Phylogeny of the genus Chrysanthemum L.: evidence from single-copy nuclear gene and chloroplast DNA sequences

Abstract

Chrysanthemum L. (Asteraceae-Anthemideae) is a genus with rapid speciation. It comprises about 40 species, most of which are distributed in East Asia. Many of these are narrowly distributed and habitat-specific. Considerable variations in morphology and ploidy are found in this genus. Some species have been the subjects of many studies, but the relationships between Chrysanthemum and its allies and the phylogeny of this genus remain poorly understood. In the present study, 32 species/varieties from Chrysanthemum and 11 from the allied genera were analyzed using DNA sequences of the single-copy nuclear CDS gene and seven cpDNA loci (psbA-trnH, trnC-ycf6, ycf6-psbM, trnY-rpoB, rpS4-trnT, trnL-F, and rpL16). The cpDNA and nuclear CDS gene trees both suggest that 1) Chrysanthemum is not a monophyletic taxon, and the affinity between Chrysanthemum and Ajania is so close that these two genera should be incorporated taxonomically; 2) Phaeostigma is more closely related to the Chrysanthemum+Ajania than other generic allies. According to pollen morphology and to the present cpDNA and CDS data, Ajania purpurea is a member of Phaeostigma. Species differentiation in Chrysanthemum appears to be correlated with geographic and environmental conditions. The Chinese Chrysanthemum species can be divided into two groups, the C. zawadskii group and the C. indicum group. The former is distributed in northern China and the latter in southern China. Many polyploid species, such as C. argyrophyllum, may have originated from allopolyploidization involving divergent progenitors. Considering all the evidence from present and previous studies, we conclude that geographic and ecological factors as well as hybridization and polyploidy play important roles in the divergence and speciation of the genus Chrysanthemum.

Figure 1. Structure of the CDS gene of Chrysanthemum lavandulifolium and the placement of primers.

Gray, black, and white boxes represent 5′ and 3′ UTR, the encoding region of the predicted plastid targeting peptide and exons, respectively. Lines represent introns, and dots indicate the long intron. Arrows indicate the locations of the primers.

Figure 2. Bayesian 50% majority consensus trees of the diploid Chrysanthemum species and their generic allies.

A: The tree of seven cpDNA loci combined. B: CDS exon gene tree. Posterior probabilities from the Bayesian inference (PP)/bootstrap percentages (BP) from the maximum parsimony analysis are shown next to the branches. “–” indicates where the BP value is <50% and the corresponding clade collapsed in the MP 50% bootstrap majority consensus tree. Main clades are indicated by ★/☆. Labels of terminal branches include species or variety name followed by population code (when available; in capital letters) and clone identities (from “a” to “e”; only for the CDS gene tree).

Figure 3. Bayesian 50% majority consensus tree of Chrysanthemum species and their generic allies.

The tree is based on the sequences of seven cpDNA loci combined. Posterior probabilities from the Bayesian inference (PP)/bootstrap percentages (BP) from the maximum parsimony analysis are shown next to the branches. “–” indicates where the BP value is <50% and the corresponding clade collapsed in the MP 50% bootstrap majority consensus tree. Labels of terminal branches include species or variety name followed by population code (when available; in capital letters). Major clades are designated by roman letters. “ZY” and “HSZ” in the terminal branch “C. yoshinaganthum + ZY + HSZ” indicate to C. morifolium cv. ZY and C. morifolium cv. HSZ, respectively.

Figure 4. Bayesian 50% majority consensus tree of Chrysanthemum, Ajania, and Phaeostigma.

The tree is based on the nuclear CDS exon+intron sequences. Posterior probabilities from the Bayesian inference (PP)/bootstrap percentages (BP) from the maximum parsimony analysis for the major clades are shown next to the nodes. “–” indicates where the BP value is <50% and the corresponding clade collapsed in the MP 50% bootstrap majority consensus tree. Labels of terminal branches contain: species or variety name followed by population code (when available; in capital letters), and then clone identities (from “a” to “e”).

Figure 5. Scanning electron microscopy (SEM) micrographs of pollen grains.

1A and 1B: Ajania purpurea; 2A and 2B: Phaeostigma salicifolium; 3A and 3B: P. variifolium; 4A and 4B: A. przewalskii; 5A and 5B: Chrysanthemum indicum; 6A and 6B: C. naktongense.