Tracing back seed and pollen flow within the crop–wild Beta vulgaris complex: genetic distinctiveness vs. hot spots of hybridization over a regional scale

Abstract

Hybrids between transgenic crops and wild relatives have been documented successfully in a wide range of cultivated species, having implications on conservation and biosafety management. Nonetheless, the magnitude and frequency of hybridization in the wild is still an open question, in particular when considering several populations at the landscape level. The Beta vulgaris complex provides an excellent biological model to tackle this issue. Weed beets contaminating sugar beet fields are expected to act as a relay between wild populations and crops and from crops-to-crops. In one major European sugar beet production area, nine wild populations and 12 weed populations were genetically characterized using cytoplasmic markers specific to the cultivated lines and nuclear microsatellite loci. A tremendous overall genetic differentiation between neighbouring wild and weed populations was depicted. However, genetic admixture analyses at the individual level revealed clear evidence for gene flow between wild and weed populations. In particular, one wild population displayed a high magnitude of nuclear genetic admixture, reinforced by direct seed flow as evidenced by cytoplasmic markers. Altogether, weed beets were shown to act as relay for gene flow between crops to wild populations and crops to crops by pollen and seeds at a landscape level.

Evaluation of rhizomania-resistance segregating sequences and overall genetic diversity pattern among selected accessions of Beta and Patellifolia. Potential implications of breeding for genetic bottlenecks in terms of rhizomania resistance

“…Since weed beets result from accidental hybridizations between wild and cultivated forms, they represent a genetically differentiated group as compared to wild beets and may be expected to display an intermediate position in the dendrogram of genetic distance. What is more, they often manifest a relatively high allelic richness, thus more closely resembling their wild counterparts than cultivated beets in this particular context (Desplanque et al 1999;Viard et al 2004;Fénart et al 2008), which was also evident in our study for the wbM population. Besides, genetic isolation and relatively high heterozygote deficiencies have been reported in many weed beet populations, suggesting their independent origins.…”

“…Its location was nevertheless the most distant within this group and as a result also visibly closer to the cluster of cultivars than that for any other wild/weedy beet examined. Several mitotypes can be potentially found within weedy populations, most probably derived from wild or ancient cultivated materials (Viard et al 2004;Fénart et al 2008). However, the most common for cultivated beets Owen CMS mitotype that was identified by us, suggests a cultivated maternal origin of the weedy accession under study.…”

“…Such introgressions may be the source of novel traits for breeding and result in increased genetic diversity on one hand. On the other, they also influence the wild beet genetic pool on the long run, thus creating potential risks for a transgene escape and/or gradual depletion in allelic richness as a consequence of swamping with more invasive forms (Bartsch et al 1999;Viard et al 2004;Arnaud et al 2009). Arnaud et al (2009) found for example that there was a dilution of crop nuclear genes in contrast to cytoplasmic pool in inland wild populations of the former French seed production areas, thus suggesting past seed escapes.…”

“…Besides, genetic isolation and relatively high heterozygote deficiencies have been reported in many weed beet populations, suggesting their independent origins. Other important features include self-fertility inheritance and substructuring within a population (Viard et al 2002(Viard et al , 2004Fénart et al 2008). It is believed that these complexes usually consist of a mixture of spatially and temporally isolated genotypes, potentially resulting from multiple recurrent hybridization events between diverse sources, including a long-lived seed bank in the field, but also cross-pollinating or selfing in a density-dependent pattern (Fénart et al 2007).…”

Transgene escape in sugar beet production fields: data from six years farm scale monitoring

“…However, these data were obtained in specially designed experiments, not in genuine agricultural conditions, and they cannot provide quantitative estimates. Other studies reported on the resulting gene flow inferred by using molecular markers (Andersen et al, 2005;Viard et al, 2004). Our paper focuses on the amount and distance of pollen dispersal within the GMO farm scale studies, and the transgene escape through seed sets of the sugar beet bolters, from 1996 to 2002.…”