86
Aging and Sex, DNA Repair in
to 71%, formation of fetuses was 5 to 16%,
and full-term development was 2 to 3%.
The low survival rate to full develop-
ment in both sheep and mice could have
three possible explanations. First, injuries
introduced by the experimental manipula-
tion of the embryos before implantation
might be deleterious to further devel-
opment. Even unreconstructed embryos
experience some increased prenatal loss
after manipulation or culture. Second, the
differentiated donor nuclei, transplanted
into the recipient oocytes, have to repro-
gram their developmental clock to zero,
and errors in this process may have
deleterious effects on fetal development.
Third, the low survival of fetuses may
be a consequence of the greater amount
of DNA damage in donor somatic cells
(
th
aninm
e
io
t
i
c
a
l
l
yp
rodu
c
edg
am
e
t
e
s
)
,
which can give rise to deleterious mu-
tations when replicating after transfer to
an oocyte.
The gene expression pro±le of about
10,000 genes in the placentas and the
livers of surviving cloned mice derived by
nuclear transfer were obtained in 2002.
The transferred nucleus came from either
an embryonic stem (ES) cell or from a
mature, differentiated cumulus cell. They
compared those gene expression pro±les
(derived from ES nucleus or cumulus cell
nucleus) with the gene expression pro±le
inth
es
am
et
i
s
su
e
s(
p
l
a
c
en
t
a
,l
i
v
e
r
)o
f
healthy mice derived from normal mating.
These comparisons showed that several
hundred (at least 4%) of the expressed
genes had pronounced dysregulation in
the cloned mice, probably accounting for
the altered phenotype of cloned individuals
(m
a
n
yc
l
o
n
e
dm
i
c
ew
e
r
eo
b
e
s
e
,e
t
c
.
)
.
Their evidence pointed to dif±culty in
reprogramming the developmental clock
back to zero, in the low percentage of
surviving mice.
In humans, about 50 to 80% of all
natural meiosis-based conceptions fail to
result in live birth. Cytogenetic studies
of spontaneous and induced abortions
and on perinatal deaths indicate that
many types of chromosome abnormalities
are present in these failed conceptions.
Chromosome abnormalities often derive
from DNA damages.
Thus, DNA damage may be a serious
problem for germ cells despite available
mechanisms for avoiding and repairing
such damage, and this problem may
be considerably greater in nonmeiosis-
derived conceptions.
5
Three Levels of Sexual Communication
Reflect (1) DNA Repair,
(2) Complementation, and (3) Selection
for Fitness
Sexual communication occurs when sig-
nals are used to promote or modulate
sexual interaction between individuals.
Sexual communication is prevalent among
organisms from bacteria to man. Sex-
ual communication occurs at three levels.
Level 1: the ±rst level of sexual commu-
nication includes signals that increase the
likelihood that two organisms will come
together for sexual interaction. Level 2: the
second level involves signals that modulate
the sexual interactions to inhibit inbreed-
ing or facilitate outbreeding. Level 3: the
third level includes signals that further
modulate the sexual interactions to pro-
mote selection among potential mating
partners based on relative ±tness. Evidence
indicates that the selective advantages of
the three levels of sexual communication
are, respectively, (1) the repair of DNA
damage, (2) the masking of mutation, and
(3) the choice of a ±t mating partner.
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