Aging and Sex, DNA Repair in
89
inbreeding is avoided to some extent.
We discussed above (Section 3.4) that the
diploid stage of a life cycle serves the
adaptive function of complementation, or
the masking of deleterious recessive muta-
tions. Optimal masking is achieved when
the diploid zygote is formed from the
union of haploid cells of two genetically
unrelated individuals, so that the reces-
sive mutations carried by each genome
are most likely to be different. However,
relatively few functions are probably ex-
pressed speciFcally in the diploid phase of
S. cerevisiae
and
S. pombe
,andth
ism
igh
t
explain why the masking of deleterious re-
cessive mutations is only weakly promoted
in these organisms.
In these yeasts, the pheromones operate
at ‘‘Level 2’’ as well, to promote ‘‘outcross-
ing,’’ in addition to acting at ‘‘Level 1’’ to
promote mating between individuals.
3.
Sexual communication in neurospora
.
Neurospora crassa
is an ascomycete, like
the yeasts
S. cerevisiae
and
S. pombe
,bu
t
its sexual phase is more elaborate than
those of yeasts.
Neurospora crassa
encodes
ag
e
n
e
mei3
, which is homologous to
E. coli recA
and
S. cerevisiae dmc1
and
rad51
.Amu
t
an
td
e
f
e
c
t
i
v
ein
mei3
has
reduced repair of DNA damage and, when
homozygous, is blocked in the zygotene
stage of meiotic prophase. This implies
that
N. crassa mei3,
like
S. cerevisiae dmc1
and
rad51
, is required for recombinational
repair of DNA damage during meiosis.
Neurospora crassa
has two stable mat-
ing types which produce sex pheromones.
The sex pheromone of one mating type
induces the formation of the female sexual
structure, the differentiated protoperithe-
cium, in the opposite mating type. Once a
protoperithecium is formed, the vegetative
conidiospores or mycelia of the opposite
mating type can act as the male partner.
Upon mating, a diploid nucleus is formed
by fusion of male and female nuclei of op-
posite mating type. This diploid nucleus
then undergoes meiosis, followed by mito-
sis, to produce eight ascospores. Each set
o
fe
igh
tspo
resisenc
losedw
i
th
inasac
,
referred to as an
ascus
, and many asci are
contained within the mature fruiting body,
termed a
perithecium
. Genes promoting as-
cus and ascospore maturation, as well as
perithecium development are expressed in
the diploid stage. It was found that 74 of
the 99 wild-collected
N. crassa
isolates from
26 populations carried one or more reces-
sive mutations in genes that expressed in
the diploid stage. They estimated that the
number of genes expressed in the diploid
stage is at least 435. Mutations in these
genes, when homozygous in the diploid
stage, cause formation of aborted asci,
ascospores with maturation defects, or bar-
ren fruiting bodies with few sexual spores.
The evolution of two distinct mating types,
which communicate via pheromones, in-
hibits inbreeding in
N. crassa
,andth
i
s
provides the advantage of masking reces-
sive mutations in the numerous genes that
express in the diploid stage.
Overall,
in
fungi,
it
appears
that
pheromones act at both the primary level of
sexual communication to promote sexual
interaction for meiotic recombinational re-
pair and the secondary level to promote
outcrossing, which allows masking of dele-
terious mutations.
5.3
Sexual Communication in Higher
Eukaryotes, Primarily for Complementation
while Repair Is an Automatic Concomitant
of Sexual Reproduction
1.
Flowering plants
. Since the diploid stage
is the most prominent stage in the life
cycle of flowering plants, the advantages of
complementation should be large. Thus,
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