A well-resolved phylogeny of <i>Epilobium</i> (Onagraceae) taxa in the British Isles finds no correlation between genomic divergence and the outcome of hybridisation

Background: Hybridisation plays a key role in plant evolution. The extent to which parental divergence can predict the outcome of hybridisation (e.g. the ability to form hybrids and hybrid fertility) has most often been studied using a small number of individuals and/or DNA barcodes which could result in erroneous measures of parental divergence. Aims: This study investigated the relationship between genetic divergence and the formation and fertility of hybrids, using Epilobium sect. Epilobium (willowherbs) of the British Isles as a model. Methods: We used reduced representation sequencing to generate a phylogeny of Epilobium, which vary in their propensity to hybridise. Genetic distance was compared to the occurrence and seed set of hybrids using the literature available on this genus. Results: A phylogeny constructed from 3522 quality-filtered SNPs from 44 samples of 10 species and four putative hybrid individuals, resolved all but one species as monophyletic. Two hybrid individuals were confirmed, and one additional hybrid was identified. All were likely to be F₁ hybrids. Conclusion: We did not detect a significant relationship between parental divergence and hybridisation likelihood and/or hybrid fertility in Epilobium sect. Epilobium, contradicting the hypothesis that increasing genetic divergence decreases hybrid formation and hybrid fertility.