Phylogeny of Spiraea (Rosaceae) based on plastid and nuclear molecular data: Implications for morphological character evolution and systematics

Spiraea (Rosaceae) is a plant genus of great horticultural value, comprising 50-80 species distributed throughout the northern hemisphere, seemingly with particular affinity to eastern Asia. Phylogenetic relationships within the genus have not been well studied and patterns of evolution of morphological characters remain unclear. Here, we present the first comprehensive phylogenetic analysis of the genus based on one nuclear and two plastid markers. Phylogenetic analysis was done using maximum parsimony, maximum likelihood, and Bayesian inference on the plastid DNA sequences (2446 bp), composed of the contiguous trnL-F region and matK, the nuclear internal transcribed spacer (727 bp), and the concatenated datasets (3173 bp). Separate analyses of the plastid, nuclear and concatenated datasets yielded five well-supported major clades within Spiraea that also received high support from morphological characters. Our results suggest that reliance on a few morphological characters in past classifications failed to recover monophyletic infrageneric taxa. All previous sections are recognized as paraphyletic or polyphyletic except section Spiraea which is nested within section Calospira. Character evolution analyses indicate that the trait, inflorescence type, was over-emphasized in earlier taxonomic treatments, which artificially divided the genus into three groups. Our analyses indicate that characters such as leaf blade margin, development of reproductive shoot, and flower color are valuable for infrageneric classification, while others, such as number of bud scale, branchlet shape, and leaf blade shape are of limited value.