Speciation and hybridization of <i>Enkianthus quinqueflorus</i> and <i>E. serrulatus</i> (Ericaceae) across a tropical–subtropical transitional zone in South China

Abstract How speciation and hybridization occur across steep environmental gradients has fascinated ecologists and evolutionary biologists for decades. Enkianthus quinqueflorus and E. serrulatus are a species pair located on the two sides of the Nanling Mountains, a previously proposed tropical–subtropical transitional zone (ecotone) of south China. In this study, we investigated its speciation and hybridization history based on DNA sequences of four chloroplast inter-genic spacers and eight nuclear genes from 44 populations. Phylogenetic analyses found clear cytonuclear discordance, indicating some E. quinqueflorus (EquiN) populations were of hybrid origin, largely corresponding to E. serotinus (especially E. tubulatus). Approximate Bayesian computation (ABC) analysis found that EquiN were derived from hybridization between E. serrulatus and the other cluster of E. quinqueflorus (EquiS) at 0.22 Mya after an initial split at 0.93 Mya, and the hybridization was also confirmed by IMa2. Ecological niche modelling indicated that E. serrulatus and EquiS had distinct ecological niches but with overlapped distribution across the late Quaternary. These results, coupled with morphological intermediacy of E. tubulatus, clearly suggest that E. quinqueflorus (EquiS) and E. serrulatus may be the products of allopatric speciation associated with refugial isolation during the late Cenozoic climate changes, and E. tubulatus could have resulted from second contact and hybridization around the Nanling Mountains between E. quinqueflorus (EquiS) and E. serrulatus. This study suggests that the Nanling Mountains are a hotspot of speciation and hybridization for woody flowering plants, and represent the tropical–subtropical transitional zone in south China, at least in terms of plant divergence and speciation.