Biology Reference
In-Depth Information
many taxa hold a wide variety of intracellular, symbiotic bacteria, these results make it increasingly
possible that parts of these modiÝcationÏrescue systems may have found their way into the nucleus
of the symbiontÔs host.
RestrictionÏmodiÝcation (RM) systems constitute by far the most universal post-segregational
killing mechanism. The selective killing is a consequence of the dilution of the DNA methylase in
plasmid-free cells, which acts as the antitoxin or rescue factor. The DNA is no longer sufÝciently
protected from the toxin in the form of the corresponding restriction enzyme. Over 4000 restriction
and putative restriction enzymes and over 1000 methylases and putative methylases have been
detected. RM systems have been identiÝed in bacteria, archaea, and bacteriophages, as well as on
plasmids and chromosomes. RM systems can move without a linked mobile element. Analyses of
whole bacterial and archaeal genome sequences show that free-living bacteria such as
Helicobacter
pylori
can carry as many as 19 different type II methylase genes alone, while bacteria with a more
intracellular lifestyle have only a few methylase genes left, e.g., three in
Rickettsia conori
, two in
Buchneria
(Lin et al., 2001; Roberts and Macelis, 2001). In the
context of CI, it is interesting that RM systems have a profound inÞuence on the genome organi-
zation of their host. RM systems can comprise more than 4% of the total host genome. Host
resistance against and host inactivation of RM systems are widespread. RM systems can pair
modiÝcation and rescue factors of independent origin. Also, RM systems can characterize and
maintain the integrity of a host strain (Handa et al., 2001; Kobayashi, 2001; Murray, 2002).
sp., and only one in
R. prowazekii
Medea Factors
For Þour beetles, Medea is the acronym for Maternal Effect Dominant Embryonic Arrest (Beeman
et al., 1992). Medea is also the mythological daughter of King Aeetes of Colchis. She married
Jason, leader of the Argonauts, after she had helped him obtain the Golden Fleece from her father.
When Jason deserted Medea for the daughter of King Creon of Corinth, Medea, in revenge,
murdered Creon, his daughter, and her own two sons by Jason and took refuge with King Aegeus
of Athens, whose wife she became. He later drove her away after her unsuccessful attempt to poison
his son Theseus. In beetles, this means that the lethality is inherited from the mother and will kill
all offspring that does not inherit a rescuing M allele from either parent. The toxin is always
cytoplasmic and therefore maternal, whereas the antitoxin can be coded on a maternal or paternal
allele. Medea breeds true through the female line and segregates in the male.
Four different Medea systems have been found in the red Þour beetle,
.
Individuals often carry multiple systems. Almost all populations carry the M-4 system. It is the
only system in North American and European populations and can be found in about half of the
populations. South American, Asian, and African populations often carry two or more systems,
whereas Australian and Indian beetles are almost devoid of Medea. The Indian populations carry
though the hybrid incompatibility factor H, which is absent from all other continents. This makes
these populations unidirectionally incompatible to non-Indian populations. Again, the incompati-
bility is temperature sensitive (Thomson et al., 1995; Thomson and Beeman, 1999). The hybrid
incompatibility factor causes the death of hybrids with a paternally derived H gene and a maternally
derived Medea factor. The H factor presumably suppresses the rescuing factor of Medea. Using
gamma irradiation, Beeman and Friesen (1999) succeeded in knocking out the gene for the cyto-
plasmic lethality factor while retaining the zygotic rescue activity of the M allele. The molecular
basis of Medea is unknown. Medea shows all the properties of a selÝsh modiÝcationÏrescue system
with a suicidal defense against invasion of competing systems. The vast distribution of Medea over
continents has inspired several models to explain its population behavior (Wade and Beeman, 1994;
Hastings, 1994; Smith, 1998; Hatcher, 2000).
A mutant of the common wall cress
Tribolium castaneum
Arabidopsis thaliana
, which is also called
(
MEA
),
MEDEA
leads to 50% abortion upon self-fertilization of heterozygotes with
(Grossniklaus et al.,
MEDEA
1998).
is a mutant of the
-
development locus
FIS1
.
FIS1/MEA
MEDEA
FERTILIZATION
INDEPENDENT
SEED
