**9. Progress and prospects of MAS in tomato breeding**

In most developing countries such as Ghana, the development of new cultivars, for example tomato, maize, groundnut, cowpeas, has been achieved through conventional plant breeding method rather than transgenic breeding. It generally comprises of sequences of imbrications of three corresponding stages:


segment is slower than the unlinked regions [76]. After identification of individuals using foreground markers, single and double recombinant individuals carrying the donor alleles as well as the recurrent parents are selected [77, 78]. The use of flanking markers are able to greatly reduce the undesirable segment of the donor parent compared to the conventional

**Figure 1.** Flowchart of foreground and background selection scheme. Source: http://passel.unl.edu/Image/siteImages/

MASFigure7Lg.jpg.

102 Recent Advances in Tomato Breeding and Production

This breeding method can take over five generations leading to increase in the number of years to develop an elite variety of a particular plant. Backcrossing is the breeding method, which involves transfer of alleles at one or more loci from a donor to an adapted variety or a desirable line [83, 84]. Recurrent backcrossing is the traditional backcrossing program based on the assumption proposed by [85] that the quantity of the recurrent parent genome is recovered at a rate of [1–(1/2)<sup>t</sup> + 1] where t is the number of generations of backcrossing. Thus, the expected recovery of the recurrent parent genome after six generations of backcrossing would be 99.2%, a situation called near-isogenic. An imperative objective of recurrent backcrossing is to reduce the effect of the donor genome, as the aim is to move just a few of its genes responsible for the target trait into the recurrent parent's genetic background. It is generally used to improve qualitatively inherited traits such as pests and diseases resistance, since the existence of target trait genes must be confirmed by individual phenotype in the successive cross-generations. Thus, individual phenotypic performance is a key indicator of the genotype, provided genes have a major effect on phenotypic performance and the phenotypic uncertainty is insignificant [86]. However, due to linkage between a target gene and nearby genes (which could code for economically undesirable traits) from the donor parent [87] and/or chance (stochastic or nonrandom positions of chiasmata), any specific backcross progeny will digress from this expectation. This digression has been experienced in couple of plants, for instance, where one tomato cultivar developed after 11 backcrosses still had the complete chromosome arm carrying the gene from the donor parent and introgressed fragments as large as 4 centimorgan (cM) found in tomato cultivars developed after 20 backcrosses, [88]. This was also found in a study conducted by [89] where the fragments around the introgressed genes in barleys diverse from about 1–14 cM in seven (7) generation backcrossed lines. Consequently, two main limitations of recurrent backcrossing approach have been identified:


reduces the required size of the backcross population and the time taken to obtain the desirable results [103, 104]. MAB has been effectively used to introgress disease-resistance gene

Marker-Assisted Selection (MAS): A Fast-Track Tool in Tomato Breeding

http://dx.doi.org/10.5772/intechopen.76007

105

The challenge associated with the utilization of MAB in the developing countries like Africa is the initial cost of developing the markers and the requisite laboratory equipment. For it to be welcomed and used effectively in these regions, the economic returns on their usage must far exceed the cost of using the conventional backcrossing. The initial cost could be funded

Tomato breeding evolved from conventional breeding where breeders directly selected for the traits of interest, to the use of morphological and physiological traits, differentiated domesticated crops from their wild ancestors. The limitations of these morphological markers gave rise to more efficient approaches with the emergence of genetic marker technologies since the turn of the nineteenth century. The discovery of DNA markers that are closely associated with the desired phenotypes has been used to track tissue, cell, chromosome or a gene in individuals and increased selection efficiency. A DNA marker is a fragment of DNA that contains large amounts of sequence information and closely linked to traits of importance. The close association of DNA markers with morphological and physiological traits has facilitated the development of several linkage maps and enhanced the selection efficiency in marker-assisted tomato breeding programs. Marker-assisted selection has been explored to increase precision and efficiency of selection for many economic traits in tomato breeding. One classical markerassisted approach in tomato breeding is marker-assisted backcrossing, which targets either the genetic background of the recurrent parent (background selection) or tracking the gene of interest (foreground selection) through the use of flanking markers. Marker-assisted backcrossing enables faster recovery of the recurrent parent genome compared with conventional backcrossing approaches. Due to the long duration of recurrent backcrossing approaches, adoption of marker-assisted backcrossing approaches will enhance selection efficiency and shorten the breeding process. The potential genetic and economic benefits of marker-assisted backcrossing

need to be compared with conventional breeding programs to determine their viability.

, Essie Blay<sup>3</sup>

3 West Africa Centre for Crop Improvement, University of Ghana, Legon, Ghana

, Joseph Adjebeng-Danquah2

, Jacinta A. Opoku1

and Hans Adu-Dapaah1

,

and improve fruit quality in tomatoes [27].

with aids from donors [105].

**10. Conclusion**

**Author details**

Agyemang Danquah3

Michael K. Osei1,3\*, Ruth Prempeh<sup>1</sup>

, Eric Danquah3

2 CSIR-Savanna Agricultural Research Institute, Tamale, Ghana

\*Address all correspondence to: oranigh@hotmail.com

1 CSIR-Crops Research Institute, Kumasi, Ghana

For the past three decades, an optimal number of molecular markers have been identified to be linked to traits of agronomic importance. These markers have been used as gene benchmarks to facilitate the introgression of genes of economic importance into elite varieties [91, 92]. Molecular markers are being used intensively to increase the efficiency of backcross breeding programs. This is what is termed as marker-assisted backcrossing (MAB) (also known as marker-assisted introgression, marker-assisted selection or molecular breeding). In the context of recurrent backcrossing, MAB amplified the pertinence of recurrent backcrossing at least in the following facets. Firstly, for traits that are simply inherited, but challenging or costly to identify phenotypically, and/or that do not have a reliable phenotypic expression under certain specific selection conditions, the efficiency of phenotypic selection is low. The use of markers for foreground selection makes the transfer of target genes feasible and economic. Secondly, quantitative traits, which are generally not targeted by a recurrent backcrossing approach, can be improved using recurrent backcrossing, if major quantitative trait loci (QTL) affecting the trait have been identified. Thirdly, markers provide an effective option to control linkage drag and to speed up the recovery of recurrent genome and make the use of genes contained in unadapted resources easier [93, 94]. Lastly, the number of backcross generations and the time required to eliminate unwanted fragments of donor parent genome to reach high level of similarity to the recurrent parent are lessened.

MAB is an accurate and an efficient process of introgression of major gene controlling a desired trait while retaining the vital features of the recurrent parent [95, 96]. MAB is the process of selecting an individual plant as the parent in a subsequent generation of a genetic improvement program using the results of DNA tests. Molecular markers used to perform DNA test are not influenced by the environment; hence, problems associated with conventional plant breeding (i.e., selection based on phenotype) are eliminated. Here, selection is concentrated on genes that control the desired traits directly and are detectable at all stages of plant growth. With the availability of an array of molecular markers [97] and genetic maps, MAB has become possible both for traits governed by single gene and quantitative trait loci (QTLs) [98]. The philosophy in marker development and implementation can be divided into three broad categories: genetic mapping [99], analyses of links between molecular markers and the trait of interest, and MAB [85, 94, 100].

Gene mapping is the method used to locate the locus of a gene and the distances between genes [101].

The closer a target gene is to another gene, the more likely they are inherited together [94, 100]. Therefore, the preferred condition for MAB is when a direct markers or gene assisted selection is used. This is a situation where molecular markers cosegregate or are closely linked with the desired trait [102]. The effective development of a marker that can be linked to a gene of interest leads to success of MAB. Hence, the assumption that the ideal distance between a molecular marker and a desirable gene initially isolated from wild germplasm be as close as 2 cM, while that of a marker and a target gene from elite into elite lines be close as 12 cM. This reduces the required size of the backcross population and the time taken to obtain the desirable results [103, 104]. MAB has been effectively used to introgress disease-resistance gene and improve fruit quality in tomatoes [27].

The challenge associated with the utilization of MAB in the developing countries like Africa is the initial cost of developing the markers and the requisite laboratory equipment. For it to be welcomed and used effectively in these regions, the economic returns on their usage must far exceed the cost of using the conventional backcrossing. The initial cost could be funded with aids from donors [105].
