The occurence of spontaneous hybridization between Brassica napus (oilseed rape) and Raphanus raphanistrum (wild radish) was investigated under different density conditions in cages and open‐field experiments. Hybrids with wild radish as the seed parent were identified by screening for herbicide resistance belonging to rape. Small seed size and intermediate morphology were used to screen for hybrids with rape as the seed parent. Leaf isozyme patterns and flow cytometry provided confirmation of hybrids. Wild radish in an oilseed rape field produced as many as three interspecific hybrids per 100 plants. This is the first report of such a spontaneous event. The frequency of hybrids is expected to range from 0.006 to 0.2% of the total seed produced, at P = 0.05. Male‐sterile oilseed rape plants surrounded by wild radish can produce up to 37 hybrids per plant. Seed production of the F1 hybrids and their F2 descendants was up to 0.4% and 2%, respectively, of that of wild radish. Gene escape from transgenic oilseed rape to wild related species is discussed.

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The soit seed bank of weeds was studied in two ticlds. The first was not Ircalcd with herbicides for nine years and had a high weed seed density. The second was treated every year and had a low weed seed density. A sludy ofthe probability distributit)ns of the weed seed populations in the soil showed that they were aggregated and appeared to have the negative binomial distribution. Transformations were therefore used to normalize the original values in order to compare the distributions and amounts of ditlerenl species at diHerent limes and locations. However, normalizations were possible only with species having an average ofat least two seeds per core. With the normalized values, it was observed that some species had similar statistical distributions in the same field. However, the distribution of a species may vary throughout the field, in time, and according to herbicide and culture rotations. Variations des parametres du stock .semencier d'udrcntices dans Ic .sol enfonction des espece.s. du temps cr du milieu Le stock semencier de mauvaises herbes a ete etudie dans le sol de deux champs. Lc premier n'a pas ete traite pendant neuf annees et presentait une forte densite de graines de mauvaises herbes. Le second ctait traite chaque annee et ne contenait qu'une faible densite de graines de mauvaises herbes. Une etude statistique de la distribution des populations de semences de mauvaises herbes dans le sol, a montre qu'elles etaient agregees et presentaient des distributions binomiales negatives. C'est pourquoi des transformations arithmctiques ont etc utilisees pour normaliser les valeurs brutes afin de pouvoir comparer les distributions et I'importance de differentes espcees en Tonction du temps et du lieu. Mais les nortnalisations n'ont etc possibles que pour les especes ayant en moyenne au moins deux graines par carotte. Avec les donnces normalisccs. il a ete observe que plusieurs especes ont des distributions statistiquement identiques dans le meme champ. Cependant la distribution d'unc meme espece peut varier a travers le champ, dans le temps et suivant ies rotations des cultures et des herbicides.

This study aims to investigate the relative plant growth and reproduction of insect-resistant and susceptible plants following the introgression of an insect-resistance Bt-transgene from Brassica napus, oilseed rape, to wild Brassica juncea. The second backcrossed generation (BC2) from a single backcross family was grown in pure and mixed stands of Bt-transgenic and non-transgenic siblings under two insect treatments. Various proportions of Bt-transgenic plants were employed in mixed stands to study the interaction between resistant and susceptible plants. In the pure stands, Bt-transgenic BC2 plants performed better than non-transgenic plants with or without insect treatments. In mixed stands, Bt-transgenic BC2 plants produced fewer seeds than their non-Bt counterparts at low proportions of Bt-transgenic BC2 plants in the absence of insects. Reproductive allocation of non-transgenic plants marginally increased with increasing proportions of Bt-transgenic plants under herbivore pressure, which resulted in increased total biomass and seed production per stand. The results showed that the growth of non-transgenic plants was protected by Bt-transgenic plants under herbivore pressure. The Bt-transgene might not be advantageous in mixed stands of backcrossed hybrids; thus transgene introgression would not be facilitated when herbivorous insects are not present. However, a relatively large initial population of Bt-transgenic plants might result in transgene persistence when target herbivores are present.