Gene Introgression in Weeds Depends on Initial Gene Location in the Crop: Brassica napus- Raphanus raphanistrum Model

Abstract

The effect of gene location within a crop genome on its transfer to a weed genome remains an open question for gene flow assessment. To elucidate this question, we analyzed advanced generations of intergeneric hybrids, derived from an initial pollination of known oilseed rape varieties (Brassica napus, AACC, 2n = 38) by a local population of wild radish (Raphanus raphanistrum, RrRr, 2n = 18). After five generations of recurrent pollination, 307 G5 plants with a chromosome number similar to wild radish were genotyped using 105 B. napus specific markers well distributed along the chromosomes. They revealed that 49.8% of G5 plants carried at least one B. napus genomic region. According to the frequency of B. napus markers (0-28%), four classes were defined: Class 1 (near zero frequency), with 75 markers covering ∼70% of oilseed rape genome; Class 2 (low frequency), with 20 markers located on 11 genomic regions; Class 3 (high frequency), with eight markers on three genomic regions; and Class 4 (higher frequency), with two adjacent markers detected on A10. Therefore, some regions of the oilseed rape genome are more prone than others to be introgressed into wild radish. Inheritance and growth of plant progeny revealed that genomic regions of oilseed rape could be stably introduced into wild radish and variably impact the plant fitness (plant height and seed number). Our results pinpoint that novel technologies enabling the targeted insertion of transgenes should select genomic regions that are less likely to be introgressed into the weed genome, thereby reducing gene flow.

Keywords: gene flow; intergeneric hybridization; introgression; oilseed rape; wild radish.

Figure 1

Genealogy of the hybrids produced under field conditions.

Figure 2

Analysis of the G5 plants having a chromosome number similar to wild radish (2n = 18). The first circle represents the 19 B. napus linkage groups (in cM) and the genetic location of the B. napus specific markers, according to Wang et al. (2011), is indicated between brackets after the marker name. The second circle indicates the frequency of each marker in the G5 plants; the color corresponds to the four statistical classes to which belongs each marker (red for the hot spot, green for medium spots, brown for low spots, and blue for cold spots). The number of plants carrying one, two, three, four, five, or all adjacent markers per chromosome is indicated in the internal pink circles.

Figure 3

Information criteria as the functions of the number of clusters K with AIC = −2log ($\widehat{L}$) + 2(2K−1) (black line) and BIC = −2log ($\widehat{L}$) + (2K–1) log(N) (blue line). $\widehat{L}$ denotes the maximized value of the likelihood function for the statistical model, and N is the loci sample size. The model with K = 4 groups of markers minimizes both AIC and BIC criteria.

Figure 4

Number of independent introgressions in the G5 plants carrying oilseed rape markers.

Figure 5

(A–E) BAC-FISH/FISH was carried out using BAC KBrH080A08 and 45S rDNA that identify the A10 and C09 chromosomes (red) and all chromosomes carrying the 45S rDNA locus (green), respectively. FISH analyses of somatic metaphase chromosomes of B. napus (A), R. raphanistrum (B), mother plant with A10 and C09 introgression (C), and two plants of its progeny (D) and (E). Chromosomes were counterstained with DAPI (blue). Bar, 5 mm.

Figure 6

(A–F) BAC-FISH/FISH was carried out using BAC KBrH117M18 and 45S rDNA, which identify the A03 and C03 chromosomes (red) and all chromosomes carrying the 45S rDNA locus (green), respectively. FISH analyses of somatic metaphase chromosomes of B. napus (A), R. raphanistrum (B), mother plant with A03 introgression (C), and three plants of its progeny (D), (E), and (F). Chromosomes were counterstained with DAPI (blue). Bar, 5 mm.