Crop production and resilience to biotic and abiotic challenges have increased significantly as a result of the creation of composite and synthetic cultivars. Because they can integrate beneficial features from several parental lines to improve yield stability, adaptability and disease resistance, composite and synthetic varieties are essential in plant breeding projects. In order to preserve genetic variability and adaptability, composite varieties are created by intercrossing a large base of genetically varied parental lines, followed by mass selection or natural selection (
Allard, 1960;
Simmonds, 1979). Conversely, regulated crossings of certain inbred lines are used to produce synthetic varieties, which combine the finest qualities of each parent to increase the stability and vigor of the hybrid (
Sprague and Eberhart, 1977;
Falconer and Mackay, 1996).
Composite cultivars are particularly beneficial for their ability to retain genetic variety, which promotes resilience to environmental changes and pests (
Jain, 2000). They are commonly utilized in self-pollinated crops such as rice and wheat, where genetic stability and flexibility are crucial for continued yield (
Allard, 1960). Conversely, synthetic varieties are more prevalent in cross-pollinated crops, such as rye and maize, where hybrid vigor or heterosis can greatly improve performance and yield (
Sprague and Eberhart, 1977). In order to prevent genetic drift and maintain the intended features through appropriate breeding and selection techniques, these variations need to be maintained strategically (
Falconer and Mackay, 1996).
The advantages of composite and synthetic varieties include their capacity to produce high and consistent yields, increased resistance to pests and diseases and improved tolerance to environmental stressors like drought and salinity (
Simmonds, 1979;
Jain, 2000). Developments in this area include the creation of high-yielding and disease-resistant varieties that have improved livelihoods and contributed to food security in many regions of the world. For example, the introduction of composite and synthetic varieties was extremely beneficial to the Green Revolution, increasing global crop production and lowering the risk of food shortages (
Borlaug, 2007).
Thus, the goal of this review is to present a thorough examination of the creation, upkeep, value and accomplish ments of composite and synthetic varieties, emphasizing their function in contemporary agriculture and potential applications in crop improvement initiatives in the future.
Objectives
This review aims to achieve the following objectives:
To explore the development methods of composite and synthetic varieties
This includes examining the historical background, genetic principles and breeding techniques used in creating composite and synthetic varieties.
To analyze the maintenance strategies of composite and synthetic varieties
This involves understanding the genetic stability, selection processes and challenges involved in preserving the desired traits of these varieties over successive generations.
To evaluate the merits of composite and synthetic varieties
This includes assessing their advantages in terms of yield potential, stress tolerance, disease resistance and genetic diversity.
To highlight the achievements and contributions of composite and synthetic varieties
This covers significant successes in crop improvement programs, their role in enhancing food security and their impact on global agricultural productivity.
To identify future prospects and challenges
This includes discussing the potential for further improvement, breeding innovations and the role of biotechnology in enhancing the performance of composite and synthetic varieties.
Literature review
Development of composite and synthetic varieties
Composite varieties
Early population breeding techniques intended to increase genetic diversity and adaptability gave rise to the idea of composite varieties. When seeds from many different but complimentary parental lines are combined and allowed to freely interbreed over the course of subsequent generations, composite types are created (
Allard, 1960). Through natural and artificial selection, this strategy allows advantageous alleles to accumulate, gradually enhancing population performance (
Jain, 2000).
According to
Allard (1960) and
Simmonds (1979), the fundamental benefit of composite varieties is that they preserve genetic variety within the population, enabling increased environmental adaptability and resilience to biotic and abiotic pressures. The following procedures are involved in the creation of composite varieties, according
Allard (1960): choosing parent lines according to desired characteristics including stress tolerance, disease resistance and yield potential. Using artificial or natural pollination to cross the parent lines. Letting the resultant population be stabilized by mass selection or natural selection over a number of generations. To guarantee trait performance and genetic integrity, monitoring and reselection are necessary.
According to
Verma and Yadav (2017), the genetic complementarily of the parental lines is essential to the success of composite varieties. A larger genetic foundation is provided by highly diversified parental lines, which improves the composite population’s ability to adapt to shifting environmental conditions. Composite wheat cultivars created in China and India, for instance, have demonstrated exceptional stability and pest and drought resistance (
Jain, 2000).
Synthetic varieties
Conversely, regulated hybridization of certain inbred lines is followed by seed multiplication to produce synthetic varieties (
Sprague and Eberhart, 1977). The objective is to preserve genetic stability and performance over generations while optimizing hybrid vigor, or heterosis. According to
Falconer and Mackay (1996), the development of synthetic varieties involves the following steps: Parental lines are chosen by combining trait performance and ability. F1 hybrids are produced through carefully regulated crossings between specific inbred strains. Propagation of seeds from the hybrids that perform the best. Cyclical evaluation and selection to improve stress tolerance and yield.
In cross-pollinated crops such as maize, rye and forage grasses, where heterosis can greatly boost production potential and stress resistance, synthetic cultivars are especially effective, according to
Sprague and Eberhart (1977). According to
Acquaah (2012), synthetic maize cultivators in Latin America and sub-Saharan Africa have consistently demonstrated increases in productivity and disease resistance.
The capacity of synthetic variety to capture and hold onto hybrid vigor is their primary advantage over composite types. However, compared to composite varieties, synthetic varieties are more vulnerable to environmental changes and disease outbreaks due to their genetic homogeneity (
Falconer and Mackay, 1996).
Table 1 shows a thorough comparison of synthetic and composite varieties based on their genetic structures, market adaptabilities, and yield results. The breeding strategy for composite varieties is different from that for synthetic varieties in terms of preserving genetic diversity; open pollinated composites use their natural breeding techniques, whereas synthetic breeds are the product of choosing their original parental lines.
Maintenance of composite and synthetic varieties
Maintenance of composite varieties
Maintaining composite varieties necessitates striking a compromise between selecting for desired features and maintaining genetic variability.
Allard (1960) asserts that mass selection, in which the top-performing plants are chosen for seed production in each generation, maintains composite types. This enhances overall performance while guaranteeing that the population maintains its genetic heterogeneity.
In composite populations, there is a risk of genetic drift and inbreeding depression.
Simmonds (1979) pointed out that in order to mitigate these consequences; superior lines must be continuously evaluated and reselected. In order to maintain genetic purity, new parental lines are frequently added to the composite population on a regular basis to replenish genetic diversity. According to
Jain (2000), it is easier to preserve composite variations in self-pollinated crops, such as rice and wheat, than in cross-pollinated crops. Greater genetic stability is ensured by self-pollination, which lowers the possibility of inbreeding depression and genetic drift. However, over time, genetic erosion may still result from environmental selection forces, requiring careful selection and monitoring.
Maintenance of synthetic varieties
Because synthetic varieties rely on hybrid vigor, they are more difficult to maintain than composite kinds.
Sprague and Eberhart (1977) state that controlled pollination and seed multiplication from the best-performing hybrids are used to sustain synthetic variations.
Sharma and Singh (2018) highlighted that loss of heterosis over many generations can result from genetic drift and recombination. Thus, it is necessary to regularly reselect parental lines in order to maintain synthetic variations.
Pollination was regulated to avoid foreign pollen contamination and trait performance was tracked to find and get rid of subpar variations.
Acquaah (2012) underlined that synthetic maize varieties have been maintained by systematic breeding programs comprising recurrent selection and hybrid seed manufacturing. This has ensured sustained yield improvements and stress tolerance throughout time.
Merits of composite and synthetic varieties
Composite varieties
In plant breeding, composite varieties offer a number of benefits
Genetic diversity: increased resilience to illnesses and pests and increased ability to respond to changes in the environment (
Allard, 1960). Yield Stability: Reliable results in a variety of settings (
Simmonds, 1979). Low Input Requirement: Because it requires fewer fertilizers and pesticides, it is appropriate for farmers with limited resources (
Patel and Patel, 2016). Enhanced tolerance to biotic and abiotic stressors, including as disease outbreaks, salt and drought (
Witcombe and Joshi, 2007).
Synthetic varieties
The following are some advantages of synthetic varieties: High Yield Potential: Using hybrid vigor to its full potential increases yield (
Ceccarelli, 1996); Uniformity: Increased consistency in plant height, maturity and yield (
Harlan and de Wet 1971). Improved Stress Resistance: By carefully choosing parental lines, improved resistance to environmental stresses and diseases (
Reif and Zhang, 2013); Improved Nutritional Quality: Certain synthetic varieties have been created to increase nutrient content (
Lübberstedt and Pixley, 2006).
Table 2 lists several benefits that composite and synthetic varieties offer over traditional breeding methods. These benefits include preserving genetic diversity, adapting to a variety of environments, and reducing reliance on multiple hybrid seeds.
Achievements in composite and synthetic variety development
The production of synthetic and composite cultivars has greatly increased agricultural production and global food security. The introduction of high-yielding synthetic and composite wheat and rice cultivars was a major factor in the Green Revolution (
Guo and li, 2018). Composite rice and wheat cultivars have decreased poverty and increased food security in India (
Taba, 2009). More maize has been produced and incomes have improved thanks to synthetic maize types created in Mexico and sub-Saharan Africa (
Mather and Jinks, 1982). Crop losses from bacterial and fungal diseases have decreased with the use of disease-resistant synthetic and composite cultivars (
Duvick and Cassman, 1999).
Challenges and future prospects
Notwithstanding the achievements, preserving and enhancing composite and synthetic variations still presents difficulties.
Genetic drift
Over time, composite varieties lose their genetic diversity (
Kumar and Soni, 2020).
Loss of heterosis
Genetic recombination causes synthetic varieties to have less hybrid vigor (
Singh and Chaudhary, 1985).
Environmental change
New risks to variety performance come from climate change and emerging pests (
Harlan, 1975).
Biotechnological Integration
Future composite and synthetic variety development may benefit from developments in CRISPR-based gene editing and molecular breeding
(Gupta et al., 2019).
