**8. Marker-based breeding and conventional breeding: Challenges and perspectives**

Marker-assisted breeding became a new member in the family of plant breeding as various types of molecular markers in crop plants were developed during the 1980s and 1990s. The extensive use of molecular markers in various fields of plant science, e.g. germplasm evalua‐ tion, genetic mapping, map-based gene discovery, characterization of traits and crop im‐ provement, has proven that molecular technology is a powerful and reliable tool in genetic manipulation of agronomically important traits in crop plants. Compared with conventional breeding methods, MAB has significant advantages:


The research and use of MAB in plants has continued to increase in the public and private sectors, particularly since 2000s. However, MAS and MABC were and are primarily con‐ strained to simply-inherited traits, such as monogenic or oligogenic resistance to diseases/ pests, although quantitative traits were also involved (Collard and Mackill, 2008; Segmagn et al., 2006; Wang and Chee, 2010). The application of molecular markers in plant breeding has not achieved the results as expected previously in terms of extent and success (e.g. re‐ lease of commercial cultivars). Collard and Mackill (2008) listed ten reasons for the low im‐ pact of MAS and MAB in general. Improvement of most agronomic traits that are of complicated inheritance and economic importance like yield and quality is still a great chal‐ lenge for MAB including the newly developed GS. From the viewpoint of a plant breeder, MAB is not universally or necessarily advantageous. The application of molecular technolo‐ gies to plant breeding is still facing the following drawbacks and/or challenges:


**8. Marker-based breeding and conventional breeding: Challenges and**

Marker-assisted breeding became a new member in the family of plant breeding as various types of molecular markers in crop plants were developed during the 1980s and 1990s. The extensive use of molecular markers in various fields of plant science, e.g. germplasm evalua‐ tion, genetic mapping, map-based gene discovery, characterization of traits and crop im‐ provement, has proven that molecular technology is a powerful and reliable tool in genetic manipulation of agronomically important traits in crop plants. Compared with conventional

**a.** MAB can allow selection for all kinds of traits to be carried out at seedling stage and thus reduce the time required before the phenotype of an individual plant is known. For the traits that are expressed at later developmental stages, undesirable genotypes can be quickly eliminated by MAS. This feature is particularly important and useful for some breeding schemes such as backcrossing and recurrent selection, in which crossing

**b.** MAB can be not affected by environment, thus allowing the selection to be performed under any environmental conditions (e.g. greenhouse and off-season nurseries). This is very helpful for improvement of some traits (e.g. disease/pest resistance and stress tol‐ erance) that are expressed only when favorable environmental conditions present. For low-heritability traits that are easily affected by environments, MAS based on reliable markers tightly linked to the QTLs for traits of interest can be more effective and pro‐

**c.** MAB using co-dominance markers (e.g. SSR and SNP) can allow effective selection of recessive alleles of desired traits in the heterozygous status. No selfing or test crossing is needed to detect the traits controlled by recessive alleles, thus saving time and accel‐

**d.** For the traits controlled by multiple genes/QTLs, individual genes/QTLs can be identi‐ fied and selected in MAB at the same time and in the same individuals, and thus MAB is particularly suitable for gene pyramiding. In traditional phenotypic selection, howev‐ er, to distinguish individual genes/loci is problematic as one gene may mask the effect

**e.** Genotypic assays based on molecular markers may be faster, cheaper and more accu‐ rate than conventional phenotypic assays, depending on the traits and conditions, and thus MAB may result in higher effectiveness and higher efficiency in terms of time, re‐

The research and use of MAB in plants has continued to increase in the public and private sectors, particularly since 2000s. However, MAS and MABC were and are primarily con‐ strained to simply-inherited traits, such as monogenic or oligogenic resistance to diseases/ pests, although quantitative traits were also involved (Collard and Mackill, 2008; Segmagn

**perspectives**

74 Plant Breeding from Laboratories to Fields

breeding methods, MAB has significant advantages:

with or between selected individuals is required.

duce greater progress than phenotypic selection.

erating breeding progress.

of additional genes.

sources and efforts saved.

With a long history of development, especially since the fundamental principles of inheri‐ tance were established in the late 19th and early 20th centuries, plant breeding has become an important component of agricultural science, which has features of both science and arts. Conventional breeding methodologies have extensively proven successful in development of cultivars and germplasm. However, subjective evaluation and empirical selection still play a considerable role in conventional breeding. Scientific breeding needs less experience and more science. MAB has brought great challenges, opportunities and prospects for con‐ ventional breeding. As a new member of the whole family of plant breeding, however, MAB, as transgenic breeding or genetic manipulation does, cannot replace conventional breeding but is and only is a supplementary addition to conventional breeding. High costs and technical or equipment demands of MAB will continue to be a major obstacle for its large-scale use in the near future, especially in the developing countries (Collard and Mack‐ ill, 2008; Ribaut et al., 2010). Therefore, integration of MAB into conventional breeding pro‐ grams will be an optimistic strategy for crop improvement in the future. It can be expected that the drawbacks of MAB will be gradually overcome, as its theory, technology and appli‐ cation are further developed and improved. This should lead to a wide adoption and use of MAB in practical breeding programs for more crop species and in more countries as well.
