Cytoplasmic Incompatibility - Insect Symbiosis - page 155

is imprinted and expressed only from the maternal genome. It encodes a SET-domain protein similar
to Enhancer of zeste of Drosophila , a member of the Polycomb group of proteins, which in animals
ensure the stable inheritance of expression pattern through cell division. Paternally inherited MEA
alleles are transcriptionally silent in the young embryo. In Arabidopsis , FIS1/MEA is likely to
repress transcription of loci of the maternally derived genome that are normally only expressed
from the paternal genome. Mutations at the DECREASE IN DNA METHYLATION 1 ( DDM1 ) locus are able
to rescue MEA by functionally reactivating paternally inherited MEA alleles during development.
The maintenance of the genomic imprint at the MEDEA locus requires zygotic ddm1 activity. Because
DDM1 encodes a putative chromatin-remodeling factor, chromatin structure is likely to be inter-
related with genomic imprinting. In animals, histone deacetylases and histone lysine methyltrans-
ferases have been implicated in genomic imprinting (Vielle-Calzada et al., 1999; Sewalt et al.,
2002). MEA leads to a paternalization of the egg, and hypomethylation in the pollen leads to
maternalization; this reversal can rescue the incompatibility phenotype without the pollen contrib-
uting a wild-type FIS1 allele (Yadegari et al., 2000; Grossniklaus et al., 2001; Spielman et al., 2001).
On the other hand, the Medea protein of Drosophila is not involved in CI. It is rather required
for embryonic dorsal-ventral and imaginal disc patterning. Medea is a functional homolog of the
human tumor-suppressor DPC4/Smad4. It mediates the signaling of the extracellular morphogen
decapentaplegic to the nucleus (Das et al., 1998; Hudson et al., 1998; Wisotzkey et al., 1998).
A Medea-like killing system has been identiÝed in mice (Peters and Barker, 1993; Hurst, 1993).
Named scat + after its pathological manifestation, it displays the familiar pattern of a modiÝcation-
and-rescue system with a cytoplasmic lethality factor and a gene-based zygotic rescue factor such
as Medea. A severe combination of anemia and thrombocytopenia causes the death in scat + mice.
The scat + locus has been mapped on chromosome 8. It is believed that scat + originated from a
spontaneous mutation in a BALB/cBY colony of the Jackson Laboratory. It is not known outside
the laboratory. The gene of the scat + locus has not been identiÝed, and the molecular mechanism
for rescue is not known either.
Cytoplasmic Male Sterility
The oldest and easily the most famous description of CI in plants that is characterized by male
sterility comes from Charles Darwin (1877). The majority of Þowering plants are hermaphroditic.
Darwin called the coexistence of hermaphroditic individuals and individuals that are fertile females
but sterile males gynodioecy. He considered this dimorphism as a transition toward dioecy, the
separation of sexual types in different individuals. Gynodioecy is the second most frequent repro-
ductive strategy after hermaphroditism in Þowering plants. Mitochondrial-borne cytoplasmic male
sterility (CMS) is the natural cause of gynodioecy and is characterized by the maternally inherited
inability to produce functional pollen without affecting the plant otherwise. This trait is now
commercially used for the production of F1 hybrid seeds in several cultivated species such as maize,
sugar beets, sunÞower, onions, rice, and wheat to eliminate labor-intensive emasculation by hand.
Molecularly, CMS is composed of a toxin and an antitoxin that are cotranscribed from the
mitochondrial genome and translated from bicistronic mRNA. The toxin in the Texas type of maize
CMS, Urf 13, is coded by a chimeric sequence composed of atp6 , subunit 6 of the ATP synthase, the
3-Þanking region of rrn26 , ribosomal RNA, an unidentiÝed sequence, and a part of the coding
sequence of rrn26 . The resulting protein is hydrophobic and localizes in the internal membrane of
the mitochondrion. The antitoxin, orf221, is an essential mitochondrial membrane protein that is also
found in wild-type mitochondria (Priolo et al., 1993; Rhoads et al., 1998). In Japanese radish, the
toxin, orf138, is made up of different radish mitochondrial sequences. The matching antitoxin, orfB,
corresponds to subunit 8 of ATP synthetase (Terachi et al., 2001). In rapeseed, the toxin contains parts
of NADH dehydrogenase subunit 3 and is rescued by subunit 6 of ATP synthase (LÔHomme et al.,
1997; Brown, 1999). The petunia toxin is composed of the 5 end of subunit 9 of ATP synthase, parts
of cytochrome c oxidase subunit II, and an unknown sequence, and this is compensated by subunit