Using linked markers to estimate the genetic age of a volunteer population: a theoretical and empirical approach.

Volunteers deriving from unharvested seeds of a crop can lead to persistent feral populations and participate in genetic exchanges across the agro-ecosystem, both between crop varieties and between crops and their wild relatives. A first step to understand the importance of volunteers is to characterize their capacity to reproduce autonomously for several generations. For that purpose, we constructed and evaluated a maximum-likelihood method to estimate the genetic age of a population deriving from one of the most common field crop type: an F1-hybrid variety. The method estimates the number of reproduction cycles that occurred since the cultivation of that variety. It makes use of genotypic data at a number of linked microsatellite loci pairs, thus exploiting the recombination of parental haplotypes, which is expected to occur as the population is reproducing. Estimates with moderate bias and variance were found for a broad range of parameter values in simulations, and the method revealed robust to some deviations from the assumptions of the underlying model. We propose a specific procedure to test the hypothesis of persistence, that is has a given volunteer population experienced more than one cycle of reproduction since the F1-hybrid state? The method was applied to both an experimental and a natural sunflower volunteer population and revealed promising, considering these ideal case studies. Possible further developments toward more complex natural systems are discussed.

The effects of the genetic background on herbicide resistance fitness cost and its associated dominance in Arabidopsis thaliana.

The advantage of the resistance conferred by a mutation can sometimes be offset by a high fitness-cost penalty. This balance will affect possible fate of the resistance allele. Few studies have explored the impact of the genetic background on the expression of the resistance fitness cost and none has attempted to measure the variation in fitness-cost dominance. However, both the fitness penalty and its dominance may modify evolutionary trajectory and outcome. Here the impact of Arabidopsis thaliana intraspecific genetic diversity on fitness cost and its associated dominance was investigated by analysing 12 quantitative traits in crosses between a mutant conferring resistance to the herbicide 2,4-D and nine different natural genetic backgrounds. Fitness cost values were found to be more affected by intraspecific genetic diversity than fitness cost dominance, even though this effect depends on the quantitative trait measured. This observation has implications for the choice of the best strategy for preventing herbicide resistance development. In addition, our results pinpoint a potential compensatory improvement of the resistance fitness cost and its associated dominance by the genetic diversity locally present within a species.

Low frequency transmission of a plastid-encoded trait in Setaria italica.

It has been claimed that engineering traits into the chloroplast will prevent transgene transmission by pollen, precluding transgene flow from crops. A Setaria italica (foxtail or birdseed millet) with chloroplast-inherited atrazine resistance (bearing a nuclear dominant red-leaf base marker) was crossed with five male-sterile yellow- or green-leafed herbicide susceptible lines. Chloroplast-inherited resistance was consistently pollen transmitted at a 3x10(-4 )frequency in >780,000 hybrid offspring. The nuclear marker segregated in the F(2), but resistance did not segregate, as expected. Pollen transmission of plastome traits can only be detected using both large samples and selectable genetic markers. The risk of pollen transmission at this frequency would be several orders of magnitude greater than spontaneous nuclear-genome mutation-rates. Chloroplast transformation may be an unacceptable means of preventing transgene outflow, unless stacked with additional mechanisms such as mitigating genes and/or male sterility.

Effect of a gap on gene flow between otherwise adjacent transgenic Brassica napus crops.

Gene flow resulting from cross pollination becomes an issue when transgenic crops are involved and the genetic modification carries a trait of ecological importance. As crop fields are often separated by a barren gap, such as an intervening roadway or unplanted area, I measured cross contamination between two herbicide-resistant transgenic fields (canola, Brassica napus) across a gap of up to 12 m. I focused on pollen exchange from the field border up to 7 m inside each field over two seasons. In the absence of a gap, I found that gene dispersal diminished rapidly with distance, with more than 40% of transgenic progeny found within the first meter from the edge of the adjacent crop. Cross contamination between fields declined more rapidly when there were intervening plants, however. Plants separated from the transgenic source by a gap of 3-4 m, yielded the same level of transgenic progeny as those separated by 1 m of crop. Both insects and wind pollinate canola, and so the explanation for my observations could involve the influence of gaps on wind patterns or on the behaviour of pollinators. The gap effect does not seem to depend only upon the variation in the density of neighbours that surrounds those plants at the crop edge versus those in the crop matrix. On the basis of this study, it is recommended that economic profit would be maximised by removing field borders after flowering rather than by leaving a surrounding gap, which would need to occupy up to threefold as much field surface to achieve the same level of containment.

The evolutionary dynamics of selfish replicators: a two-level selection model.

The aim of the present paper is to study the evolutionary dynamics of selfish replicators in a constant genetic background. Selfish replicators are viewed as alleles at a single locus, having a pleiotropic effect. Infinitely many alleles are possible; they act on individual fitness and have various levels of ability to distort segregation. This results in a two-level process of selection, including inter-individual selection (effect on individual fitness) and intra-individual selection (ability to distort segregation). The model takes other parameters into account, such as dominance, inbreeding and inbreeding depression. The system can have two different behaviours. (1) In some cases, evolutionary cycles are possible. The cycles correspond to an alternation of phases with predominant inter-individual selection, corresponding to major-effect mutations, and phases with predominant intra-individual selection, corresponding to small-effect mutations. (2) For other values of the parameters, a synthetic fitness can be defined: this absolute allelic fitness is estimated as a function of one's fitness due to both inter-individual and intra-individual selection. During the course of evolution, the synthetic fitness increases. The optimisation of a synthetic fitness is the most general process. The optimised value is essentially homologous to the value optimised for resource allocation to male and female function in hermaphrodites (female function being homologous to the effect on individual fitness, and male function being homologous to distortion ability). The relative importance of both behaviours is discussed. It is argued that repeated sequences causing some human degenerative hereditary diseases may follow a two-step evolutionary process: a progressive increase in number of sequences accompanied by a decrease of the individual fitness would be followed by massive elimination of such sequences. But in general the optimisation of the synthetic fitness seems to be more likely.

A method to determine the mean pollen dispersal of individual plants growing within a large pollen source.

Pollen dispersal has been recently focused on as a major issue in the risk assessment of transgenic crop plants. The shape of the pollen dispersal of individual plants is hard to determine since a very large number of plants must be monitored in order to track rare longdistance dispersal events. Conversely, studies using large plots as a pollen source provide a pollen distribution that depends on the shape of the source plot. We report here on a method based on the use of Fourier transforms by which the pollen dispersal of a single, average individual can be obtained from data using large plots as pollen sources, thus allowing the estimation of the probability of long-distance dispersal for single plants. This method is subsequently implemented on simulated data to test its susceptibility to random noise and edge effects. Its conditions of application and value for use in ecological studies, in particular risk assessment of the deliberate release of transgenic plants, are discussed.

Outcrossing and hybridization in wild and cultivated foxtail millets: consequences for the release of transgenic crops.

Outcrossing rates within the wild green foxtail, Setaria viridis, and the cultivated foxtail millet, S. italica, are very low. However, spontaneous interspecific hybridizations in the experimental garden occurred in both directions at rates ranging from 0.002% to 0.6% according to plant density and distance between parents. Offtypes found in farmers' fields where foxtail millet is cultivated were shown to have originated from such interspecific crosses. Differences in the EcoR1 patterns of chloroplast DNA between cultivated and wild plants indicated that reciprocal crosses do occur in the field. These findings indicate that even a largely selfing cultivated species may exchange genetic information with wild relatives at rates that may cause problems if transgenic cultivars are released.

The cost of herbicide resistance measured by a competition experiment.

The cost of resistance has been measured by a competition experiment over a range of densities, in the absence of herbicide treatment, on two nearly isogenic lines of Foxtail millet, differing in a chloroplastic resistance to herbicide. Three characters have been measured: shoot height, shoot weight, and seed production. Sensitive individuals were better competitors despite a larger decrease in production under within-biotype competition. The cost of resistance was density dependent and increased with density. The cost was higher when measured on seed production and reached 65% at the higher density for resistant individuals. This is compatible with the low frequency or the absence of that gene in natural populations. This work illustrates that the cost is easiest to observe when high levels of constraints are used.