Biology Reference
In-Depth Information
mice), a female mates with several different males during her lifetime. A maternally-
derived gene in a current offspring is thus more likely to have copies in future offspring
(because the mother remains the same) than a paternally-derived gene (because
different offspring can have different fathers). Therefore, paternal genes in offspring are
predicted to demand more maternal resources than are maternal genes in the same
offspring. Genomic imprinting would be favoured under these conditions, enabling
genes to play conditional strategies depending on whether they were derived from the
mother or father (Moore & Haig, 1991).
Two antagonistic genes in mice support this idea.
Insulin-like growth factor 2
(
lgf2
) is
paternally-imprinted (expressed only when inherited from the father). It encodes
IGF-II, an insulin-like polypeptide that plays a role in extracting resources from the
mother during pregnancy. When expression of this paternal allele is experimentally
inactivated, offspring are 60% their normal weight at birth, whereas inactivation of
the maternal allele has no effect on birth weight. Counteracting the effects of
Igf2
is a
maternally-imprinted gene, the
insulin-like growth factor 2 receptor
(
lgf2r
). This encodes
a receptor that degrades the product of
lgf2
, thus reducing the resource transfer from
mother to offspring. When expression of the maternal allele is inactivated, offspring
are 20% larger than normal at birth, whereas inactivation of the paternal allele has no
effect on birth weight (Haig, 1997).
Thus, there is a tug of war between offspring and mother, with the paternally-
imprinted
lgf2
gene functioning to extract extra resources from the mother and the
maternally-imprinted
lgf2r
gene functioning to resist extra investment. It is not yet
clear whether other examples of genomic imprinting have evolved in response to
parent-offspring conflict (Haig, 2004).
Antagonistic
genes in mice: a
tug-of-war
between
offspring and
mother
Conflict resolution
Charles Godfray (1995) has made a useful distinction between two types of model of
parent-offspring conflict. So far, we have considered a 'battleground' model that defines
the zone of conflict (Fig. 8.8). Our last two examples provide evidence for this
battleground; tugs of war between parent and offspring over sex ratios in social
hymenopteran societies, and between parent and offspring genomes over maternal
resources during mouse pregnancy. However, there are also 'resolution' models that try
to predict how the conflict might be resolved. If evolution leads to a continuing arms
race, then the battleground may still be evident. However, if a stable resolution has been
reached, then the original difference in parent and offspring optima may be concealed
by the two parties having now become coadapted to each other's strategies.
Nestling begging displays provide a good example of how the conflict might lead to
a stable resolution. In theory, an offspring should increase its demand with need.
However, if it pays offspring to demand more than the parental optimum, parents
should require an honest, unfakable demonstration of need, otherwise they will be
tricked into providing too much investment. In theory, an evolutionarily stable
resolution to this conflict can be achieved if begging nestlings suffer a fitness cost
from soliciting care (Godfray, 1991, 1995). This might explain why nestling begging
is so exuberant, with loud calls, stretching and colourful gapes. Just as a female
demands an honest signal from a male in mate choice (Chapter 7), so a parent might
'Battleground'
models define the
conflict,
'resolution'
models predict
the outcome
