A suspected introgressive population of Ilex aquifolium and Ilex x altaclerensis, Satlburn, Yorkshire, UK

This post introduces to a well-established and complex population of self-sown Ilex x altaclerensis (Highclere holly) observed in Saltburn Valley Gardens, Yorkshire, UK.

Ilex aquifolium - Ilex x altaclerensis hybrid in Saltburn Valley Gardens
Self-sown holly with definite hybrid characteristics, including large, broad and flat leaves.

We are familiar with the native holly, Ilex aquifolium, a small tree whose spiny evergreen foliage and small red berries are associated with end-of-year festivities. Ilex x altaclerensis was born of a marriage only possible as a result of human agency, when it took the Vicorian’s fancy to grow Ilex perado (Madeira holly) in glasshouses and allowed it to cross pollinate with the native I. aquifolium accidentally growing in the wild near that greenhouse. The resulting hybrids remain popular planted hollies.

In most cases, when growing side by side Ilex aquifolium and Ilex x altaclerensis are relatively distinct (See picture above). However, looking more closely among trees in Saltburn Valley Garden, I also found a series of specimens with confusing morphologies suggesting a full-range gradient of intermediates between typical Ilex x altaclerensis and typical Ilex aquifolium (See picture below). This variety in the hybrid population may be a sign of introgressive hybridisation, or introgression.

A range of leaf size and shapes found in Saltburn Valley Gardens: each leaf is a representative leaf for an individual tree.

Introgression is a biological term used to describe the process leading hybrids to re-hybridise with one of their parents. Such population tend to include specimens with the full spectrum of intermediates between typical hybrid and ‘pure’ species, blurring boundary between species.

Introgression is under the spotlight in nature conservation because it is claimed to threaten the integrity of some native species. For example, there are concerns that the charismatic English bluebell could be outcompeted by garden escape hybrids (between the English bluebell and Spanish bluebell). However this threat proved unfounded in recent research, despite the potential for introgresssion.

More controversially, I would happily argue that introgression is an opportunity for native species to become more global-change adapted. For example, a species may assimilate additional genes and characteristics that will render it more resilient to environmental change over time. Following this logic, the introgressive population of hollies from Saltburn Valley Garden may represent novel biodiversity fit to face human-induced environmental change.

Such cases of introgression are interesting case studies to better understand what lays ahead. In fact introgression between natives and non-native is expected to become increasingly common with the rise of the Anthorpocene, a period of earth history where we are seeing a big reshuffle in species distribution as well as changing environmental conditions such as increase CO2 in the atmosphere, climate warming and disruption of nutrient cycles.

In the absence of DNA studies, my suspicion of intr0gression between Ilex aquifolium and Ilex x altaclerensis are based on morphological observation. In Saltburn Valley Gardens, the best vegetative identification criteria between the two taxa can be summarised with the three-choices key as follows:

  • Leaves flat/plane, with small forward-pointing spines more or less adpressed to leaf margin, length:width ratio <2, often dull. __________________________Ilex x altaclerensis
  • Leaves showing one or more of the following characteristics: irregularly undulated, mixture of forward-adpressed and other types of spines, leaves unusually large (>12cm in length), hybrid vigor in terms of yearly growth _________________________ Ilex x altaclerensis
  • Leaves strongly undulated (wavy margin, significantly more three-dimensional than blade thickness, forming folds when pressed flat), spines patent or backward pointing (spines nearer apex often forward pointing but not small and adpressed to leaf margin), length:width ratio >2. ____________________________Ilex aquifolium

Natives and hybrids can often show individual leaves, branches or whole trees with spineless leaves.

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