Cyto-nuclear incompatibilities across a bluebell hybrid zone in northern Spain do not prevent genome-wide introgression of neutral markers

Abstract

AbstractThe bluebells Hyacinthoides hispanica (Mill.) Rothm. and H. non-scripta (L.) Chouard ex Rothm. form a hybrid zone in Spain and represent a natural experiment for the study of gene flow between species. The results of this study provide not only important insights to obtain empirical evidence regarding the mechanisms of gene flow enabled by hybridization but also to improve conservation assessment of the anthropogenic hybridization zone amongst bluebells occurring in Britain. We developed genome-wide genetic markers for amplicon-based resequencing from individuals across the hybrid zone, mapped morphological changes across the zone and determined the fitness of hybrids in laboratory crosses. We revealed significant clines across the zone at 61% of single nucleotide polymorphisms of the nuclear genes, most of which have a relatively shallow slope (mean slope 0.051 km–1, mean width of 78.4 km). In contrast, there was a rapid change in organellar haplotypes (slope = 0.238 km–1, mean width of 16.78 km). Thirteen nuclear genes had clines coincident with the organellar transition, and a subset had products directly involved in cyto-nuclear interactions. The location of this hybrid zone, the wide transitions in morphology and allele frequency all suggest that the zone is long established, and probably formed after the last glacial maximum. The high fitness of interspecific crosses is consistent with ongoing gene flow across the zone, although cyto-nuclear incompatibilities appear to prevent the movement of some alleles and coordinated activity between cellular compartments.