Growth, Differentiation and Sexuality

190 H.D. Osiewacz and A. Hamann
arial hyphae, which grow into the air, is significantly
reducedinagedcultures.Microscopicanalysesrevealed
that the hyphal tips of senescent cultures
show a curved and undulated phenotype and frequently
burst (Delay 1963; Esser and Tudzynski
1980). In the case of the intensively studied wildtype
strains, mean life span is about 25 days when
grown on rich medium at 27 ◦ C. Other wild-type
strain s have other characteristic mean life spans,
demonstrating that life span control is genetically
determined. In addition, environmental conditions
like nutrient supply or incubation temperatures affect
the onset of senescence.
Aging in P. anserina was found to be strongly
dependent on the type of carbon source supplied
in the growth medium. Carbon sources like
glucosehavebeendemonstratedtoshortenlife
span, whereas others (like acetate or glycerol)
increase life span (Tudzynski and Esser 1979;
Maas et al. 2004). In addition, low glucose concentration
(0.05%) in the culture medium clearly
increases life span. Also, metabolic inhibitors
(e.g. the DNA intercalating compound ethidium
bromide, and the inhibitors of mitochondrial
protein synthesis chloramphenicol, kanamycin,
neomycin, streptomycin, puromycin and tiamulin)
which are added to the culture medium result
in an increased life span (Esser and Tudzynski
1977; Tudzynski and Esser 1977; Belcour and
Begel 1980; Koll et al. 1984). Antioxidants like
glutathion are also capable of prolonging life span
in P. anserina (Munkres und Rana 1978). Not only
compounds added to the culture medium but also
the incubation conditions have rather early been
described to influence life span. Cold treatment
was found to lead to the rejuvenation of cultures
(Marcou 1961). A life-shortening effect of higher
incubation temperature is observed not only in
wild-type strains but also in mutants (Turker et al.
1987).
B. Mitochondrial DNA Instabilities
Early experimental data revealed that both nuclear
as well as extrachromosomal genetic traits
play a crucial role in the genetic control of senescence
in P. anserina (Marcou 1961; Esser and Keller
1976; Tudzynski and Esser 1979). A major impact
on the onset of senescence has been assigned to
the occurrence of a covalently closed, circular DNA
species, termed plDNA or α-senDNA (Stahl et al.
1978; Cummings et al. 1979). This element was
demonstrated to accumulate in mitochondria of
senescent cultures. In juvenile cultures it is an integral
part of the mitochondrial DNA (mtDNA),
the first intron of the gene Cox1 coding for the cytochrome
c oxidase (COX; Osiewacz and Esser 1984;
Cummings et al. 1985). During aging of wild-type
cultures, the pl-intron becomes liberated and amplified.
This process is accompanied by deletions
of large parts of the high-molecular weight mtDNA
(Belcour et al. 1981; Kück et al. 1981). Since these
reorganizations are almost quantitative, the majority
of the mtDNA molecules are extensively rearranged
in senescent wild-type cultures (Belcour
et al. 1981; Kück et al. 1981, 1985a). In addition
to these specific rearrangements which occur reproducibly
during aging of wild-type strains, other
types of rearrangements are observed. They occur
between short dispersed repeats and are less specific
and frequent, resembling recombination processes
found in pathological situations and during
aging in a number of different systems (Linnane
et al. 1989; Osiewacz and Hermanns 1992; Wallace
1992, 2001; Cottrell et al. 2000; Samuels et al.
2004).
ThetermplDNA,for“plasmid-likeDNA”(Stahl
et al. 1978), refers to the structure of the amplified
element which resembles the structure of typical
circular plasmids in bacteria. senDNA refers to the
stage of a culture, the senescent stage, in which
this element accumulates. It should be stressed that
plDNA and α-senDNA are identical elements of an
oligomeric series of molecules with a size of 2539 bp
of the monomer (Osiewacz and Esser 1984; Cummings
et al. 1985). Other additional DNA species
which are also termed senDNAs (e.g. βsenDNA,
γsenDNA) are clearly different. They accumulate
in senescing cultures of P. anserina but have a variable
size and originate from a different region of
the mtDNA (Jamet-Vierny et al. 1980; Belcour et al.
1981, 1986; Kück et al. 1981; Wright et al. 1982;
Cummings et al. 1985, 1987; Koll et al. 1985).
On the basis of various experimental datasets,
it was suggested that the age-related mtDNA rearrangements,
and specifically the accumulation of
the plDNA, are a prerequisite for aging of P. anserina
cultures (Belcour et al. 1982; Stahl et al. 1982;
Vierny et al. 1982; Koll et al. 1985; Schulte et al.
1988; Osiewacz et al. 1989). However, some mutant
strains which, due to specific mutations, live
longer than the wild-type strain were later demonstrated
to senesce despite not having accumulated
plDNA in the senescent stage (Borghouts et al. 1997;
Silar et al. 1997). Thus, it has to be concluded that