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(American Journal of Botany. 2003;90:571-578.)
© 2003 Botanical Society of America, Inc.

Population Biology

Frequency-dependent fitness of hybrids between oilseed rape (Brassica napus) and weedy B. rapa (Brassicaceae)1

Thure P. Hauser2,4, Christian Damgaard3 and Rikke B. Jørgensen2

2Plant Research Department, Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark; 3National Environmental Research Institute, Vejlsøvej 25, 8600 Silkeborg, Denmark

Fitness of interspecific hybrids is sometimes high relative to their parents, despite the conventional belief that they are mostly unfit. F1 hybrids between oilseed rape (Brassica napus) and weedy B. rapa can be significantly more fit than their weedy parents under some conditions; however, under other conditions they are less fit. To understand the reasons, we measured the seed production of B. napus, B. rapa, and different generations of hybrid plants at three different densities and in mixtures of different frequencies (including pure stands). Brassica napus, B. rapa, and backcross plants (F1 x B. rapa) produced many more seeds per plant in pure plots than in mixtures and more seeds in plots when each was present at high frequency. The opposite was true for F1 plants that produced many more seeds than B. rapa in mixtures, but fewer in pure stands. Both vegetative and reproductive interactions may be responsible for these effects. Our results show that the fitness of both parents and hybrids is strongly frequency-dependent and that the likelihood of introgression of genes between the species thus may depend on the numbers and densities of parents and their various hybrid offspring in the population.

Key Words: backcross plants • Brassicaceae • density • fitness • frequency • interspecific hybrids • seed production






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