Ancient hybridization and chloroplast capture in the paleotropical genus Dasymaschalon (Annonaceae)

Abstract

Dasymaschalon (Annonaceae) comprises 27 species of small trees, with a center of diversity in continental Southeast Asia. Unlike most genera of Annonaceae, which have six petals per flower, Dasymaschalon flowers only have three petals; these petals are considered homologous with the outer petals of other Annonaceae species due to their alternate position relative to the sepals. Despite the apparently synapomorphic loss of inner petals, Dasymaschalon was found to be non-monophyletic in a previous phylogenetic study based on chloroplast DNA sequences. To test hypotheses of generic monophyly and the possibility of chloroplast capture due to hybridization, we reconstructed a multi-locus phylogeny based on plastid and nuclear DNA sequence data (matK, psbA-trnH, ndhF, rbcL, trnL-F, ITS, ETS, PHYA, PHYC; ca. 8.5kb). Our results show incongruence between plastid and nuclear topologies: in the plastid tree, the majority of Dasymaschalon species form a strongly supported clade, but three species are recovered in a clade which is more closely related to Asian species of Friesodielsia; in the nuclear tree, however, Dasymaschalon is monophyletic. Detailed analysis of patterns of topological incongruence and DNA sequence variation suggests intergeneric hybridization between Dasymaschalon and Friesodielsia in continental Southeast Asia. These results highlight the importance of integrating plastid and nuclear DNA sequence data in Annonaceae phylogenetics, which has heavily relied on chloroplast DNA sequence data in the past.