A summary of hybrids detected in the genus Hedera (Araliaceae) with the provision of three new names

Abstract

Hedera L. (Araliaceae) is a northern temperate genus of around twelve, largely allopatric, species. No hybrids have been confirmed from the few areas where species distributions naturally intersect. The intergeneric hybrid × Fatshedera, introduced in 1910, is believed to be the first known occurrence of hybridisation involving an extant species of Hedera. The existence of a hybrid within Hedera was established by the detection of H. helix × H. hibernica in 2005. Here, aberrant plants recently observed in the UK, USA and Spain, from cultivated and naturalised populations, are shown to be hybrids based on their morphological characters and chromosome counts. Three new infrageneric hybrids in Hedera are described. The names H. × sepulcralis, H. × cazorlensis and H. × nessensis are provided for hybrids between H. hibernica and H. algeriensis, H. helix and H. maroccana, and H. iberica and H. hibernica respectively. The distinguishing features of these plants are discussed and a key allowing identification of the two hybrids known to have arisen in the UK is given. These discoveries reveal the potential for hybridisation within Hedera and geographical separation, therefore, appears to be the main barrier to crossing between species in the wild. As species come increasingly into contact through cultivation and naturalisation, it is predicted that more hybrids will appear. These hybrids highlight a previously unrecognised risk of introgression among these morphologically similar species.

Keywords:

• chromosome number
• interfertility
• invasive species
• spontaneous hybridisation

Acknowledgements

We are particularly grateful to Marta Bényei-Himmer and Mária Höhn for discussing their work on ivy and sending specimens of triploid hybrids and Richard Sanford for assisting us with the Latin diagnoses.