Prickly holly leaves are a traditional Christmas decoration, from
wreaths adorning homes, to greeting card scenes. Yet, look closer at a
holly tree and while some leaves are prickly, others are not. Scientists
writing in the Botanical
Journal of the Linnean Society believe variations within a
single tree are the combined result of herbivore activity and molecular
responses to environmental change.
“The ability of an organism to change its characteristics in response to
environmental variations is known as phenotypic plasticity and it is a
key driving factor in the evolution of a species,” said Dr Carlos
Herrera from National Research Council of Spain (CSIC) in Seville. “In
plants this is often seen in eye-catching changes to leaves and flowers
related to variable growing conditions. Every gardener knows that leaves
produced in deep shade and under full sun are often very different in
size and shape.”
However, this variation of leaf forms can also take place within a
single tree of many different species, and it is known as heterophylly.
Dr Herrera partnered with Ms Pilar Bazaga, also from CSIC, to explore
this phenomenon in European holly (Ilex aquifolium) a pioneer
species, with a strong ability to accommodate to changing conditions.
“Heterophylly is often witnessed in holly trees, where some leaves are
prickly, a defense against herbivores, while others are non-prickly,
with smooth margins and no defense,” said Dr Herrera. “We wanted to find
out if this variation was a response to environmental changes and if
this took place without wider genetic change, that is, without
alteration of the organism’s DNA sequence.”
“Heterophylly is a widespread phenomenon occurring in many different
types of plants,” Dr Mike Fay, Chief Editor of the Botanical
Journal of the Linnean Society. “By coincidence it is also a
conspicuous feature of ivy (Hedera helix), another plant
associated with Christmas decorations.”
Such change is known as epigenetics and to explore the biological
mechanics behind this process the scientists turned to methylation, a
chemical modification of DNA which does not alter the DNA sequence of an
organism, but can have decisive consequences.
DNA methylation profiles, heterophylly and herbivory were studied in 40
holly trees from a forest in South Eastern Spain. 39 were found to be
heterophyllous, with branches displaying prickly and non-prickly leaves
in neighbouring positions.
The team then explored the feeding activity of browsing deer and goats
to see if this was the environmental factor driving this genetic
diversity. The team found a significant relationship between recent
feeding and the growth of prickly leaves, noting that under the height
of 2.5 meters, the average reach of an adult red deer, leaves were
consistently pricklier.
The results revealed a clear link herbivore activity, phenotypic
plasticity and epigenetic changes involving modifications in the
methylation status of cytosine, one of the building blocks of DNA
molecules. This supports the theory that epigenetic variation alone can
be a source of variation in natural plant populations that does not
require changes in the DNA sequence.
“An increasing number of studies support the idea that the presence of
spines and prickles in plants is a response to herbivore activity, and
our research suggests this is the case with holly,” concluded Dr
Herrera. “The ability of plants to respond to environmental changes
through quick epigenetic modifications makes also one to feel a bit more
optimistic about plant survival in a quickly changing world.”
