1. ¹Plant Research Department, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark
2. ²Department of Ecology and Genetics, Institute of Biology, University of Aarhus, Building 540, Ny Munkegade, DK-8000 Aarhus C, Denmark
3. ³Department of Terrestrial Ecology, DMU, Vejlsøvej 25, DK-8600 Silkeborg, Denmark

Correspondence: T P Hauser, Plant Research Department, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark. E-mail: thure.hauser@risoe.dk

Received 20 December 2001; Accepted 7 June 2002.

Abstract

The likelihood that two species hybridise and backcross may depend strongly on environmental conditions, and possibly on competitive interactions between parents and hybrids. We studied the paternity of seeds produced by weedy Brassica rapa growing in mixtures with oilseed rape (B. napus) and their F₁ hybrids at different frequencies and densities. Paternity was determined by the presence of a transgene, morphology, and AFLP markers. In addition, observations of flower and pollen production, and published data on pollen fertilisation success, zygote survival, and seed germination, allowed us to estimate an expected paternity. The frequency and density of B. napus, B. rapa, and F₁ plants had a strong influence on flower, pollen, and seed production, and on the paternity of B. rapa seeds. Hybridisation and backcrossing mostly occurred at low densities and at high frequencies of B. napus and F₁, respectively. F₁ and backcross offspring were produced mainly by a few B. rapa mother plants. The observed hybridisation and backcrossing frequencies were much lower than expected from our compilation of fitness components. Our results show that the male fitness of B. rapa, B. napus, and F₁ hybrids is strongly influenced by their local frequencies, and that male fitness of F₁hybrids, when pollinating B. rapa seeds, is low even when their female fitness (seed set) is high.