Unraveling extensive paraphyly in the genus Hydrangea s.l. with implications for the systematics of tribe Hydrangeeae

Several representatives of Hydrangeaceae tribe Hydrangeeae are very popular in the horticultural trade for their inflorescences with attractive marginal flowers, resulting in many hybrids and cultivars. However, despite previous morphological and molecular studies, the phylogenetic relationships between the nine currently recognized genera in this clade are still largely unclear, and the widely used infrageneric classification of the genus Hydrangea by McClintock (which was mainly based on the study of herbarium specimens) requires critical reevaluation. We present a partially well resolved and well supported phylogeny of the tribe Hydrangeeae based on maximum parsimony and Bayesian analyses of multiple coding and non-coding chloroplast regions (rps16-trnK spacer, trnK intron, trnK exon, matK gene and trnK-psbA spacer). All genera, sections and subsections were included making it a highly representative sampling for the tribe Hydrangeeae. Hydrangea is paraphyletic with respect to the eight other genera of Hydrangeeae, requiring a classification update. The subsections Heteromallae and Petalanthe of section Hydrangea are monophyletic, the subsections Americanae, Asperae and Macrophyllae are paraphyletic. Of the two subsections of section Cornidia, one, Polysegia, is monophyletic whereas the other Monosegia, is paraphyletic. The necessary nomenclatural changes based on these results might have a certain effect in the trade but also might encourage breeders to produce "intergeneric" hybrids between traditionally recognized genera in Hydrangeeae.

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