• Premise of the study: Hybridisation between species with contrasting ploidy is usually considered rare in nature due to strong ploidy related postzygotic reproductive isolating barriers. However, genomic sequencing has revealed previously overlooked examples of natural cross-ploidy hybridisation, suggesting this phenomenon may be more common than once thought. Here, we investigate potential cross-ploidy hybridisation in British eyebrights (Euphrasia, Orobanchaceae), a group where thirteen putative cross-ploidy hybrid combinations have been reported based on morphology.   • Methods: We analysed a contact zone between diploid E. rostkoviana and tetraploid E. arctica in Wales. We sequenced part of the internal transcribed spacer of nuclear ribosomal DNA (ITS1) and used Genotyping by Sequencing (GBS) to look for evidence of cross-ploidy hybridisation and introgression. • Key results: All variant sites in the ITS1 region were fixed between diploids and tetraploids, indicating a strong barrier to hybridisation. Clustering analyses of 356 SNPs generated using GBS clearly separated samples by ploidy and revealed strong genetic structure (FST = 0.44). However, the FST distribution across all SNPs was bimodal, indicating potential differential selection on loci between diploids and tetraploids. Demographic inference with dadI suggested potential gene flow – with this limited to around one or fewer migrants per generation. • Conclusions: Our results suggest recent cross-ploidy hybridisation is rare or absent in a site of secondary contact in Euphrasia. While a strong ploidy barrier prevents hybridisation over ecological time-scales, such hybrids may form in stable populations over evolutionary time-scales and may allow for cross-ploidy introgression to take place.

Genotyping by sequencing data were generated for diploid and tetraploid Euphrasia species using the enzyme ApeKI. Sanger sequencing was also performed, amplifying the ITS2 region.