Abstract
Natural hybridization is a common key evolutionary process and a
long-standing topic in the plant taxon. Hybrid zones, where two closely
related species interact, can provide an avenue to insight the process,
illuminating the maintenance of the taxon diversity. Incomplete
isolation barriers between Ligularia species generally form
hybrid zones. In this study, we used the double digest restriction-site
associated DNA sequencing technology (ddRAD-seq) to examine genetic
structure and estimate introgression in four newly discovered hybrid
zones between L. tongolensis and L. cymbulifera. Our
results showed high differentiation between parental species, whereas
pairwise FST between parents and their hybrids was low,
further corroborating sympatric site form hybrid zones. Moreover, most
F1 hybrid individuals were observed within the four hybrid zones
implying the presence of substantial barriers to interbreeding. An
analysis of genomic clines indicated that a large fraction of loci
deviated from a model of neutral introgression in the four hybrid zones,
of which most loci exhibited selection favouring L. cymbulifera
genotypes. Our analysis demonstrated bidirectional but asymmetric
introgression appearing in the four hybrid zones. The different habitats
among four hybrid zones may affect isolation barriers between both
species. Natural hybridization with post-zygotic isolation barriers may
significantly contribute to the diversification of Ligularia in
the HMR.