are a relatively homogeneous group of animals, probably monophyletic. They have
evolved a rather unique and successful alternative to the way of life of small unicellular
protozoa, on the one hand, and larger multicellular animals, on the other.
It can reasonably be argued that the ciliates should be classified as a separate
phylum of animal-like protists.
Major distinguishing characteristics:
= two morphologically (and functionally) different types of nucleii.
(Evolutionary footnote: Nuclear dimorphism is almost unique to ciliates. One other
group, among the Foraminifera, shows nuclear dimorphism. In this case, the macronuclei
do not divide, but can only be produced from micronuclei. This feature is also found in a primitive group of ciliates known as karyorelictid ciliates.)
Spasmonemes and myonemes
. Contractile organelles, with very fast
contraction energized by calcium ion binding, not by ATP hydrolysis. (Probably related
to contractile protein filaments in dinoflagellate flagella and in rootlets of other
flagella--centrins, caltractins ?. But use as a primary contractile system is unique to ciliates.)
. A special kind of cytoskeleton that provides a scaffolding for the cilia and determines
the form of the body.
Minor distinguishing characteristics
Transverse division, without involvement of centrioles, is useful for distinquishing
from some other organisms covered with flagella, such as Opalina
Usually, but not always (eg Suctoria) covered with cilia.
Usually large -- 100 to 10000 x volume of typical unicells.
Four kinds you should know about
: easy to maintain and demonstrate in classes, so well known. First ciliate to be
used extensively for genetic studies, because its sexual process (conjugation) could
be controlled. P. caudatum
is large enough to be used for electrophysiological studies. Normally feeds on
bacteria, somewhat difficult to culture it in absence of bacteria (axenic culture),
very difficult to culture on chemically defined media.
: easy to grow on chemically defined medium, and therefore has been used for many
biochemical research projects. Now being used for genetics, because methods for
controlling conjugation so that it occurs synchronously in a large population have
been worked out for Tetrahymena thermophilia
.. Relatively easy to isolate the two kinds of nuclei. Smaller than Paramecium
: large, particularly useful for grafting and micromanipulations because morphology
of the infraciliature is revealed by bands of blue pigment.
: contractile stalk is good example of spasmoneme.
: two functionally distinct kinds of nucleii.
is specialized for sexual exchange (no nucleoli)
is specialized for transcription (have nucleoli to package rRNA into ribosomes)
Information flow in these cells is:
Micronuclear DNA --> macronuclear DNA --> RNA --> protein
Micronuclear DNA --> macronuclear DNA --> RNA --> protein
In comparison with multicellular animals, the micronuclear DNA functions analogously
to the DNA of germ cells
that are set aside for sexual reproduction. The macronuclear DNA functions analogously
to the DNA of somatic cells
In typical ciliates, the macronucleus is large and highly polyploid (many copies of
genome) In T. thermophilia
, about 46N. In others, may be several hundred copies. (Macronucleus also contains
The result, and significance, of this is presumably a greater rate of transcription
than would be possible with a single diploid nucleus, and presumably this greater
rate of transcription is essential for the large absolute growth rate of these large,
active cells. However, these ideas are not well tested. But lets consider a hypothetical
A large ciliate may have 10000 x volume, mass of unicell. If its doubling time were
the same, we would expect that its energy metabolism and the rate of DNA->RNA->protein
would also have to be 10000x.
There is a large body of data indicating that the relationship between size and energy
metabolism for similar animals is less than a direct proportionality between metabolism
and size. Instead it is proportional to mass0.75. (see Schmidt-Nielson's Animal Physiology text, for example). The large ciliate
would then be expected to have 1000 times the energy metabolism of a unicell. If
its growth efficiency is similar, its doubling rate would be 0.1x (10x doubling time).
We then might expect that it would have 1000 times as much DNA for DNA->RNA transcription.
These predictions are reasonably close to what is observed.
Micronuclei divide by mitosis, within nuclear envelope, when the ciliate divides.
Their mitosis appears unusual -- the chromosomes do not condense and display themselves
as conveniently as is typical for eukaryotes. However, the results appear to be
typical. (No nucleoli in micronuclei.)
A macronucleus develops from a micronucleus, by a process that obviously must involve
DNA replication, but is not well understood. There is clear evidence that not all
of the DNA sequence information in the micronuclear DNA is found in the macronuclear
DNA. In Tetrahymena
only about 10% is missing, but in some ciliates (hypotrichs such as Stylonicha
) 90-95% appears to be missing.
The macronucleus divides when the animal reproduces asexually, by cell division.
This macronuclear division does not
involve mitosis. The mechanism is unknown, and it is not known how uniform segregation
is achieved. (It appears to be much better than random, but not as perfect as mitosis.)
Surgical removal of the macronucleus kills the cell, at least in all the species where
this experiment has been done. I.e., development of a macronucleus from a micronucleus
can only occur at a particular time after conjugation; there is no permanent capability for regenerating a macronucleus.
Micronuclei participate in sexual exchange (conjugation
Conjugation is described briefly in your text. I will only emphasize some features:
0. In Tetrahymena
conjugation can be induced by starvation and repressed by shaking the cultures.
This allows control and synchronization of conjugation in a population.
1. The purpose of conjugation is not reproduction, or increase in numbers, so calling
it sexual reproduction is probably not a good idea, even though we may not end up
with normal animals until 4 daughters have been produced from a conjugating pair.
2. Conjugation begins with two animals adhering to each other. After apparently
normal meiosis, all but one of the daughter micronuclei degenerate, in each member
of the pair.
(In Paramecium caudatum
and Tetrahymena thermophilia
, which normally have 1 micronucleus, 3 of the 4 meiotic products degenerate. P. aurelia
normally has 2 micronucleii, and 7 of the 8 meiotic products degenerate.)
(This elimination process resembles elimination of meiotic products during oogensis
(Apparently one of the meiotic products becomes localized in a special region
near the cell membrane where it is somehow protected from the action of enzymes that
are responsible for degeneration of the others.)
Then, something happens that is not found in typical multicellular animals.
The surviving (haploid) micronucleii undergo mitotic division, resulting in two identical
haploid micronucleii in each conjugating cell. One of each pair of identical haploid micronucleii then migrates to the other conjugating cell, and nuclear fusion occurs.
(There is a meshwork of microtubules in the region of cell fusion that appears to
be involved in this process, but the details of the mechanism are unknown.) The
two resulting micronucleii in each cell then fuse to produce a diploid micronucleus.
The two conjugating cells will then have diploid micronucleii that are genetically identical.
3. These diploid micronucleii are the source of new macronucleii in the cells resulting
from conjugation. We might expect that all of the daughter cells should have identical
macronucleii, even if they inherit different cytoplasms. But actually its not that simple.
4. Conjugation can only occur between individuals of different mating type. Mating
type can be thought of as a self-recognition system that prevents conjugation between
What is the optimal number of mating types? More types = greater probability that
another individual will be a potential partner, but more than two types requires
perfect symmetry in the conjugation or mating events.
Mating type appears to be established during gene rearrangements that occur during
the development of new macronucleii. This implies that the daughter macronucleii
produced by conjugation may not be identical, even if the micronucleii are identical.
However, the usual result is that the daughter macronucleii all have the same mating
type, but it may not be the same as either parent.
5. Conjugation cannot occur again immediately after conjugation. There appears to
be a maturation process, requiring about 50 cell divisions in Tetrahymena
, that is required before conjugation is possible. In some species, if conjugation
is prevented, the strain ages and dies. The mechanisms of these maturation and aging
processes are unknown. One suggestion is a gradual accumulation of deleterious mutations. One the other hand, there are a few laboratory strains of ciliates, especially
, where the micronucleii have been lost, and the strains survive indefinitely without
conjugation. This implies that the mechanisms for macronuclear division must be