Have you ever wondered why holly leaves are prickly?

A study by Herrera and Bazaga (2012) looked into the very interesting link between mammalian browsing and the European holly tree (Ilex aquifolium) defensive response of producing prickly leaves. Research has already taken place to suggest this link, with this particular study looking into a “three-way link between herbivory, phenotypic plasticity and epigenetic changes in plants.”

We should probably start by looking at the definitions of some of the terms used:

Epigenetics: this is the study of how the way your genes work can be changed by behaviours and the environment without any changes to DNA sequences.

Plasticity: the ability to adapt to environmental or habitat changes.

Phenotype: an observable trait or characteristic, for example, eye colour.

Phenotypic plasticity: when an organism is able to produce different phenotypes in response to environmental changes.

Heterophylly: where environmental conditions can cause significant variation in leaf shape or form on a single plant.

DNA methylation: an epigenetic change where methyl groups are added to the DNA molecule – this can change the expression of a DNA segment without changing the DNA sequence.

If you have taken a closer look at a holly tree and realised that not all of the leaves actually are prickly, this is an example of heterophylly as mentioned above. If you look closely at the photograph of holly below you will be able to see some very rounded “nonprickly” leaves and prickly leaves with tough spines.


The purpose of the study was to find out if there was a link between the heterophylly of holly, the eating of the holly by herbivores and how the trees were able to make changes to leaf shape so quickly in response to this.

Scientists studied 40 trees in South-eastern Spain. 39 trees were heterophyllous and one tree had only prickly leaves. As you would expect, the trees with branches and leaves closer to the ground showed more signs of browsing damage from herbivores such as deer and goats. They also found there was a significant correlation between the trees which had been used for browsing and the proportion of prickly leaves found closer to the ground. It was noted that this it was not just a general structure or “architectural effect” of the tree.

The DNA of the trees was analysed for traces of a process called DNA methylation and it was found that there was a relationship between the herbivory, prickly leaf growth and methylation taking place in the DNA. This suggests that the eating of the holly triggers a change in the DNA molecule to change the expression of the DNA segment i.e. prickly leaves, without making any changes to the sequence.

The study highlights the importance of the findings for plant conservation. It shows the ability to respond quickly to environmental changes without having to wait for traits to be passed on through actual changes to the DNA.

They say you learn something new every day, a new Christmas quiz fact!