Classical Genetics in Paramecium

Contributed by Bob Hinrichsen, Department of Biology, Indiana University of Pennsylvania

Paramecium has been utilized as a model genetic organism for almost seventy years. This began with the landmark paper describing mating types in Paramecium aurelia, which for the first time allowed crossbreeding to be done in an unicellular organism (Sonneborn, 1937). Since that time, the genetics of Paramecium have been expanded to include numerous species (Preer, 1969).

Each species of Paramecium has a pair of complementary mating types; e.g. Paramecium tetraurelia is comprised of mating types VII (O) and VIII (E). The discovery of complementary mating types allowed for reliable conjugation experiments to take place, leading to the use of Paramecium as a convenient genetic organism. The cytology of the mating process (conjugation) is well established (Beale, 1954). Despite the fact that Paramecium has a single highly polyploid macronucleus and two diploid nuclei, typical Mendelian ratios are obtained. However, starved cells of the P. aurelia complex that are greater than twenty cell divisions from the last nuclear reorganization undergo a process much like conjugation, but with no exchange of nuclei between cells. This process, termed autogamy, causes all loci to become homozygous (Preer, 1968). This is an important event for geneticists because it allows for recessive mutations to be expressed following mutagenesis. This allowed for the isolation of a large number of genetic mutants that have been invaluable to Paramecium research (e.g., see Sonneborn, 1975). A further genetic event that has been utilized in the study of Paramecium is macronuclear regeneration (Preer, 1968). This results from the abortive development of a new macronucleus, and results in a macronucleus with the parental genotype, and the micronuclei with the genotype of the offspring.

Mutagenesis has been accomplished by a number of different mutagens. X-ray was initially used as a mutagen (Kimball, 1949), and g-ray mutagenesis has also been successful (Takahashi, et al., 1985). The most common chemical mutagen is nitrosoguanidine; however, nitrogen mustard, triethylene melamine, among others, has also been used (Sonneborn, 1975). These mutagens have little effect when exposure is during S phase, but their effectiveness rises sharply with exposure during the G1 and G2 phases of the cell cycle. The combination of mutagenesis and subsequent induction of autogamy allows for the rapid selection of recessive mutants.

Furthermore, the dual nature of the macro- and micronuclei in Paramecium offers the opportunity to study a variety of other genetic processes. These include amitosis of the macronucleus (Preer and Preer, 1979), caryonidal inheritance due to the reorganization of macronuclear DNA during development (Amar, 1994), and macronuclear inheritance (Sonneborn and Schneller, 1979;Meyer and Keller, 1996).

Finally, the study of organellar genetics is well established in Paramecium. Mitochondrial inheritance was shown in cells displaying mutations allowing erythromycin, chloramphenicol and spiramycin resistance (Adoutte 1974). In addition, the first example of cytoplasmic inheritance in animals was shown in Paramecium for the symbiotic bacteria (Preer and Preer, 1984).



References

Adoutte A. 1974. Mitochondrial Mutations in Paramecium: Phenotypical Characterization and Recombination. In: The Biogenesis of Mitochondria. Ed. A. Kroon and C. Saccone. Academic Press, New York. pp. 263 – 271.

Amar L. 1994. Chromosome End Formation and Internal Sequence Elimination as Alternative Genomic Rearrangements in the Ciliate Paramecium. J. Mol. Biol. 236: 421 – 426.

Beale G. 1954. The Genetics of Paramecium aurelia. Cambridge University Press, New York.

Kimball R. 1949.The Induction of Mutations in Paramecium aurelia by beta radiation. Genetics 34: 210 – 222.

Meyer E and Keller A. 1992. A Mendelian Mutation Affecting Mating Type Determination also Affects Developmental Genomic Rearrangements in Paramecium tetraurelia. Genetics. 143: 191 – 202.

Preer J. 1968. Genetics of Protozoa. Res. Protozool. 3:133 – 278.

Preer J. and Preer L. 1979. The Size of Macronuclear DNA and its Relationship to Models for Maintaining Genic Balance. J. Proto. 26: 3-26.

Preer J. and Preer L. 1984. Endosymbionts of Protozoa. In: Bergy’s Manual of Systemic Bacteriology Vol I. ed. N.R. Kreig. Williams and Wilkins, Baltimore. Pp. 795 – 811.

Sonneborn TM. 1937. Sex, Sex Inheritance and Sex Determination in P. aurelia. Proc. Nat. Acad. Sci. USA 23: 378-395.

Sonneborn TM. 1975. Paramecium aurelia. In: Handbook of Genetics, Vol. 2. Ed. R. King. Plenum, New York. Pp. 469-594.

Sonneborn TM and Schneller M. 1979. Dev. Genet. 1: 21 – 46.

Takahashi M, Haga N, Hennessey T, Hinrichsen RD, Hara R. 1985. A gamma ray-induced non-excitable membrane mutant in Paramecium caudatum: a behavioral and genetic analysis. Genet. Res. 46:1-10.