Growth, Differentiation and Sexuality

the amplification of plDNA is not a prerequisite
for senescence but rather a modulator of life span.
The amplification of this element appears to speed
up aging of cultures. At this point, it needs to be
emphasized that long-lived strains of this plDNAless
type have not been isolated from nature but
only in the laboratory. Under natural conditions,
they seem not to be able to survive and, thus, the
specific mtDNA rearrangements demonstrated in
wild-type strains appear in fact to be the reason
why the cultures age and die. These mechanisms
are probably a major adaptation of P. anserina to its
natural niche (herbivorous dung), which requires
fast reproduction of cultures because the substrate
dries out fast and thereafter is not suited for further
growth.
The molecular mechanisms by which the
amplification of plDNA leads to an acceleration of
the aging process have been characterized in some
detail. This is dependent on the transposition of
the pl-intron. These transposition events can occur
either to the position directly downstream of the
first CoxI exon (“homing-like” transposition) or
to other acceptor sites in the mtDNA molecule
(“ectopic” transposition). As a result, mtDNA
molecules are generated which contain two or
more tandem or dispersed copies of the 2.5-kbp
intron sequence (Sellem et al. 1993; Borghouts et al.
2000). Subsequent homologous recombination
events between these duplicated sequences result
in the formation of circular plDNA or circular
DNA molecules containing other parts of the
mtDNA. The intron transposition itself appears
to proceed via a reverse transcriptase step and
depends on the activity of a protein encoded by
an open reading frame on the pl-intron (Osiewacz
and Esser 1984; Kück et al. 1985b; Michel and Lang
1985; Fassbender et al. 1994).
Fungal Senescence and Longevity 191
C. Mitochondrial Functions
and mtDNA Instabilities
During aging of P. anserina cultures, a decline of
respiratory efficiency is observed (Belcour and
Begel 1980; Frese and Stahl 1992). Such a decline
caneasilybeexplainedbythefactthatnormalprotein
turnover, including the turnover of proteins of
the respiratory chain, requires a functional set of
genes encoding the corresponding proteins. Since
the mtDNA encodes parts of the mitochondrial
respiration chain, the age-related accumulation
of deleted mtDNA molecules interferes with
the turnover of respiratory chain proteins. As
a consequence, the generation of ATP declines. At
the same time, incomplete reduction of oxygen at
impaired respiratory chains leads to an increased
generationofthesuperoxideanion,atoxicreactive
oxygen species (ROS) which is normally generated
at complex I and complex III of the respiration
chain (Fig. 10.1A). The superoxide anion itself is
part of a reaction chain which ultimately leads to
the formation of the highly toxic hydroxyl radical,
capable of damaging various biomolecules in the
cell (Fig. 10.1B). In principle, the level of ROS
formation is rather low in undamaged juvenile mitochondria,
and it is reduced by certain scavengers
like the superoxide dismutases. However, during
aging ROS levels increase significantly to reach
thresholds which cellular scavenging systems can
no longer handle. One reason is that damage of respiration
chains which is not “repaired” by protein
turnover leads to the generation of increasingly
more ROS. A kind of “vicious circle” of reactions
finally results in the death of the corresponding
systems. In this scenario, there are different
components involved which affect the time by
which the system declines. These are (1) the level of
Fig. 10.1.A,B Generation of the superoxide anion
in the mitochondrial respiration chain. A Superoxide
anions are generated at complex I (NADH
dehydrogenase) and at the transfer of electrons
from ubiquinol (UQ) to complex III (cytochrome
c reductase). Complex IV (cytochrome c oxidase,
inhibited by cyanide) deficiency leads to the
induction of an alternative oxidase (AOX, inhibited
by salicylhydoxamic acid, SHAM) which
receives the electrons directly from ubiquinol,
circumventing complex III. B Generation of the
reactive oxygen species hydroxyl anion through
manganese superoxide dismutase (MnSOD, inP.
anserina PaSOD2) and Fenton reaction. Complex
III Ubiquinol–cytochrome c oxidoreductase,
complex V ATP synthase, cytc cytochrome c