Matthew Robbins, The Ohio State University
The inbred backcross (IBC) population was proposed by Wehrhahn and Allard (1965) as a way of identifying genes or quantitative trait loci (QTL) that contribute to a quantitatively inherited trait. This is accomplished by developing a population that collectively contains most of the genome of a donor parent, divided among each individual line in the population. The majority of the genome of each line is from the recurrent parent, with a small portion from the donor parent. IBC breeding has also been employed for the introgression of exotic germplasm to improve quantitative traits in crop plants. This method has been utilized in bean (Bliss, 1981; Sullivan and Bliss, 1983), oilseed rape (Butruille et al., 1999), rice (Lin et al., 1998), cucumber (Robbins et al., 2008) and tomato (Hartman and St Clair, 1999; Doganlar et al., 2002; Kabelka et al., 2002; Kabelka et al., 2004; Yang et al., 2005, Robbins et al., 2009) for classical breeding and QTL studies.
Development of an IBC Population
The first stage of generating an IBC population (Fig. 1, steps 1–3) is similar to generating a backcross breeding population. One distinction is that many individuals are backcrossed to the recurrent parent to generate an IBC population. The second stage (Fig. 1, step 4) is similar to single-seed descent to generate recombinant inbred lines (RILs).
- An inbred donor parent is crossed to an inbred recurrent parent to produce an F1, which is fully heterozygous.
- The F1 is backcrossed to the recurrent parent to generate the BC1.
- A large number of BC1 individuals are backcrossed to the recurrent parent to generate the BC2 generation. Seed is saved from each individual. Each line is backcrossed to the recurrent parent for several generations. The total number of backcross generations, including the BC1 generation, is called k.
- Individuals in the BCk population are self-pollinated until they reach homozygosity (usually five or more generations) using the single-seed descent method. The IBC population consists of all of the individual backcross-inbred lines.
Figure 1. Schematic illustrating the development of an inbred backcross (IBC) population. Figure credit: Matthew Robbins, The Ohio State University.
An important consideration in creating an IBC population is the number of backcrossing generations. More backcrossing ensures that the IBC lines will be more like the recurrent parent, since the percentage of the genome from donor parent is reduced by half with each generation of backcrossing (see article on backcrossing). However, the probability of recovering the genes from the donor parent is reduced by half each generation due to the backcrossing process. The probability of recovering the gene(s) from the donor parent is (1/2)k+1 for a single gene and (1/2)2k+2 for two unlinked genes.
Advantages of an IBC Population
- An immortal population. Each line in an IBC population is inbred and can be propagated simply by self-pollination.
- The population can be replicated. Since each entry of the population is a line and not an individual, traits can be measured on a plot basis rather than an individual plant basis. This allows the population to be evaluated in multiple environments over years, which increases the precision of trait measurements.
- A breeding friendly population. Since the majority of the genome of each entry in an IBC population is from the recurrent parent, which is typically an elite line, IBC lines can directly be used in crosses with minimal germplasm improvement.
- Mapping quantitative traits. The structure of IBC populations makes them a good population for mapping quantitative traits using single factor analysis.
- Simultaneous discovery and introgression. Quantitative traits can be mapped and introgressed in the same population.
Disadvantages of an IBC Population
- Time. Developing an IBC population requires a minimum of eight generations (Fig. 1).
- Limited ability to study epistatic interactions. Since only a small part of the donor genome is represented in each line, it is difficult to study the interaction of multiple, unlinked genes from the donor parent.
- Not amenable to some QTL mapping methods. Because the structure of an IBC population is not a simple segregating population, the algorithms of the majority of QTL mapping software are not designed to work with this population type. It is not practical to use interval or composite interval mapping methods on an IBC population.
- Bliss, F. A. 1981. Utilization of vegetable germplasm. HortScience 16: 129–132.
- Butruille, D. V., R. P. Guries, and T. C. Osborn. 1999. Linkage analysis of molecular markers and quantitative trait loci in populations of inbred backcross lines of Brassica napus L. Genetics 153: 949–964. (Available online at: http://www.genetics.org/cgi/content/full/153/2/949) (verified 23 Sept 2010).
- Doganlar, S., A. Frary, H. M. Ku, and S. D. Tanksley. 2002. Mapping quantitative trait loci in inbred backcross lines of Lycopersicon pimpinellifolium (LA1589). Genome 45: 1189–1202. (Available online at: http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=0831-2796&volume=45&issue=6&startPage=1189) (verified 23 Sept 2010).
- Hartman, J. B., and D. A. St.Clair. 1999. Combining ability for beet armyworm (Spodoptera exigua) resistance and horticultural traits of selected Lycopersicon pennellii-derived inbred backcross lines of tomato. Plant Breeding 118: 523–530.
- Kabelka, E., B. Franchino, and D. M. Francis. 2002. Two loci from Lycopersicon hirsutum LA407 confer resistance to strains of Clavibacter michiganensis subsp. michiganensis. Phytopathology 92: 504–510. (Available online at: http://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO.2002.92.5.504) (verified 23 Sept 2010).
- Kabelka, E., W. Yang, and D. M. Francis. 2004. Improved tomato fruit within an inbred backcross line derived from Lycopersicon esculentum and L. hirsutum involves the interaction of loci. Journal of the American Society of Horticultural Science 129: 250–257.
- Lin, S. Y., T. Sasaki, and M. Yano. 1998. Mapping quantitative trait loci controlling seed dormancy and heading date in rice, Oryza sativa L., using backcross inbred lines. Theoretical and Applied Genetics 96: 997–1003.
- Robbins, M. D., M. D. Casler, and J. E. Staub. 2008. Pyramiding QTL for multiple lateral branching in cucumber using inbred backcross lines. Molecular Breeding 22: 131–139.
- Robbins, M. D., A. Darrigues, S. Sim, M.A.T. Masud, and D. M. Francis. 2009. Characterization of hypersensitive resistance to bacterial spot race T3 (Xanthomonas perforans) from tomato accession PI 128216. Phytopathology 99: 1037–1044. (Available online at: http://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO-99-9-1037) (verified 27 Sept 2010).
- Sullivan, J. G., and F. A. Bliss. 1983. Expression of enhanced seed protein content in inbred backcross lines of common bean. Journal of American Society of Horticultural Science 108: 787–791.
- Yang, W., E. J. Sacks, M. L. Lewis-Ivey, S. A. Miller, and D. M. Francis. 2005. Resistance in Lycopersicum esculentum intraspecific crosses to race T1 strains of Xanthomonas campestris pv. vesicatoria causing bacterial spot of tomato. Phytopathology 95: 519–527. (Available online at: http://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO-95-0519) (verified 27 Sept 2010).
- Wehrhahn, C., and R. W. Allard. 1965. Detection and measurement of the effects of individual genes involved in the inheritance of a quantitative character in wheat. Genetics 31: 109–119.
Development of this lesson was supported in part by the National Institute of Food and Agriculture (NIFA) Solanaceae Coordinated Agricultural Project, agreement 2009-85606-05673, administered by Michigan State University. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the United States Department of Agriculture.