Populations of weedy crop-wild hybrid beets show contrasting variation in mating system and population genetic structure

Abstract

Reproductive traits are key parameters for the evolution of invasiveness in weedy crop-wild hybrids. In Beta vulgaris, cultivated beets hybridize with their wild relatives in the seed production areas, giving rise to crop-wild hybrid weed beets. We investigated the genetic structure, the variation in first-year flowering and the variation in mating system among weed beet populations occurring within sugar beet production fields. No spatial genetic structure was found for first-year populations composed of F1 crop-wild hybrid beets. In contrast, populations composed of backcrossed weed beets emerging from the seed bank showed a strong isolation-by-distance pattern. Whereas gametophytic self-incompatibility prevents selfing in wild beet populations, all studied weed beet populations had a mixed-mating system, plausibly because of the introgression of the crop-derived Sf gene that disrupts self-incompatibility. No significant relationship between outcrossing rate and local weed beet density was found, suggesting no trends for a shift in the mating system because of environmental effects. We further reveal that increased invasiveness of weed beets may stem from positive selection on first-year flowering induction depending on the B gene inherited from the wild. Finally, we discuss the practical and applied consequences of our findings for crop-weed management.

Keywords: Beta vulgaris; genetic structure; invasive species; outcrossing rate; timing of flowering; weediness.

Figure 1

Variation in the average kinship coefficient (F_ij) between pairs of weed beet individuals according to (log-transformed) geographical distance for populations A, B, C and D. Ten distance classes were defined in such a way that the number of pairwise comparisons within each distance interval was constant. Dashed lines depict the 95% (two-tailed) confidence interval for the null hypothesis of complete spatial randomness of genotypes. Sp statistics and their significance are also indicated for each population. NS, nonsignificant; *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 2

(A) Variation in first-year flowering (bolting rate) and population-level outcrossing rate (t_m) in weed beets for populations A, B, C and D. Bolting rates were estimated in the greenhouse after 28 weeks in noninductive conditions (temperature ranging from 18.5 to 30.5°C; 16 h/8 h day/night period). The vertical error bars refer to the standard errors for both rates. (B) Variation in bolting rate and individual-level outcrossing rate (t_m) according to weed beet classification: ‘in-row bolters’ found within the row of cultivation and ‘out-row bolters’ found outside the row of cultivation. The vertical error bars refer to the standard errors for both rates.

Figure 3

Relationship between individual-level outcrossing rates (t_m) and the local density measured as the number of flowering plants within a radius of 20 m around each individual within populations A, B, C and D. Correlation coefficients associated with regression lines are equal to 0.201, −0.132, 0.268 and 0.062 for population A, B, C and D respectively. Logistic regressions suggested no effect of local density on individual outcrossing rates (all at P > 0.05 with χ²₁ = 0.51, 0.23, 0.82 and 0.18 for population A, B, C and D respectively).