Is hybridisation with non-native congeneric species a threat to the UK native bluebell Hyacinthoides non-scripta?

Societal Impact StatementHybridisation is an important evolutionary force in plants, but it can potentially lead to genetic swamping and extinction of one or both parental species. The threat of extinction is of particular concern if hybridisation occurs between native and introduced species, especially when the native is of national importance. The widespread occurrence of non-native bluebells in the United Kingdom has raised concerns that the iconic native bluebell could be at risk due to extinction by hybridisation from introduced non-native bluebells. This study determines the taxonomic identity of non-natives and investigates the amount of hybridisation occurring in natural and semi-natural UK bluebell populations. The widespread occurrence of a non-native bluebell taxon in the UK has raised concerns that the iconic native bluebell H. non-scripta (Asparagaceae) could be at risk due to extinction by hybridisation from introduced non-native congeners. Understanding the nature of this threat requires quantification of the extent of hybridisation between the native and non-native taxa. An additional complication is taxonomic uncertainty regarding the identity of the non-native bluebells in the United Kingdom that are colloquially referred to as the 'Spanish' bluebell (H. hispanica).We collected 501 bluebell samples from 56 populations in the United Kingdom (H. non-scripta and non-natives) and the Iberian Peninsula (H. hispanica). The samples were assayed for variation at 1871 nuclear and 17 plastid single nucleotide polymorphisms.Our genetic analyses demonstrated that non-native bluebells in the United Kingdom are not H. hispanica but the hybrid between H. hispanica and H. non-scripta. Moreover, they supported the hypothesis that Portugal is the country of origin of the first H. hispanica introductions to the United Kingdom. The frequency of hybrids was about 16%. Backcrosses between the (hybrid) non-native bluebell and the native H. non-scripta were primarily found in public parks. Of the sampled individuals for H. non-scripta from natural habitats, only 2% showed evidence of introgression.Although hybridisation might be frequent in locations where non-native bluebells have been introduced, we found no evidence of large-scale introgression in natural H. non-scripta populations. Therefore, our results do not support concerns of an 'extinction by hybridisation' scenario.