Is there hybridisation between diploid and tetraploid Euphrasia in a secondary contact zone?

• 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 barr...