Classical Genetics in Paramecium
Contributed by Bob Hinrichsen, Department of Biology, Indiana University
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
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,
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).
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,
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: Bergys 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.