium have, after conjugation, a period of immaturity during which the cells cannot mate. The duration of immaturity is measured by the number of cell generations, which remains fairly constant, although duration in time varies with rate of cell reproduction. Genic involvement is shown by mutants with reduced periods of immaturity. In three different groups of Paramecium species, the cytoplasm of immature cells apparently contains the same substance which represses mating activity when injected into sexually mature cells. The immaturity-inducing substance seems to be absent from sexually mature cells, as brei made not only from mature cells in the stationary phase (mating-reactive cells), but also from those in the log phase (mating-non-reactive cells), does not repress mating activity when injected into mature cells. Variations in the amount of the substance during immaturity suggest that it controls the duration of the period. We have isolated and partially characterized the substance-a single protein called immaturin. The activity of immaturin is dose dependent and associated with a heat-labile protein of molecular weight (MW) 10,000.

Perturbance analysis of nuclear determination in Tetrahymena I: background, rationale, and illustrative example, employing temperature responses

A model based on comparative considerations, is presented for the nuclear determination of mating type in Tetrahymena thermophila. The model proposes a system of three binary control elements, each capable of stable persistence in one of two states. A general method is proposed for evaluating the model and for assigning particular mating types to particular compound states. Preliminary assignments of mating types are made from the responses of nuclei to temperature differentials.

The genetics of mating type in the suctorian Tokophrya lemnarum

The two clones produced by the two members of a conjugating pair are normally of the same mating type, about 50% of such synclones being mating type I, 50% type II. This ratio suggests (1) that mating type is determined by a pair of alleles at one locus or by a pair of mating type determining chromosomes, and (2) that one mating type is homozygous, the other heterozygous or hemizygous. Mating type ratios are unaffected by temperature or parental age. Exceptions are of three types: (1) mates that produce one clone of type I, the other of type II, (2) mates which fuse permanently and yield a clone pure for either mating I or mating type II, and (3) conjugation of three mates one of which is regularly non-viable, the two survivors producing either two clones of type I, two clones of type II or one of each type. The bases of these exceptional results remain obscure.