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 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.