Exploring the driving forces promoting speciation is a key question in evolutionary biology. Cycas bifida and C. micholitzii are two leaflets dichotomously divided Cycas species that have morphological similarities. Their distribution areas are separated by the RRFZ, a biogeographical barrier for Cycas species. The hypothesis for explaining their divergence is geographical isolation or convergent evolution. Three chloroplast DNA fragments, five low copy nuclear genes and 16 microsatellite loci were used to test this hypothesis and also to reveal their population genetics. The two species had high genetic differentiation but low gene flow with a deep divergence occurred in the late Miocene. The long-term geographical isolation not convergent evolution could explain for the divergence of the two species revealed from significant IBD testing, Barrier analysis and Niche consistency detection. Subsequently, each species made its own response to the Pleistocene climate fluctuations: a weak bottleneck effect in C. bifida and a population expansion in C. micholitzii. These findings imply that the role of geographic isolation rather than convergent evolution facilitates them divergence by constraining gene flow. Maybe they have gene flow or introgression with their neighboring distribution of Cycas species, making them genetically closer to neighboring Cycas species.

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.

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.