marker-assisted selection in wheat - ictsd

76Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishTable 4Percentage of introgressed loci (I%), at the heterozygous state, of 37 BC 4 plants that were derivedfrom four different BC 1 plants (Nos. 3, 11, 16, and 27)PlantnumberBC 1 BC 2 BC 3 BC 4Numberof lociI% NumberofplantsNumberof lociI% Number Numberof plants of lociI% NumberofplantsNumberof lociI%globalI%target/non-targetNo. 3 646 55 1 479 14 1 467 8 9 456 5 10/4No. 11 654 64 1 446 28 1 403 13 1 408 10 29/6No. 16 681 67 2 464 31 3 420 15 21 428 9 25/6No. 27 668 63 1 471 26 1 433 15 6 439 10 25/6Mean 62 26 14 8 21/5Note: The number of plants and of loci analysed at each generation are given. At the BC 4 generation, the percentage ofintrogression is also differentiated between target and non-target (as defined in Table 3) regions.Moderate deviations were observed fromtheoretical transmission values (62, 26, 14and 8 percent compared with 50, 25, 12.5and 6.25 percent at the BC 1 , BC 2 , BC 3 andBC 4 stages, respectively), with a bias infavour of a higher rate of G. barbadenseallele transmission. This bias was probablydue to the selection pressure imposedat least in the BC 3 and BC 4 generations.Throughout the BC 1 and BC 2 generationsthat have undergone no deliberate selection,the introgression of G. barbadensealleles (at the heterozygous state) coveredthe complete genome fairly well, i.e.introgressed segments were found on allof the 26 chromosomes (not shown). Thisresult contradicts the findings of Jiang et al.(2000) who detected important deficienciesin donor (G. barbadense) allele transmissionin a population of 3 662 BC 3 plantsoriginating from 21 BC 1 plants.After combining the SSR and AFLPmarker data, it was observed that the introgressionrate differed between target andnon-target regions. When averaged overthe 37 BC 4 plants, the percentage of introgressedloci (8 percent genome-wide) wasmuch lower in the non-target regions(5 percent) than that reached within targetregions (21 percent) (Table 4). The differentBC 4 plants introgressed between three andsix QTL-rich target regions in differentcombinations. As an illustration of theselection pressure applied through the useof molecular markers, Figure 3 shows thegraphical genotype of two BC 4 individualsas well as that of the BC 1 plant (No. 16)from which these individuals were derived.The two BC 4 plants had a common BC 1ancestor but originated from two differentBC 2 plants. In this particular example,starting from a common BC 1 plant (No.16) which harboured 13 out of 19 possibleQTL-rich regions, the two BC 4 plants(Nos. 104 and 419) derived from it partly orcompletely retained respectively five (c16,c23top, c23bot, c25 and A03) and four (c6,c25, c26 and A01bot) genomic regions carryingfavourable alleles. The other regionscarrying QTL on c3, c4, c23, c20, A01 andA03, which had been introgressed andwere heterozygous in the BC 1 plant, hadreturned to the homozygous G. hirsutum/G. hirsutum state. The percentages of introgressedloci in target and non-target regionsin these two examples were 29 and 10 percent,and of 29 and 5 percent in the two BC 4plants (Nos. 104 and 419) respectively.This example shows that, at least insome cases, the process used was efficientin selecting for chromosomal regionsof interest (foreground selection), whileallowing the rest of the genome to returntowards that of the recurrent parent.