Explain the meaning of combined selection and discuss its relevance in marker-assisted backcrossing.

 

Combined selection, in the context of plant breeding, refers to the simultaneous use of both phenotypic and genotypic information for selecting desired individuals in a breeding population. This approach integrates traditional selection based on observable traits (phenotypes) with marker-assisted selection (MAS) based on molecular markers linked to target genes or genomic regions of interest. Combined selection offers several advantages over using either method alone, as it leverages the strengths of both phenotypic and genotypic selection strategies.

In marker-assisted backcrossing (MAB), combined selection plays a crucial role in introgressing specific genes or genomic regions from a donor parent into an elite recurrent parent while minimizing the linkage drag of unwanted donor alleles. Here's how combined selection is relevant in MAB:

·         Initial Selection of Donor Parents: Combined selection begins with the identification of suitable donor parents that carry the desired traits or alleles of interest. While phenotypic evaluation helps identify donor parents with desirable traits, MAS allows breeders to confirm the presence of target genes or markers linked to these traits in the donor genome. This ensures that only donors with the desired genetic background are chosen for backcrossing.

·         Marker-Assisted Introgression: During backcrossing, combined selection facilitates the introgression of target genes or genomic regions from the donor parent into the recurrent parent. Molecular markers linked to the target traits are used to track the presence of the donor alleles in the progeny at each backcross generation. This allows breeders to select individuals with the highest probability of carrying the target alleles while retaining the genetic background of the recurrent parent.

·         Phenotypic Confirmation: While MAS provides valuable information about the presence of target alleles, phenotypic evaluation is still essential to confirm the expression of the desired traits in the progeny. Combined selection involves assessing the performance of individuals for target traits under field or controlled conditions. This ensures that the selected progeny not only carry the target alleles but also exhibit the desired phenotypic characteristics.

·         Selection for Recurrent Parent Genome Recovery: In addition to introgressing target alleles, combined selection in MAB also involves selecting for the recovery of the recurrent parent genome. Breeders aim to minimize the retention of donor genomic segments outside the target region to reduce the negative effects of linkage drag. Molecular markers linked to recurrent parent alleles are used to monitor the recovery of the recurrent parent genome and facilitate the selection of progeny with maximum recurrent parent genetic background.

Overall, combined selection in marker-assisted backcrossing enhances the efficiency and precision of gene introgression by integrating phenotypic evaluation with marker-based selection. This approach ensures the successful transfer of target genes or genomic regions from donor parents into elite breeding lines while maintaining the desired genetic background and minimizing undesirable donor alleles.