Genes determine the variation in plant traits. Different versions of a gene
can lead to different traits. However, genes are not the only determinant of
such traits, and researchers are learning more about another contributor:
epigenetics. Epigenetic factors are things that regulate genes, altering
their expression, and like genes they can be inherited from generation to
generation, even though they are independent of the actual DNA sequences of
One epigenetic mechanism is DNA methylation, which can turn genes on or off
with the addition of chemical tags called methyl groups. Genes that share
the identical DNA sequence, but have different patterns of methylation are
called epialleles. Several studies have shown that epialleles can cause
differences in traits between plant subpopulations. Mary Gehring's team at
the Whitehead Institute has described evidence that epialleles alone can
lead to different heritable traits in plants.
In plants, methylation states of genes change most frequently during seed
development, when genes are switched on or off to progress development of
the organism. During this period, a conflict of interest arises in the
genome of each seed between the parts inherited from its mother and father.
Mother plants produce seeds fertilized by different fathers at the same
time. The mother wants to give an equal share of nutrients to each seed to
have many smaller seeds. But the father's interest is only for its seed to
get the most nutrients and grow larger. This conflict plays out through an
epigenetic mechanism called imprinting, in which, through differential
methylation between the father's and mother's copies of a gene, one parent's
copy is silenced in the offspring so that only the other parent's version of
the gene is expressed.
Gehring and her team found that when the strain loses its paternal
imprinting, the timing of seed development is affected and the plant ends up
with smaller seeds. This is consistent with the theory of imprinting: When
the father's genes have the advantage, the seeds are larger than when both
parents' genes are equally expressed.