Mass and Recurrent Selections, Combining Ability - 1 | Agriculture Optional Notes for UPSC PDF Download

Population Approach to Breeding of Self -Pollinated Crops

• Homozygosity through Self-Fertilization: Self-fertilization in F1 hybrids quickly leads to increased homozygosity. After several generations, approximately 94% of genes become homozygous. Even by F2, around half of the genes are homozygous. This process separates the progeny from a hybrid into various purelines.
• Limitations of Traditional Breeding Methods: Breeding methods relying on self-pollination (e.g., pedigree and bulk methods) have limitations. Recombination is mainly limited to two or, at most, three generations. Furthermore, there is little opportunity to alter the genotype of the segregants further.
• Population Breeding Solution: To address these challenges, a population breeding approach is proposed. In this approach, exceptional F2 plants are mated with each other through various means, which reintroduces heterozygosity in the progeny. This offers more opportunities for recombination and combines desirable genes from different F2 plants, facilitating the accumulation of favorable genes. This, in turn, increases the chances of obtaining transgressive segregants. This intermating process can be repeated multiple times.
• Similarity to Recurrent Selection: The population breeding approach resembles recurrent selection used in cross-pollinated crops. An alternative is to intermate F3 or later generation progenies, enabling more effective selection of desirable progenies compared to F2. Selection in F2, particularly for traits like yield, is of limited value. Therefore, choosing F3 or F4 progenies for selection is more preferable. The intermating of selected plants can continue for two or more generations.
• Historical Perspective: This population approach was initially suggested by Palmer in 1953. While not widely used today, it may become more relevant in the future as the improvements achieved through the pedigree method become less significant. The population approach is conceptually similar to recurrent selection commonly employed in cross-pollinated crops and is often referred to as such.
• Challenges in Recurrent Selection for Self-Pollinated Crops: A significant challenge with recurrent selection in self-pollinated crops is the labor-intensive process of making a large number of required crosses by hand, including emasculation and pollination.
• Overcoming Cross-Making Challenges: The difficulties of making numerous crosses by hand can be circumvented by employing genetic or cytoplasmic male sterility. When genetic male sterility is utilized, selection focuses on the male sterile (ms ms) plants in each generation. Seeds from these selected male sterile plants are typically harvested in bulk. Progeny from these plants are expected to include both male sterile (ms ms) and male fertile (Ms ms) plants in roughly equal proportions. Furthermore, seeds produced on male sterile plants are achieved through pollination by male fertile plants in the population. This approach effectively ensures intermating among the plants in the population and eliminates the laborious processes of hand emasculation and pollination.
• Evidence from Recurrent Selection: Recurrent selection techniques have yielded positive results in crops like tobacco and soybean. For instance, in tobacco, Matzinger and colleagues selected plants before flowering and intermated them. This led to a linear response, with a 4.9% reduction in plant height and a 7% increase in leaf number per selection cycle over five cycles. Importantly, there was no observed reduction in variability as a consequence of selection. In soybean, Brim and collaborators conducted six cycles of recurrent selection for increased protein content in two segregating populations and three cycles of selection for yield and high oil content in another segregating population. These efforts resulted in a 0.33% and 0.67% increase in protein content per cycle, a 5.3% increase in yield per cycle, and a 0.3% increase in oil content per cycle. These findings demonstrate the effectiveness of recurrent selection in improving yield and yield-related traits in self-pollinated crops.
• Diallel Selective Mating Scheme (DSM): In 1970, Jensen proposed a comprehensive breeding scheme designed to fulfill three fundamental functions of a versatile breeding program. Firstly, it allows for the development of selfing series, such as F2, F3, at every stage of the breeding program. This enables the isolation of purelines that can be used as commercial varieties. Secondly, the scheme involves intermating among selected plants or lines at each stage. The progenies resulting from these intermatings form the basis for the subsequent stages of the selfing series within the breeding program.
• Two-Directional Progress: The breeding program progresses in two distinct directions: vertically, through the selfing series, leading to the isolation of commercial varieties; and horizontally, through intermating among selected plants or lines, generating the recurrent selection series. Importantly, new germplasm can be introduced at any stage of the program by intermating it with some of the selected plants from that stage. This allows for the retention and creation of significant variability for effective selection over multiple cycles and the introduction of new genes if desired. This comprehensive breeding scheme is known as the Diallel Selective Mating Scheme (DSM) and is designed to serve both short-term and long-term breeding objectives.
• Multipopulation Approach: Breeders can create more than one such population for a crop, with each population tailored to fulfill specific objectives. However, this approach has not been widely used, mainly due to the challenges associated with making the large number of crosses required. Jensen has proposed the use of male sterility to overcome this challenge, similar to its use in the recurrent selection scheme discussed earlier. Additionally, it's worth noting that DSM is considerably more complex than the simple pedigree method, which remains the preferred breeding method for self-pollinated crops.

Merits of population Approach

• The population approach offers increased chances for recombination, facilitated by the reinstatement of heterozygosity through the interbreeding of chosen plants.
• This method aids in amassing favorable genes within the population, a result of the intermating of selected plants from segregating generations.

Demerits of Population Approach

• The effectiveness of this method relies on the identification of desirable plants in the F2 generation and subsequent segregating generations. This can be particularly challenging, if not impossible, for complex traits like yield, which exhibit low heritability. To mitigate this, later-generation progenies (e.g., F3 or F4) and replicated yield data may be employed.
• Another drawback of this approach is the necessity for interbreeding selected plants. In some crops, this intermating can pose a significant limitation due to the difficulty and time-consuming nature of crossing in many self-pollinated species.
• The time required to develop a new variety through the population approach is generally longer than that needed using the pedigree method.
• There is a lack of compelling evidence supporting the advantages of the population approach. Critics have argued that increased recombination might be detrimental as it could disrupt desirable linkages. However, this criticism assumes that most new gene combinations resulting from recombination will be inferior to existing ones, which is not always the case in crop improvement, where the goal is to create new and desirable gene combinations.