WALLANDER, EVA. Botanical Institute, Göteborg University, P. O. Box 461, SE 405 30 Göteborg, Sweden. - Phylogenetic relationships among the Ashes of the world, Fraxinus (Oleaceae).
The genus Fraxinus, the Ashes, in Oleaceae, consists of about
50 tree and shrub species. The most recent classification of the genus
by Lingelsheim (1920) identified two sections: Ornus (subsections
Euornus and Ornaster), and Fraxinaster (subsections Dipetalae,
Pauciflorae, Sciadanthus, Melioides, and Bumelioides). Ornus is
characterized by terminal inflorescences, insect pollinated Euornus
have showy petalous flowers and wind pollinated Ornaster are
apetalous. The species of section Fraxinaster are wind pollinated and
characterized by apetalous flowers in lateral inflorescences (except
F. dipetala that has two petals). The two main subsections
Melioides and Bumelioides differ in that both calyx and corolla are
wanting in Bumelioides. In an attempt to establish phylogenetic
relationships among all species of Fraxinus, DNA sequences from
the nuclear ITS region in 94 specimens, representing 43 of the 49
presently recognized taxa of the genus, were analyzed by cladistic
methods. The result shows that three major and two smaller clades can
be distinguished, roughly corresponding to Lingelsheim's subsections,
but relationships between them depend heavily on the choice of
outgroup: (1) Old World subsection Bumelioides, including North
American F. nigra as sister to F. mandshurica and F.
platypoda. F. xanthoxyloides, in monotypic Sciadanthus, is
sister to Bumelioides. (2) New World subsection Melioides and F.
cuspidata. (3) Xerophytic New World Pauciflorae. (4)
F.dipetala, in monotypic Dipetalae, is joined by F.
anomala and F. quadrangulata in a strongly supported clade,
sister group to the rest of Fraxinus in some of the trees. (5)
Section Ornus is clearly monophyletic, but its subsection Euornus is
paraphyletic because subsection Ornaster is derived from it. Section
Fraxinaster, as circumscribed by Lingelsheim, is paraphyletic no
matter how the cladograms are rooted. Thus, in Fraxinus, wind
pollination appears to be ancestral and insect pollinated species are
derived, and within the insect pollinated group, adaptations to wind
pollination have evolved again.
Key words: anemophily, Fraxinus, ITS, Oleaceae, phylogeny, wind pollination