Phylogenetic relationships in Bupleurum (apiaceae) based on nuclear ribosomal DNA its sequence data

Abstract

Background and aims: The genus Bupleurum has long been recognized as a natural group, but its infrageneric classification is controversial and has not yet been studied in the light of sequence data.

Methods: Phylogenetic relationships among 32 species (35 taxa) of the genus Bupleurum were investigated by comparative sequencing of the ITS region of the 18-26S nuclear ribosomal DNA repeat. Exemplar taxa from all currently accepted sections and subsections of the genus were included, along with outgroups from four other early branching Apioideae genera (Anginon, Heteromorpha, Physospermum and Pleurospermum).

Results: Phylogenies generated by maximum parsimony, maximum likelihood, and neighbour-joining methods show similar topologies, demonstrating monophyly of Bupleurum and the division of the genus into two major clades. This division is also supported by analysis of the 5.8S coding sequence alone. The first branching clade is formed by all the species of the genus with pinnate-reticulate veined leaves and B. rigidum with a unique type of leaf venation. The other major clade includes the remaining species studied, all of which have more or less parallel-veined leaves.

Conclusions: These phylogenetic results do not agree with any previous classifications of the genus. Molecular data also suggest that the endemic Macaronesian species B. salicifolium is a neoendemic, as the sequence divergence between the populations in Madeira and Canary Islands, and closer mainland relatives in north-west Africa is small. All endemic north-west African taxa are included in a single unresolved but well-supported clade, and the low nucleotide variation of ITS suggests a recent radiation within this group. The only southern hemisphere species, B. mundii (southern Africa), is shown to be a neoendemic, apparently closely related to B. falcatum, a Eurasian species.

Fig. 1. Strict consensus tree of 128 most‐parsimonious trees of 772 steps derived from equally weighted maximum parsimony analysis of sequences of the ITS region of Bupleurum and outgroup (OG) taxa (CI = 0·58; RI = 0·85). Bootstrap percentages appear above the branches (boldface); jackknife percentages are given in brackets; values <50 % are not indicated. Decay indices are below the branches in italics. Clade A = subgenus Penninervia; clade B = subgenus Bupleurum; clade C = the ‘NW African’ group; clade D = ‘Macaronesian’ group; clade E = the ‘Iberian and Balearic’ group; clade F = the ‘Ranunculoides’ group; clade G = the ‘Trachycarpa’ group; clade I = the ‘Perfoliata’ group.

Fig. 2. One of 128 most‐parsimonious trees of 772 steps derived from equally weighted maximum parsimony analysis of sequences of the ITS region of Bupleurum and outgroup (OG) taxa (CI = 0·58; RI = 0·85). Branch lengths are proportional to the number of substitutions (see scale bar). Numbers represent bootstrap values; values <50 % are not indicated. The letters A–I represent the same Bupleurum groups referred to in Fig. 1.

Fig. 3. Single tree obtained from neighbour‐joining (NJ) analysis of substitution rates on sequences of the ITS region of Bupleurum and outgroup (OG) taxa. Branch lengths are proportional to distances (see scale bar) estimated from the two‐parameter method of Kimura (1980), with substitution rates assumed to follow gamma distribution (∝ = 0·5). Numbers above branches are NJ bootstrap estimates; values <50 % are not indicated. The letters A–I represent the same Bupleurum groups referred to in Fig. 1.