Chloroplast sequences and nuclear microsatellites reveal extensive hybridization among sympatric species of the genus Machilus postprint

Abstract: Plants of the genus Machilus Nees (Lauraceae) are evergreen trees and are common dominant species in subtropical evergreen broad-leaved forests, with significant economic value. However, there is considerable debate regarding the classification within the genus and the phylogenetic relationships among species. To explore the potential reasons behind this, this study selected 12 sympatric species of Machilus and analyzed 72 chloroplast fragments and 10 nuclear microsatellite loci. The results were as follows: (1) A total of only 48 variable sites were found across the 44 025 base pairs of the 72 chloroplast fragments, indicating extremely low polymorphism. (2) The 10 nuclear microsatellite markers showed good universality across the 12 species of Machilus, with high polymorphism, a large number of shared alleles among different species, and very few private alleles. (3) A high-support consensus phylogenetic tree could not be obtained based on chloroplast DNA, while microsatellite markers supported the taxonomic status of M. oreophila, M. rehderi, and M. thunbergii. However, those relationships among other species were confused and there was incongruence between nuclear and plastid phylogenies. This confusion is likely attributed to frequent interspecific hybridization and gene flow that have blurred the boundaries between species. Further research is needed to integrate morphological data and multiple molecular markers, and to develop effective analytical tools to understand the complex phylogenetic relationships within the genus Machilus.