Triploid Bridges in Overcoming Ploidy Barriers in Cardamine amara

Polyploidisation is a significant reproductive barrier, yet recent genetic evidence indicates interploidy admixture is more common than previously thought. While theoretical models and controlled crosses suggest the triploid bridge hypothesis, suggesting that intermediate ploidy hybrids can facilitate gene flow, comprehensive evidence combining experimental and genetic evidence remains scarce.In this study, we investigated the rates and directions of gene flow within a diploid-autotetraploid contact zone of Cardamine amara, a species notable for its abundant triploids in natural populations. We employed cytotyping of over 500 wild individuals, conducted reciprocal interploidy crosses, and inferred gene flow using genome-wide sequencing of 84 individuals.Our findings reveal that triploids, though infrequent (5%) in mixed-ploidy populations, arise through both interploidy hybridisation and the fusion of reduced and unreduced gametes from diploids. Despite their rarity (<1%), these triploids can backcross with parental cytotypes, producing viable offspring, including euploid hybrids (32%). Coalescent simulations further demonstrated significant bidirectional introgression between sympatric diploids and tetraploids, with a pronounced asymmetry favoring gene flow towards the tetraploid cytotype.In conclusion, even though triploids are infrequently formed, they play a crucial role in overcoming reproductive barriers imposed by polyploidization, particularly when they persist locally, for example through clonality. By integrating genomic data and experimental crosses, we provide compelling evidence for extensive bidirectional interploidy gene flow mediated by the triploid bridge in natural populations.