Clinal variation of two Arabidopsis species along an altitudinal gradient
Background and Aims Even though plants frequently hybridize, hybrid zones seem much rarer in plants than animals. Hybrid zones are evident from clinal variation in, for example, genetic structure, and once identified, they are a magnifying glass on the speciation process, particularly the strength of pre- and postzygotic barriers to gene flow. This study introduces the single hybrid zone between Arabidopsis arenosa and Arabidopsis lyrata known so far, found along an altitudinal gradient in the eastern Austrian Forealps. Methods We sampled 27 populations across the edge of the eastern Austrian Alps to screen for the presence of clinal variation in four datasets: genetic data (ddRADseq), genome size, morphological characters (96 characters), and environmental preference (estimated from phytosociological relevés). Key Results We found spatially concordant gradients in all four datasets, indicating the existence of a bimodal hybrid zone between tetraploids of Arabidopsis arenosa and Arabidopsis lyrata in the eastern Austrian Forealps along an altitudinal gradient, and potentially pointing toward a coincidence of barrier effects. Conclusions Even though introgression between these two Arabidopsis species has been documented in the past, this is the first evidence of a hybrid zone. As postzygotic barriers between the tetraploids of both species have previously been shown to be relaxed, it is probably prezygotic barriers, such as flower phenological differences, that prevent extensive merging of the two genomes along this hybrid zone.