Biology Reference
In-Depth Information
CI systems are usually associated with means of horizontal transmission but have sometimes
made their way (back) onto the chromosome and perhaps into the nucleus, so that there now exists
a continuum of addiction systems ranging from plasmid systems, bacterial chromosomal systems,
mitochondrial, and, perhaps, chloroplast systems in plants, chromosomal invertebrate systems, and
Wolbachia
-based arthropod systems.
Table 14.1
provides an overview. The plasmid systems are the
best-studied systems at the molecular level, whereas
-induced CI is the most prominently
investigated system at the phenotypic and population levels. CI systems are also known as
toxinÏantitoxin, poisonÏantidote, and modiÝcationÏrescue systems. When the cytoplasmic factors
are genes, they are also described as cytoplasmic genes, extrachromosomal genes, extranuclear
genes, plasmagenes, or plasmons. These genes may be part of plasmids, viruses, endosymbionts,
organelles, or chromosomes. Because cytoplasm is almost universally inherited through the female
sex only, CI can be nuclear coded as long as the respective factors are expressed in the cytoplasm
of the egg cell. This inevitable and exclusive association with the maternal cytoplasm during
reproduction predestines these systems to manipulate the sex ratio of their hosts.
Wolbachia
I
S
NTRACELLULAR
YSTEMS
Intracellular systems have evolved in at least three very disparate backgrounds. Intracellular addic-
tion systems are very widespread in bacteria and presumably part of the great majority of plasmids,
especially low-copy-number plasmids. They have also been described for plasmids of archaea,
suggesting that the pairing of toxin and antitoxin in a functional cassette can be regarded as a very
basic and successful evolutionary invention. A nuclear-coded cytoplasmic modiÝcation-and-rescue
system called
has been detected in wild populations of a beetle species. So far it is unique
in invertebrates, but very likely this only reÞects our ignorance of invertebrate natural history.
Incompatibility systems become obvious only in crosses between different populations. However,
only in exceptional cases are different wild populations reared in the same lab and then mated with
each other. For this simple reason most CI systems will escape detection. Host-based CI seems to
be rare in invertebrates, but a unique system of cytoplasmic male sterility is common in plants.
With the exception of a possible mutation in an inbred laboratory mouse strain called
Medea
scat
, CI
+
systems have so far not been described in vertebrates.
Addiction Systems
The two essential components of intracellular systems in prokaryotes are a stable, long-lived toxin,
poison, or modiÝcation factor expressed at low levels and a labile, short-lived antitoxin, antidote,
or rescue factor expressed at high levels. The survival of the plasmid-bearing cells requires an
autoregulation of the toxin expression (Engelberg-Kulka and Glaser, 1999). The modiÝcation factors
are usually small proteins, but the rescue factors can be small, unstable antisense RNA molecules
as well. The system relies on the fact that a progeny cell will always inherit cytoplasm during a
cell division. Should a cell fail to inherit the cytoplasmic gene, a plasmid, for example, it will still
have inherited the toxinÏantitoxin components. Because the antitoxin is shorter lived than the toxin,
this cell will eventually end up with the more stable component, the toxin, which will then kill the
cell, as the doomed cell does not have the gene to synthesize the antitoxin. The short half-life of
the antitoxins may be due to a low thermodynamic stability that keeps the antitoxins close to
unfolding, after which they are rapidly degraded by cellular proteases such as Lon, ClpPX, ClpPA,
or RNAses where the rescue factors are antisense RNAs. Originally, it was thought that these
systems would directly stabilize the vertical transmission of the plasmids carrying them by adding
an adaptive value, which led for some time to the term plasmid stabilization/stability system;
however, addiction systems are pure selÝsh systems (Cooper and Heinemann, 2000).
The Ýrst
(now Ñcontrolled cell death,Ò originally Ñcouples cell divisionÒ) system, encoded
by the F plasmid of
ccd
Escherichia coli
, was identiÝed in 1994 (Dao-Thi et al., 2002). The CcdB
