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

Premise of the study: Strong postzygotic reproductive isolating barriers are usually expected to limit the extent of natural hybridisation between species with contrasting ploidy. However, genomic sequencing has revealed previously overlooked examples of natural cross-ploidy hybridisation in different flowering plant genera, 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 (ITS) of nuclear ribosomal DNA and used Genotyping by Sequencing (GBS) to look for evidence of cross-ploidy hybridisation and introgression.

Key results: Common variant sites in the ITS 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 aI 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 potentially allow for cross-ploidy introgression to take place.