[Show abstract][Hide abstract] ABSTRACT: Like conventional crops, some GM cultivars may readily hybridize with their wild or weedy relatives. The progressive introgression of transgenes into wild or weedy populations thus appears inevitable, and we are now faced with the challenge of determining the possible evolutionary effects of these transgenes. The aim of this study was to gain insight into the impact of interspecific hybridization between transgenic plants and weedy relatives on the evolution of the weedy phenotype.
Experimental populations of weedy birdseed rape (Brassica rapa) and transgenic rapeseed (B. napus) were grown under glasshouse conditions. Hybridization opportunities with transgenic plants and phenotypic traits (including phenological, morphological and reproductive traits) were measured for each weedy individual. We show that weedy individuals that flowered later and for longer periods were more likely to receive transgenic pollen from crops and weed × crop hybrids. Because stem diameter is correlated with flowering time, plants with wider stems were also more likely to be pollinated by transgenic plants. We also show that the weedy plants with the highest probability of hybridization had the lowest fecundity.
Our results suggest that weeds flowering late and for long periods are less fit because they have a higher probability of hybridizing with crops or weed × crop hybrids. This may result in counter-selection against this subset of weed phenotypes, and a shorter earlier flowering period. It is noteworthy that this potential evolution in flowering time does not depend on the presence of the transgene in the crop. Evolution in flowering time may even be counter-balanced by positive selection acting on the transgene if the latter was positively associated with maternal genes promoting late flowering and long flowering periods. Unfortunately, we could not verify this association in the present experiment.
PLoS ONE 01/2011; 6(2):e14649. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It has long been supposed that variation in mating phenology leads to assortative mating, but its inherent frequency dependence has not been examined. When plants in a population vary in their flowering schedule, the phenotypic (and genetic) composition of the mating pool changes over the season; this causes phenological assortative mating even if pollen is exchanged at random during each interval of the season. Phenotype frequencies govern this temporal shift in the mating pool and this makes phenological assortative mating frequency dependent. We studied phenological assortative mating in four steps. First, we derived a method to estimate the phenotypic correlation between mates from flowering schedules; this correlation, symbolized by ρ, is the standard measure of assortative mating. Next, in a one-locus, two-allele system, we showed that the correlation between mates decreases as one or the other allele approaches fixation, but increases as the population deviates from Hardy-Weinberg propor-tions. Third, we showed that unlike assortative mating based on fixed preferences, the level of phenological assortment in one generation influences the level of assortment in the next, such that ρ reaches an equilibrium value that depends on allele frequencies. Finally, we contrasted the effects of frequency-dependent, phenological assortative mating on directional selection to the effects of fixed levels of assortative mating. When allelic effects were additive, frequency dependence slightly accelerated the selection response, compared with a fixed ρ. When one allele was dominant, phenological assortment slightly decelerated the selection response. Similarities between assortative mating through phenology and through habitat preference can cause the latter also to be frequency dependent.
[Show abstract][Hide abstract] ABSTRACT: The inevitable escape of transgenic pollen from cultivated fields will lead to the emergence of transgenic crop-wild plant hybrids in natural patches of wild plants. The fate of these hybrids and that of the transgene depend on their ability to compete with their wild relatives. Here we study ecological factors that may enhance the fitness of genetically modified hybrids relative to wild plants for a Bacillus thuringiensis ( Bt) transgene conferring resistance to insects. Mixed stands of wild plants and first-generation hybrids were grown under different conditions of herbivore pressure and density, with Bt oilseed rape ( Brassica napus) as the crop and B. rapa as the wild recipient. Biomass and fitness components were measured from plant germination to the germination of their offspring. The frequency of transgenic seedlings in the offspring generation was estimated using the green fluorescent protein marker. The biomass of F(1) Bt-transgenic hybrids relative to that of wild-type plants was found to be sensitive to both plant density and herbivore pressure, but herbivore pressure appeared as the major factor enhancing their relative fitnesses. In the absence of herbivore pressure, Bt hybrids produced 6.2-fold fewer seeds than their wild neighbors, and Bt plant frequency fell from 50% to 16% within a single generation. Under high herbivore pressure, Bt hybrids produced 1.4-fold more seeds, and Bt plant frequency was 42% in the offspring generation. We conclude that high-density patches of highly damaged wild plants are the most vulnerable to Bt-transgene invasion. They should be monitored early to detect potential transgene spread.
Theoretical and Applied Genetics 09/2004; 109(4):806-14. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It has been argued from first principles that plants mate assortatively by flowering time. However, there have been very few studies of phenological assortative mating, perhaps because current methods to infer paternal phenotype are difficult to apply to natural populations. Two methods are presented to estimate the phenotypic correlation between mates-the quantitative genetic metric for assortative mating-for phenological traits. The first method uses individual flowering schedules to estimate mating probabilities for every potential pairing in a sample. These probabilities are then incorporated into a weighted phenotypic correlation between all potential mates and thus yield a prospective estimate based on mating opportunities. The correlation between mates can also be estimated retrospectively by comparing the regression of offspring phenotype over one parent, which is inflated by assortative mating, to the regression over mid-parent, which is not. In a demonstration experiment with Brassica rapa, the prospective correlation between flowering times (days from germination to anthesis) of pollen recipients and their potential donors was 0.58. The retrospective estimate of this correlation strongly agreed with the prospective estimate. The prospective method is easily employed in field studies that explore the effect of phenological assortative mating on selection response and population differentiation.
American Journal of Botany 06/2004; 91(6):825-36. · 2.59 Impact Factor