Plant Gene and Trait 2024, Vol.15, No.1, 33-43 http://genbreedpublisher.com/index.php/pgt 34 This study will discuss the current status of MAS in cassava breeding, identify bottlenecks and challenges, and discuss the practical application and role of MAS in cassava improvement. The biosafety aspects of molecular tools used in cassava biotechnology will also be considered, as the use of selectable and gradeable markers must be evaluated for safety in terms of toxicity, allergenicity, pleiotropy, horizontal gene transfer and their overall impact on food, feed and environmental safety (Petersen et al., 2005). This study aims to provide a comprehensive overview of how MAS can be effectively implemented in cassava breeding programmes to achieve improvements in crop varieties that contribute to food security and economic development in regions where cassava is a major crop. 2 Theoretical Basis of Marker-assisted Selection 2.1 Definitions and types of genetic markers Genetic markers are tools used in the field of molecular biology and genetics to identify a specific DNA sequence of an organism that can be located at a specific location on a chromosome, usually associated with certain genetic traits or able to reveal patterns of transmission of genetic information. Genetic markers have important biological significance due to their wide application in disease research, genetic diversity research, genetic relationship analysis, and plant and animal breeding. There are many types of genetic markers, common ones include restricted fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), simple sequence repeats (SSR, also known as microsatellites), single nucleotide polymorphism (SNP), cleaved amplified polymorphic sequence (CAPS), and randomly amplified polymorphic DNA (RAPD). Each markup type has its own unique characteristics and scope of application. RFLP is an early commonly used genetic marker, which depends on the difference in the cutting pattern of DNA fragments under the action of a specific restriction enzyme, and it has a high resolution, but the operation process is cumbersome and expensive. AFLP technology combines the advantages of PCR and restriction enzyme cleavage to generate a large number of markers without specific sequence information and is suitable for species with wide genetic diversity. SSR markers are composed of short nucleotide sequences, such as (CA)_n repeats. This marker with high polymorphism and genetic stability is very suitable for the analysis of population genetic structure and genetic linkage map. SNP is the most popular genetic marker at present, which can provide extremely high resolution and is suitable for fine genetic analysis and precise breeding. SNP detection technology is becoming more and more efficient and economical. CAPS labeling is a simple and effective method to identify single nucleotide variation by revealing the restriction enzyme cleavage polymorphism of PCR amplified fragments. RAPD received initial attention due to its simple operation, but its application has been limited due to its low repeatability and specificity. 2.2 Basic principles and workflow of marker-assisted selection Mark-assisted selection (MAS) is a key technique in modern crop improvement, which uses molecular markers to directly select individuals with desired genetic characteristics, thereby improving breeding efficiency and accuracy. The basic principle of MAS is based on genetic markers that are closely related to the target trait, which are usually DNA sequences such as single nucleotide polymorphisms (SNPS) or simple sequence repeats (SSR) (Hasan et al., 2021). By identifying markers that carry beneficial alleles, breeders can select those individuals who perform best in advance, without the need for phenotypic assessment. The MAS workflow includes several key steps: The first is the development and validation of genetic markers, a step that requires the identification of markers associated with target traits through molecular biology techniques. The second is the construction of populations and the screening of molecular markers, which involves genotyping in large populations using selected markers to determine which individuals carry beneficial genetic information. Statistical and genetic analysis tools are then used to establish the strength of the association between markers and trait expression, which often requires quantifying trait locus (QTL) mapping (Figure 1) (Hasan et al., 2021).
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