Potential gene flow from crops to wild relatives remains a central issue in discussions of the ecological risks involved in introducing genetically modified (GM) plants (Ellstrand et al., 1999; Andersson & de Vicente, 2010). Once a gene has dispersed from a crop to a wild relative it is beyond control, and this process may be irreversible. Care is thus required. Transgenes for herbicide resistance may hold no selective advantage outside agricultural fields but, for instance, a Bt gene that reduces herbivory may well be selected for in the wild (Vacher et al., 2004). The same issue pertains for transgenes that are currently developed to enhance plant growth under suboptimal conditions (Warwick et al., 2009). Introgression involves the incorporation of genes from one species into the gene pool of another. For introgression to occur, gene exchange between species must first result in viable hybrids. Secondly, these newly introduced alleles must be picked up by selection, perhaps aided by drift, and sweep through the population of the receptor species. Whether both aspects of introgression occur is still a matter of debate. Here, we review these outstanding issues using the genus Brassica as a model system.
Brassica napus L. (oilseed rape) has been cropped in Western Europe since the Middle Ages (Zohary & Hopf, 2000). The species crosses readily with Brassica rapa L. (Devos et al., 2009), a common self-incompatible weed occurring throughout Europe and North America. Hybrids resulting from such crosses and backcrosses to B. rapa have up to nine unpaired C-chromosomes (explained below) derived from B. napus. In a recent review, Andersson & de Vicente (2010) concluded that these unpaired C-chromosomes impose little or no fitness cost (see also Légère, 2005). If this view were correct, then wild B. rapa populations growing in an area where B. napus is frequently cropped should accumulate C-chromosomes. Surprisingly, population surveys of B. rapa in the UK (Wilkinson et al., 2000, 2003) and the Netherlands (Luijten & de Jong, 2010, 2011) only picked up F1 hybrids. By contrast, F1 and backcross hybrids were found in a Danish agricultural field converted to organic farming (Hansen et al., 2003). Hansen et al. (2001) suggested that backcrossing occurs much more frequently than hybridization between B. rapa and B. napus. Warwick et al. (2008) found F1 hybrids and backcrossed plants in B. rapa populations growing along the margins of GM canola fields in Canada; exact fractions were not given in this publication.
In this paper we ask under what scenarios C-chromosomes can persist in B. rapa populations, using a population genetic model that integrates detailed knowledge on hybrid crosses from the plant breeding literature, thereby highlighting discrepancies between different observations and interpretations. Our model has multiple hybrid classes that are characterized by the number of C-chromosomes that plants possess. Several earlier models in the field of crop-to-wild gene flow used F1, BC1, BC2 etcetera as different hybrid classes (for instance, Hooftman et al., 2007). Our model differs from those models in the sense that we link the presence of C-chromosome to reduced fitness rather than attributing an average fitness to BC1, BC2 etcetera.
We first outline some peculiarities about the Brassica system. An ancient cross between B. rapa (AA genome, 2n = 20) and Brassica oleracea L. (CC, 2n = 18) has resulted in the allopolyploid B. napus with the AACC (2n = 38) genome (U, 1935). Crosses between wild B. rapa and B. napus produce AAC hybrids with 2n = 29. Ten pairs of chromosomes belong to the A set and nine unpaired chromosomes to the C set. When an AAC hybrid is backcrossed to B. rapa, all chromosomes from the A genome are paired and each gamete contains a set of 10 A-chromosomes. The nine chromosomes from the C genome, however, are unpaired. These univalent C-chromosomes are passed on in crosses, resulting in plants with various C-chromosome numbers. Such plants can be grown and raised to maturity in the glasshouse. Homeologous recombination between A- and C-chromosomes can occur, starting with meiosis in the F1 hybrid, but its frequency is still under discussion (Leflon et al., 2006; Howell et al., 2008; Nicolas et al., 2009). In the model we ignore this aspect and keep A- and C-chromosomes strictly separate.
The chance that a specific unpaired C-chromosome is passed on to a gamete is theoretically 50%. An AAC hybrid contains all nine C-chromosomes, so the frequency of additional C-chromosomes in BC1 offspring (AA × AAC) is expected to follow a binomial distribution with a mean of 4.5. Although results from some studies have confirmed the binomial distribution (Mikkelsen et al., 1996), others have not (Table 1). Lu & Kato (2001) showed that the number of C-chromosomes per gamete was in close accordance with a binomial distribution, while in surviving embryos the distribution was skewed to the left (Table 1). They concluded that this biased transmission of C-chromosomes occurred during the embryo stage, as a result of either lower fertilization capacities of aneuploid gametes or higher abortion rates of aneuploid embryos. Indeed, seed set in AAC × AA crosses is typically very low (4.5%; Lee & Namai, 1992). However, Tomiuk et al. (2000) challenged the idea of gametic selection causing biased transmission. They argued that C-chromosomes form multivalents in the gametes and this clustering may also cause biased transmission, even without selective embryo abortion. Whether or not there is gametic selection against C-chromosomes can be ascertained by counting the total number of C-chromosomes passed on to the offspring in the AAC × AA cross. This involves summing all chromosome numbers in the BC1, weighted for the frequency at which they occur (Table 1). Instead of the expected transmission of 4.5 C-chromosomes (50%), only 3.6 C-chromosomes (40%) were passed on to the next generation in the data set of Lu et al. (2002). However, 50% were passed on in several other studies, which suggests no selection against gametes carrying C-chromosomes (Mackay, 1977; Fantes & Mackay, 1978; Leflon et al., 2006; see Table 1).
|Observed %||6.3||7.1||12.1||7.8||17||18.4||16.3||9.2||3.5||2.1||47.0||Leflon et al. (2006)1|
|Observed %||6||13||10||6||9||17||16||15||6||3||49.8||Fantes & Mackay (1978)|
|Observed %||16.1||13.9||10.4||13.6||9.8||9.1||8.3||7.5||6.1||5.4||40.0||Lu et al. (2002)|
|Observed %||28||38||4||3||3||4||9||0||3||8||26.3||Hansen et al. (2001)3|
Using Table 1, Lu et al. (2002) computed, with and without biased transmission, the distribution of the number of C-chromosomes in backcrosses of F1 hybrids (AAC) to B. rapa. In this paper we extend their idea to a large outcrossing population of B. rapa in which there is continuous input of C-chromosomes through crossing with an oilseed rape crop. We first consider the simplest case with nonbiased transmission of unpaired C-chromosomes and a binomial distribution of C-chromosomes in the gametes. Next we consider biased transmission of univalent C-chromosomes, by using data including gametic selection (Lu et al., 2002) or only the clustering of C-chromosomes in gametes (Fantes & Mackay, 1978). Finally, we incorporate weak and strong selection against adult plants carrying C-chromosomes in our model. For all cases, we examine how the shape of the frequency distribution of C-chromosomes in the recipient B. rapa population develops and ask how much selection is needed to eliminate C-chromosomes from the population.