Select Annals of Botany Volume 101 Number 9 2008

Plants that colonize new and/or transient habitats may find themselves ‘out on their own’ with very few individuals of the same species in the vicinity. Opportunities for outbreeding are then very restricted and it is therefore not surprising that many colonizing and pioneer species are self-fertile. Indeed, so common is this correlation that it is often assumed that all such species are mainly inbreeders. However, it is dangerous to make unsupported assumptions of this kind, as nicely shown by Friedman and Barrett (Toronto, Canada, pp. 1303–1309) in respect of the North American annual Ambrosia artemisiifolia. The authors’ experiments were models of efficiency and clarity. Firstly, they grew plants in arrays of differing densities. Planting density had some effect on the amount of pollen received: plants grown at the highest density received the most pollen. However, even at the lowest densities, enough pollen was received to achieve good seed set. Study of multi-locus allozyme markers in the progeny showed that at all planting densities A. artemisiifolia behaved as an obligate outbreeder; outcrossing rates were at or very close to 1.0. The predominance of outbreeding was confirmed by the very poor seed set observed in isolated plants. Secondly, when plants were pollinated by hand, stigmatic surfaces were receptive to both self and non-self pollen. However, the self pollen either failed to germinate or, if it did germinate, the pollen tube did not penetrate very far down the style. Indeed, the stylar tissue exhibited a typical self-incompatibility reaction, namely the synthesis of callose, a b1-3 glucan. So, how does an obligate outbreeder act as a colonizing species? The authors suggest two main factors. The first is the ability to deposit seed banks, providing a long-lived potential source of colonizing individuals. The second is the production of very large amounts of wind-blown pollen, maximizing the chance of outbreeding except for completely isolated plants.