Home | Images | Classification of Oleaceae and links to genera | Classification of Fraxinus and links to species

PhD thesis by Eva Wallander

Wallander, Eva (2001) Evolution of wind-pollination in Fraxinus (Oleaceae) – an ecophylogenetic approach. PhD thesis. Göteborg University, Sweden. ISBN 91-88896-37-4.

Email me know if you want a copy (2 MB pdf available).

This thesis is based on the following four papers


Studies in Fraxinus (ash) and other genera of Oleaceae have revealed interesting trends and patterns in the evolutionary history of reproductive traits. They contribute empirical data for hypotheses under which conditions anemophily (wind-pollination) and dimorphic breeding systems, such as dioecy and the rare androdioecy, may evolve.

The study includes a molecular phylogeny of all 25 genera of the family Oleaceae, based on two noncoding chloroplast regions (the intron of rps16 and the trnL-F region), and of 40 of the 43 recognised species of the genus Fraxinus, based on the internal transcribed spacers of the nuclear ribosomal DNA (ITS-1 and ITS-2, and the intervening 5.8S gene).

The reproductive ecology of three species of Fraxinus, F. excelsior, F. ornus, and F. longicuspis, representing different combinations of pollination and breeding systems, were studied in the field in order to gain insight in their adaptations to wind and/or insect-pollination and compare different reproductive strategies.

Based on estimates of phylogenetic relationships in combination with reproductive data, I have shown that anemophily has three separate origins in Fraxinus and that it is preceded by ambophily (both wind- and insect-pollination). Dioecy is correlated with anemophily and has evolved after the origin of anemophily in three instances. In one case, dioecy has originated via androdioecy, which in turn has a hermaphroditic origin. This is the first study that shows the evolution of dioecy from hermaphrodites via the rare androdioecious pathway. In the other wind-pollinated taxa, dioecy has evolved twice, independently, from polygamous ancestors.

In subtribe Oleinae, sister group to Fraxinus, there are five genera with species that are adapted to wind-pollination. Here, anemophily is believed to have four separate origins. Many species have inconspicuous, whitish to yellowish-green flowers, which appear to attract unspecialised pollinators by offering pollen as a reward. This is believed to be an exaptation (traits that facilitate or are a prerequisite) for ambophily, which in turn may have selected for anemophily. Many of the taxa of Oleinae are hermaphrodites, but dioecy and androdioecy are common breeding systems, especially among the anemophilous or ambophilous species.

The interpretations of the results are that ambophily and a breeding system with unisexual males, either androdioecy or polygamy, were some of the exaptations for the evolution of anemophily. Dioecy, on the other hand, has followed in evolutionary sequence and is interpreted as an adaptation that evolved in response to anemophily and is adaptive for increasing the efficiency and function of that system. In wind-pollinated flowers there are different structural and productivity demands for optimal male and female function that are hard to compromise between in one flower. Thus, the selection for sexual specialisation leads to dioecy through total or partial reduction of the opposite sex in the resulting unisexual flowers.

Key words: androdioecy, ambophily, anemophily, breeding system evolution, dioecy, floral evolution, Fraxinus, Oleaceae, pollination system evolution.