Competition Affects the Production of First Backcross Offspring on F 1 -hybrids, Brassica Napus × B. Rapa

Summary Interspecific F 1 -hybrids may arise in fields with transplastomic oilseed rape where B . rapa occurs as a weed. Spilled seeds, including transplastomic F 1 -hybrids with B. rapa , may germinate, which creates an opportunity for production of transplastomic BC 1 with B . rapa as father (BC 1r ). Field trials were made with three different proportions of B . napus , B . rapa and F 1 -hybrids and three different densities. Contrary to most studies on how plant competition affects introgression between oilseed rape and B . rapa , this study focused on offspring produced on F 1 -hybrids, where the F 1 -hybrids had oilseed rape as maternal parent. We estimated the BC 1r production in all combinations of proportion and density, and found that B . rapa sired from 0.6–7.8% of the offspring. At the proportion with the highest abundance of F 1 -hybrids the entire paternity was assessed. There was a significant density effect on the production of BC 1r but the effect differed among proportions. Both the highest and lowest frequencies of BC 1r were obtained at high plant density. Neither the proportion nor density affected the number of BC 1r per square-meter significantly. Biomass components decreased significantly from low to intermediate density, whereas a further increase in density only affected the thousand-seed weight significantly. On the basis of the results from the present experiment we conclude that introgression of transgenes from transplastomic oilseed rape to B . rapa seems most likely at current field densities of B . napus , and when B . rapa is an abundant weed.