Academic journal article Genetics

Gene Introgression in Weeds Depends on Initial Gene Location in the Crop: Brassica napus–Raphanus Raphanistrum Model

Academic journal article Genetics

Gene Introgression in Weeds Depends on Initial Gene Location in the Crop: Brassica napus–Raphanus Raphanistrum Model

Article excerpt

IN spite of multiple reproductive barriers between species, interspecific hybridization is considered a major evolutionary force in plants due to their ability to produce homoploid (same chromosome number as the parents) or polyploid (whole genome duplication of the F1 hybrid genome) hybrids, or to introgress genomic regions from one species to another. The latter situation is of particular importance in the evolution of crop-weed complexes (Rieseberg and Willis 2007; Nosil 2008; Ellstrand et al. 2013; Gressel 2015). Many studies have demonstrated that closely related plant species are able to hybridize under natural conditions (Jenczewski et al. 2003; Andersson and De Vicente 2010; Ellstrand 2014). The rate of interspecific hybridization depends on sympatry and flowering period overlap, the relatedness between species, and their ploidy level. When interspecific or intergeneric hybrids produce fertile progeny after recurrent natural pollinations by the weed, the question remains of the occurrence of crop gene introgression within the genome of wild species.

Genome structure was proposed as one of the main barriers to introgression (Stewart et al. 2003). It has been shown that particular combinations of parental genomes are selected for the emergence of viable hybrid, offspring generating new species, which can present a transgressive phenotype better adapted to new ecological niches (Rieseberg and Willis 2007; Heredia and Ellstrand 2014). This indicates that some genomic regions may not present the same likelihood of being maintained after an interspecific hybridization event (Hufford et al 2013; Barb et al 2014). Stewart et al (2003) and Ellstrand et al (2013) suggest that chromosomal blocks are the unit of introgression, which can be linked with domestication alleles influencing the stability of the introgressed weed. This situation was demonstrated for gene flow from Lactuca sativa to L. serriola, with the identification of hotspots or cold regions (Hooftman et al 2011). Such data are missing concerning intergeneric hybrids from species with different ploidy levels, where it depends on the occurrence of intergenomic recombination between the parental genomes. It is thus likely that the genomic regions presenting the highest sequence similarity between two species have the highest probability to recombine in interspecific hybrids (Benavente et al. 2008). However, it is still unknown whether the location of a gene within a genome will modify its probability to be introgressed in the genome of a weed belonging to a closely related genus. For this purpose, experiments at the whole genome level are essential, and will aid in deciphering whether some genomic regions are more likely to be introgressed. Results from such studies will determine the best gene location, using new technology to target specific sites (Lu et al. 2016), in order to prevent crop gene escape in weeds.

There are several reasons why the Brassiceae tribe (46 genera) (Al-Shehbaz 2012) is a good model system to study the putative impact of initial gene location in a crop, and transfer to a weedy related genome. First, oilseed rape (Brassica napus, AACC, 2n = 4x = 38), an allopolyploid species arising from the hybridization between B. rapa (AA, 2n = 2x = 20) and B. oleracea (CC, 2n = 2x = 18), is one of the main transgenic crops cultivated worldwide (mainly for herbicide tolerance: www.isaaa.org), and has several closely related weedy species belonging to the same tribe (Murat et al 2015). Second, gene flow between oilseed rape and its most frequent weeds has already been assessed (Chevre et al. 2004; Fitzjohn et al 2007; Liu et al. 2013). In B. rapa, one of the progenitors of B. napus, it has been shown that gene introgression can occur in two generations (Mikkelsen et al 1996). It is expected that introgression through homologous recombination occurs more easily when the transgene is located on the A genome (shared between B. napus and wild B. rapa). …

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