Molecular evidence for natural intergeneric hybridization between Liquidambar and Altingia

Since its establishment, a hybrid origin for Semiliquidambar has been proposed based on morphological intermediacy and sympatric distribution with Altingia and Liquidambar. This hypothesis, however, has lacked convincing molecular evidence. In this study, two nuclear genes, pin2 and cab4, and a chloroplast gene, matK, from Semiliquidambar cathayensis and its putative parental species Liquidambar and Altingia in Jianfengling, Hainan, and Heishiding and Nanling, Guangdong, China, were sequenced to test this hypothesis. Our results showed that L. formosana and L. acalycina were closely related and constituted an inseparable clade in the phylogenetic trees of both pin2 and cab4 genes. Phylogenetic analyses revealed two types of sequences for S. cathayensis, which were clustered with its putative parents, L. formosana-L. acalycina and A. obovata in Jianfengling, and with L. formosana-L. acalycina and A. chinensis in Heishiding and Nanling. The partial chloroplast matK gene sequences showed four nucleotide substitutions between L. formosana and A. obovata in Jianfengling; the sequences of the two individuals of S. cathayensis were identical with those of A. obovata. No diagnostic chloroplast markers including matK and three other chloroplast genes were found to distinguish L. formosana and A. chinensis in Heishiding and Nanling. Molecular data clearly demonstrated that S. cathayensis is of intergeneric hybrid origin between L. formosana-L. acalycina and A. obovata or A. chinensis and that A. obovata functions as the maternal parent in the hybridization event in Jianfengling, Hainan.