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Reproductive Biology |
2Institut für Umweltwissenschaften, Universität Zürich, Winterthurerstrasse 190, 8057 Zurich, Switzerland; 3Fachgebiet Geobotanik, Technische Universität München, Am Hochanger 13, D-85350 Freising-Weihenstephan, Germany
In small, fragmented populations of self-incompatible plant species, genetic drift and increasingly close relationships between plants may restrict the number of genetically different pollen donors, the availability of compatible mates, and the opportunity for pollen competition and selection. These restrictions may reduce the siring success or increase the probability of inbreeding depression in the offspring. To test if this was the case, we hand-pollinated maternal plants in small and large populations of the rare, endemic plant Cochlearia bavarica (Brassicaceae) with pollen from one, three, or nine donors from the same population or with nine donors from a different population. In one additional population of intermediate size, maternal plants were hand-pollinated with ten donors located at a distance of 1, 10, 100, or 1000 m. We then recorded seed and offspring characters. On average, offspring from small populations were smaller than normal and fewer survived to maturity. Increasing the number of pollen donors had a positive effect on reproductive success in small and large populations, but at the highest pollen diversity this occurred at the expense of slightly reduced offspring fitness. Because the total amount of transferred pollen was held constant, these effects could not be attributed to increasing pollen load. Rather, the increasing pollen diversity may have increased the chances of selecting a particularly "good" donor for fertilization—an example of a sampling effect of diversity. Pollen from outside a population or from 10–100 m away resulted in higher reproductive success and greater offspring size. Effects of population size and pollination treatments on reproductive success and offspring fitness were additive. Apparently, there is no obvious size threshold above which the potential of inbreeding depression can be ignored in C. bavarica.
Key Words: Brassicaceae • Cochlearia bavarica • endemic plant species • inbreeding depression • Munich, Germany • pollen competition • pollen diversity • pollination distance • population size • sampling effect
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