Recent hybridization burst in diploid-tetraploid Cardamine contact zones in the Apennines: are allotriploids threatening the persistence of parental species?

Abstract

Differences in ploidy levels represent a significant barrier to interspecific gene flow, but growing evidence from natural and experimental systems suggests that this barrier may not be as strong as commonly believed. In this study, we aimed to examine patterns of interploidy hybridization between two ecologically similar and partially sympatric wetland perennials in the central Apennines: diploid Cardamine apennina and allotetraploid C. amporitana. Our detailed chromosomal, flow cytometric, and genomic (RADseq) analyses revealed a strikingly high frequency of allotriploids across multiple sites, with no evidence of backcrossing, later-generation hybrids, or higher-level allopolyploids. Contrary to expectations, clonal propagation is not the primary mechanism underlying the observed abundance of triploids. Instead, a high hybridization rate with recurrent, bidirectional, and polytopic hybrid formation, together with the presumed longevity of triploid individuals, appears to explain their persistence. Our findings also raise two key issues: the possible triggers of this apparently recent burst of hybridization, and the potential long-term consequences of frequent allotriploid formation for the survival of the parental (sub)endemic species in the Apennines. From a broader perspective, this study underscores the evolutionary significance and dynamics of species contact zones.