1 Introduction

Maize (Zea mays L.), also known as corn, is one of the most important cereal crops globally, serving as a staple food for millions of people and a critical feedstock for livestock. Originating in Central America, maize has been domesticated and cultivated for thousands of years, leading to the development of numerous varieties adapted to diverse climatic and soil conditions. Its significance extends beyond nutrition; maize is also a key industrial crop used in the production of biofuels, bioplastics, and other value-added products. The crop's versatility and economic value underscore its importance in global agriculture and food security (Wilkes, 2007).

The breeding of maize has evolved considerably over the centuries, employing both conventional and modern techniques to enhance desirable traits such as yield, disease resistance, and stress tolerance. Conventional breeding methods, including mass selection, backcrossing, and hybridization, have been the cornerstone of maize improvement. These methods rely on the natural genetic variation within maize populations and involve selecting individuals with favorable traits to propagate the next generation. Hybrid breeding, which involves crossing two genetically diverse inbred lines to produce a hybrid with superior traits, has been particularly successful in increasing maize yields and improving agronomic performance (Dreher et al., 2003).

In recent decades, advances in genetic engineering have introduced new possibilities for maize improvement. Techniques such as transgenic modification and CRISPR/Cas9 genome editing allow for the precise manipulation of maize DNA to introduce or enhance specific traits. Genetic engineering enables the transfer of genes between unrelated species, significantly broadening the genetic base available for crop improvement. This technology has been used to develop maize varieties with traits such as herbicide resistance, insect resistance, and enhanced nutritional content (Shou, 2003).

This study aims to provide a comprehensive comparison between conventional breeding and genetic engineering in maize. It will explore the historical context and methodologies of each approach, evaluate their respective impacts on yield, stress tolerance, and disease resistance, and assess their roles in enhancing the nutritional quality of maize. Furthermore, the study will discuss the economic and ecological implications of these breeding techniques, considering factors such as cost-effectiveness, speed of development, and sustainability. By synthesizing current research and case studies, this study seeks to highlight the strengths and limitations of conventional breeding and genetic engineering, offering insights into future directions for maize improvement. Ultimately, the goal is to inform breeding strategies that can ensure food security and agricultural sustainability in the face of global challenges.

2 Conventional Breeding in Maize

2.1 Historical development of conventional breeding

The history of maize breeding is deeply rooted in traditional agricultural practices that date back thousands of years. Native Americans initially domesticated maize from its wild ancestor teosinte, through careful selection for desirable traits such as larger kernels and more robust plant architecture (Wilkes, 2007). Early farmers relied on mass selection, which involves selecting the best plants based on observable traits and using their seeds for the next planting season.

The advent of scientific breeding methods in the early 20th century revolutionized maize improvement. Researchers such as George Shull and Edward East developed the concepts of hybrid vigor (heterosis) and inbred lines, leading to the creation of hybrid maize. Hybrid maize exhibited superior yields and uniformity compared to open-pollinated varieties, making it a cornerstone of modern agriculture. The widespread adoption of hybrid maize in the United States during the mid-20th century significantly boosted maize productivity and cemented its role as a staple crop (Dreher et al., 2003).

2.2 Techniques and methods

2.2.1 Selection

Selection is the oldest form of plant breeding and involves choosing plants with desirable traits to propagate the next generation. There are two primary types of selection: mass selection and pure line selection. In mass selection, seeds from the best-performing plants are mixed and sown together, which can lead to gradual improvements in traits such as yield, disease resistance, and drought tolerance. Pure line selection, on the other hand, involves selecting the best plants and self-pollinating them to produce homozygous lines, which are then evaluated for performance (Dreher et al., 2003).

2.2.2 Hybridization

Hybridization is the process of crossing two genetically distinct inbred lines to produce hybrid offspring with superior traits. This technique exploits hybrid vigor, where the resulting hybrids exhibit greater vigor, higher yields, and improved stress resistance compared to their parents. Hybrid maize breeding involves several steps: developing inbred lines through repeated self-pollination, evaluating their performance, and finally, crossing the best inbreds to produce hybrids (Dreher et al., 2003).

2.2.3 Mutation breeding

Mutation breeding involves exposing seeds to physical or chemical mutagens to create genetic variations. These mutations can lead to new traits that may not exist in the natural population. Selected mutants with desirable characteristics are then propagated and integrated into breeding programs. Although mutation breeding is less common in maize compared to other techniques, it has been used to develop traits such as disease resistance and improved nutritional content (Goldstein et al., 2019).

2.3 Achievements and success stories

Conventional breeding has achieved numerous successes in maize improvement. One of the most notable achievements is the development of high-yielding hybrid maize varieties, which have significantly increased maize production worldwide. For example, the introduction of single-cross hybrids in the mid-20th century resulted in maize yields doubling in the United States within a few decades (Wang et al., 2020).

Another success story is the development of Quality Protein Maize (QPM), which contains higher levels of essential amino acids lysine and tryptophan. This improvement addresses the nutritional deficiencies in populations relying on maize as a staple food. QPM varieties were developed using conventional breeding techniques and have been widely adopted in developing countries to combat malnutrition (Tandzi et al., 2017).

Conventional breeding has also played a crucial role in developing maize varieties resistant to diseases and pests. For instance, resistance to maize streak virus, a major disease in sub-Saharan Africa, has been successfully incorporated into maize varieties through traditional breeding methods (Figure 1) (Masuka et al., 2017).

Figure 1 Maize streak virus (MSV) score plotted against year of entry into regional trials of (a) early open-pollinated varieties (OPVs) and (b) intermediate-late OPVs from regional trials between 1999 and 2011 (Adopted from Masuka et al., 2017) Image caption: These two graphs illustrate the relationship between MSV scores and the year of entry into regional trials. Graph a shows a slight increase in MSV scores from 1998 to 2012, but the change is not significant (y=2.3139+0.0074x, ns), with data points widely dispersed. Graph b shows a significant decrease in MSV scores (y=3.3998-0.0552x, p<0.001), indicating that MSV scores significantly decrease over the years, with data points more concentrated along the downward trend. This suggests that over time, MSV scores have a statistically significant decreasing trend (Adopted from Masuka et al., 2017)

2.4 Limitations and challenges

Despite its successes, conventional breeding faces several limitations and challenges. One significant challenge is the time required to develop new varieties. The process of creating inbred lines and evaluating hybrids can take several years, delaying the introduction of improved varieties. Additionally, conventional breeding relies heavily on the natural genetic variation present in maize populations, which may limit the range of achievable improvements (Morris et al., 2003).

Another limitation is the environmental dependency of trait expression. Genotype by environment interactions can affect the performance of bred varieties, making it challenging to predict how new hybrids will perform under different growing conditions. This variability necessitates extensive field trials across multiple environments, further extending the breeding timeline (Burger et al., 2008).

Moreover, conventional breeding is often resource-intensive, requiring significant investments in labor, land, and facilities for field trials and seed production. These costs can be prohibitive, especially for public breeding programs and smaller seed companies with limited budgets (Jumbo et al., 2011).

In conclusion, while conventional breeding has been instrumental in the development and improvement of maize, it faces challenges that limit its efficiency and scope. Addressing these challenges through technological advancements and complementary breeding techniques will be essential to meet the future demands of global agriculture.

3 Genetic Engineering in Maize

3.1 Historical development of genetic engineering

The history of genetic engineering in maize dates back to the late 20th century when scientists began to explore the potential of recombinant DNA technology. The first genetically modified (GM) maize plants were developed in the early 1980s using methods such as Agrobacterium-mediated transformation and particle bombardment. These techniques allowed for the introduction of foreign genes into the maize genome, enabling the development of transgenic maize with improved traits (Dunder et al., 1995).

One of the earliest successes in genetic engineering was the development of Bt maize, which expresses a gene from the bacterium Bacillus thuringiensis. This gene produces a protein toxic to certain insect pests, significantly reducing the need for chemical pesticides and leading to widespread adoption of Bt maize in the mid-1990s (Wisniewski et al., 2002). Since then, genetic engineering has expanded to include traits such as herbicide resistance, drought tolerance, and enhanced nutritional content.

3.2 Techniques and methods

3.2.1 Recombinant DNA technology

Recombinant DNA technology involves the manipulation of DNA to create new genetic combinations that are not found in nature. In maize, this technology typically involves the insertion of genes that confer desirable traits, such as resistance to pests or tolerance to herbicides. The process begins with the identification and isolation of the gene of interest, which is then inserted into a plasmid vector. This vector is introduced into maize cells using methods like Agrobacterium-mediated transformation or particle bombardment (Yadava et al., 2017).

Agrobacterium-mediated transformation utilizes the natural ability of Agrobacterium tumefaciens to transfer DNA into plant cells. The bacterium carries a plasmid with the gene of interest, which integrates into the plant genome, allowing for stable genetic modification. Particle bombardment, or biolistics, involves shooting microscopic particles coated with DNA into plant cells. This method is particularly useful for maize, which can be challenging to transform using Agrobacterium due to its recalcitrant nature (Hong et al., 2019).

3.2.2 CRISPR-Cas9 and other genome editing tools

CRISPR-Cas9 has revolutionized genetic engineering by enabling precise, targeted modifications to the genome. This system uses a guide RNA to direct the Cas9 enzyme to a specific DNA sequence, where it creates a double-strand break. The cell's natural repair mechanisms then fix the break, allowing for the introduction or deletion of specific genes. This technology offers several advantages over traditional recombinant DNA methods, including higher precision, fewer off-target effects, and the ability to make multiple simultaneous edits (Hue et al., 2018).

Other genome editing tools, such as zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), also allow for precise genetic modifications. These technologies have been used to develop maize varieties with enhanced traits, such as improved drought tolerance and resistance to various pests and diseases (Yang and Yan, 2021).

3.3 Achievements and success stories

Genetic engineering has led to several significant achievements in maize improvement. One of the most notable successes is the development of Bt maize, which has dramatically reduced the need for chemical insecticides and increased yields by protecting crops from pest damage. This has had profound economic and environmental benefits, especially for smallholder farmers in developing countries (Wisniewski et al., 2002).

Another success story is the development of herbicide-resistant maize varieties. These varieties allow farmers to control weeds more effectively with fewer herbicide applications, reducing labor costs and promoting more sustainable farming practices. Glyphosate-resistant maize, for example, has been widely adopted and has significantly improved weed management in maize cultivation (Hong et al., 2019).

Genetic engineering has also been instrumental in developing drought-tolerant maize varieties. By introducing genes that help plants manage water stress, researchers have created maize that can maintain yields under adverse conditions. This is particularly important in regions prone to drought, where crop failure can have devastating consequences for food security (Hue et al., 2018).

3.4 Limitations and challenges

Despite its successes, genetic engineering in maize faces several limitations and challenges. One of the main challenges is public perception and regulatory hurdles. Genetically modified organisms (GMOs) are often viewed with suspicion by the public, and stringent regulatory requirements can delay the approval and commercialization of new GM maize varieties (Lemaux, 2008).

Another limitation is the potential for unintended effects. While genome editing technologies like CRISPR-Cas9 offer high precision, off-target mutations can still occur, leading to unintended consequences. This necessitates thorough screening and validation of edited plants to ensure their safety and efficacy (Fu et al., 2021).

Additionally, the high cost and technical expertise required for genetic engineering limit its accessibility, particularly for resource-poor farmers and developing countries. While the technology holds great promise, ensuring its benefits are widely distributed remains a significant challenge (Khan et al., 2012).

Genetic engineering has significantly advanced maize breeding, providing solutions to some of the most pressing challenges in agriculture. However, addressing the limitations and overcoming the challenges associated with this technology will be crucial to fully realize its potential for improving maize production and sustainability.

4 Comparative Analysis

4.1 Efficiency and precision

Conventional breeding and genetic engineering differ significantly in their efficiency and precision. Conventional breeding methods, such as selection and hybridization, rely on the natural genetic variation present within maize populations. These methods are relatively imprecise because they involve crossing plants and selecting offspring with desirable traits over multiple generations, which can introduce unwanted traits along with the desired ones (Burger et al., 2008). In contrast, genetic engineering, particularly with tools like CRISPR-Cas9, allows for precise modifications at specific locations within the genome. This precision minimizes the introduction of unwanted traits and speeds up the development of new varieties (Hue et al., 2018).

4.2 Time and resource investment

The time and resources required for developing new maize varieties are considerably different between conventional breeding and genetic engineering. Conventional breeding is time-consuming, often taking 7-10 years to develop a new variety due to the multiple generations needed for selection and stabilization (Figure 2) (Morris et al., 2003) . Genetic engineering, however, can significantly shorten this timeline by directly introducing desired traits into the plant's genome, potentially reducing the development time to 3-5 years. The initial setup and technological investments for genetic engineering are higher, but the long-term benefits in speed and precision often justify these costs (Yadava et al., 2017).

Figure 2 Comparison of the flow of annual research net benefits generated by conventional and marker-assisted inbred line conversion schemes (Adopted from Morris et al., 2003) Image caption: The marker-assisted scheme (MAS-A) features larger up-front investment costs (larger negative annual net benefits in years 1-6), but because marker-assisted selection accelerates the line conversion process, varieties based on the converted line are released earlier and adoption by farmers occurs sooner. Positive annual net benefits generated by the marker-assisted line conversion scheme are therefore moved forward in time (Adopted from Morris et al., 2003)

4.3 Trait improvement and yield

Both conventional breeding and genetic engineering have achieved significant improvements in maize traits and yield. Conventional breeding has successfully increased yields through hybridization and selection, focusing on traits like disease resistance, drought tolerance, and nutrient efficiency. Quality Protein Maize (QPM), which addresses nutritional deficiencies, is a notable success of conventional breeding (Tandzi et al., 2017). Genetic engineering has introduced traits such as Bt toxin for pest resistance and glyphosate resistance for herbicide tolerance, which have significantly enhanced yield stability and reduced reliance on chemical inputs (Wisniewski et al., 2002).

4.4 Environmental impact

The environmental impacts of conventional breeding and genetic engineering are complex and context-dependent. Conventional breeding generally maintains broader genetic diversity within crop populations, which can enhance ecosystem resilience. However, it often relies on extensive field trials and the use of chemical inputs to manage pests and diseases. Genetic engineering can reduce environmental impact by decreasing the need for chemical pesticides and herbicides through the introduction of resistant traits (Hong et al., 2019). Nevertheless, there are concerns about potential gene flow from genetically modified crops to wild relatives and non-target species, which could have unforeseen ecological consequences (Weber et al., 2007).

4.5 Socio-economic considerations

The socio-economic implications of conventional breeding and genetic engineering in maize are significant. Conventional breeding is often more accessible to resource-poor farmers and public breeding programs due to lower initial costs. It supports local agricultural practices and biodiversity by utilizing locally adapted varieties (Jumbo et al., 2011). Genetic engineering, while potentially more cost-effective in the long term, requires substantial initial investment in technology and expertise, which can be prohibitive for small-scale farmers. Moreover, the patenting of genetically modified seeds by private companies can lead to issues of accessibility and control over seed distribution, impacting farmer autonomy and seed sovereignty (Khan et al., 2012).

4.6 Regulatory and safety issues

Regulatory frameworks for conventional breeding and genetic engineering are markedly different. Conventional breeding is generally well-accepted and subject to standard agricultural regulations. Genetic engineering, however, faces stringent regulatory scrutiny to ensure safety and environmental compatibility. Concerns about the long-term health impacts and ecological risks of genetically modified organisms (GMOs) have led to rigorous testing and approval processes that can delay the release of new GM varieties (Lemaux, 2008). Additionally, public perception and acceptance of GMOs play a critical role in the adoption and success of genetically engineered crops.

5 Case Studies

5.1 Successful conventional breeding programs

Conventional breeding has achieved numerous successes in maize improvement through the careful selection of desirable traits over many generations. One notable example is the development of Quality Protein Maize (QPM), which was bred to address protein deficiencies in populations that rely heavily on maize as a staple food. QPM varieties contain higher levels of essential amino acids lysine and tryptophan, significantly improving the nutritional value of maize (Tandzi et al., 2017).

Another successful program is the participatory plant breeding initiative in Gujarat, India, which produced improved maize varieties through collaboration between farmers and breeders. This program resulted in the development of the variety GDRM-187, which showed better yield and grain quality than local landraces and was well-received by farmers for its early maturity and adaptability to local conditions.

Conventional breeding efforts have also focused on developing maize varieties that are adapted to organic farming systems. Studies have shown that specific lines selected for organic conditions can perform comparably to those bred for conventional systems, demonstrating the versatility and effectiveness of traditional breeding methods (Burger et al., 2008).

5.2 Successful genetic engineering programs

Genetic engineering has revolutionized maize breeding by introducing traits that were difficult or impossible to achieve through conventional methods. One of the earliest and most impactful examples is the development of Bt maize, which incorporates a gene from the bacterium Bacillus thuringiensis. This gene produces a protein toxic to certain insect pests, significantly reducing the need for chemical pesticides and leading to increased yields and lower environmental impact (Wisniewski et al., 2002).

Another significant achievement is the development of herbicide-resistant maize varieties, such as those resistant to glyphosate. These varieties allow farmers to manage weeds more effectively and with fewer herbicide applications, promoting more sustainable agricultural practices (Hong et al., 2019).

Recent advancements in CRISPR-Cas9 genome editing have further enhanced the potential of genetic engineering in maize. For instance, the BREEDIT pipeline combined multiplex genome editing with traditional breeding techniques to improve complex traits like yield and drought tolerance. This program successfully increased leaf length and width, demonstrating the potential for CRISPR to accelerate breeding processes and achieve significant genetic gains (Lorenzo et al., 2022).

5.3 Comparative case studies

A comparative analysis of conventional breeding and genetic engineering in maize reveals unique advantages and limitations for each approach. Conventional breeding, exemplified by the development of QPM and participatory breeding programs in India, relies on the natural genetic variation within maize populations. These programs have produced varieties that are highly adapted to specific local conditions and nutritional needs, demonstrating the effectiveness of traditional methods in addressing complex agricultural challenges (Tandzi et al., 2017).

On the other hand, genetic engineering has achieved rapid and precise trait enhancements, as seen with Bt maize and herbicide-resistant varieties. These engineered crops have provided significant economic and environmental benefits by reducing the need for chemical inputs and increasing yield stability (Wisniewski et al., 2002; Hong et al., 2019).

In a head-to-head comparison, marker-assisted selection (MAS) has been highlighted as a bridge between conventional and genetic engineering approaches. MAS allows breeders to select plants with desirable traits more efficiently by using molecular markers, enhancing the precision and speed of traditional breeding methods. Case studies at CIMMYT have shown that MAS can be cost-effective and time-saving compared to conventional breeding, particularly when visual selection is challenging (Dreher et al., 2003).

Ultimately, both conventional breeding and genetic engineering play crucial roles in maize improvement. The choice of method depends on the specific goals, available resources, and the context in which the breeding program operates. Integrating both approaches, along with advancements like MAS and CRISPR, can provide a comprehensive strategy for addressing the diverse challenges in maize cultivation.

6 Future Prospects

6.1 Integration of conventional and genetic approaches

The future of maize improvement lies in the integration of conventional breeding and genetic engineering techniques. Combining these approaches can maximize the strengths of each method while compensating for their individual limitations. Conventional breeding excels in leveraging natural genetic diversity and producing stable, locally adapted varieties. In contrast, genetic engineering provides the precision and speed necessary to introduce specific traits rapidly. Integrating marker-assisted selection (MAS) with traditional breeding can enhance the efficiency of selecting desirable traits by using molecular markers to track genes of interest (Mwamahonje and Mrosso, 2016).

Moreover, CRISPR-Cas9 and other genome editing tools can be employed alongside conventional methods to introduce or modify genes with high precision, thus accelerating the breeding process. This integration allows for the development of maize varieties that combine multiple beneficial traits, such as high yield, disease resistance, and stress tolerance (Hue et al., 2018).

6.2 Innovations in breeding technologies

The rapid advancement of breeding technologies promises to revolutionize maize improvement. Innovations such as genomic selection, which uses genome-wide markers to predict the breeding value of individuals, can significantly accelerate the breeding cycle and increase the accuracy of selecting high-performing plants (Andorf et al., 2019).

Doubled haploid (DH) technology, which produces completely homozygous lines from a single cross within two generations, has become a cornerstone in maize breeding. This technology reduces the time required to develop pure lines and enhances the efficiency of breeding programs. Combining DH technology with CRISPR and other genetic engineering tools could lead to rapid development of maize varieties with complex trait improvements (Fischer and Edmeades, 2010).

Additionally, advancements in phenotyping technologies, such as high-throughput phenotyping platforms and remote sensing, enable breeders to assess plant traits more accurately and efficiently. These tools, coupled with big data analytics and artificial intelligence, can provide deeper insights into the genetic basis of complex traits and optimize breeding strategies (Rosa et al., 2021).

6.3 Policy and ethical considerations

As breeding technologies evolve, policy and ethical considerations will play a crucial role in their implementation and acceptance. Regulatory frameworks must adapt to accommodate new technologies, ensuring that genetically engineered crops are safe for the environment and human health. This involves stringent testing and monitoring to prevent unintended consequences and ensure transparency in the approval process (Barrows et al., 2014).

Ethical considerations include the accessibility of advanced breeding technologies to smallholder farmers and developing countries. Ensuring equitable access to these innovations is essential for addressing global food security challenges and preventing technological disparities. Public and private sectors must collaborate to create policies that support the dissemination of new varieties and technologies to all farmers, regardless of their economic status (Khan et al., 2012).

Intellectual property rights (IPR) also pose significant challenges. While protecting the investments of researchers and companies is important, it is equally vital to ensure that IPR does not hinder the free exchange of germplasm and the adoption of beneficial technologies. Developing balanced IPR policies that encourage innovation while promoting accessibility and sharing of genetic resources is crucial (Jauhar, 2001).

In conclusion, the future of maize breeding lies in the synergistic integration of conventional and genetic engineering approaches, leveraging innovations in breeding technologies, and addressing policy and ethical challenges. By adopting a holistic and inclusive approach, the potential of these technologies can be harnessed to meet the global demands for food security and sustainable agriculture.

7 Concluding Remarks

This research has explored the comparative aspects of conventional breeding and genetic engineering in maize. Conventional breeding has a long history of success, exemplified by the development of hybrid maize and Quality Protein Maize (QPM), which have significantly improved yields and nutritional quality. Conventional methods, such as mass selection, hybridization, and mutation breeding, continue to play a crucial role in maize improvement. However, these methods are often time-consuming and resource-intensive.

Genetic engineering, on the other hand, offers precise and rapid modification of the maize genome. Technologies like CRISPR-Cas9 and recombinant DNA have enabled the development of traits such as pest resistance, herbicide tolerance, and enhanced nutritional content. Genetic engineering has demonstrated significant achievements, including the development of Bt maize and glyphosate-resistant varieties, which have contributed to higher yields and reduced chemical inputs.

The integration of both approaches can maximize the benefits of each, combining the genetic diversity and adaptability of conventional breeding with the precision and efficiency of genetic engineering.

Future research should focus on integrating conventional and genetic engineering approaches to leverage the strengths of both methods. Specific recommendations include:

Development of Integrated Breeding Programs: Establish programs that combine conventional breeding with marker-assisted selection and genome editing to enhance efficiency and precision in developing new maize varieties.

Exploration of Genomic and Phenomic Data: Utilize advanced genomics and phenomics tools to better understand the genetic basis of complex traits and optimize breeding strategies.

Sustainable Agricultural Practices: Research should aim to develop maize varieties that are resilient to climate change and capable of thriving under sustainable agricultural practices, reducing the environmental impact of maize cultivation.

Public and Private Sector Collaboration: Foster collaborations between public research institutions and private companies to share resources, knowledge, and technologies, ensuring that advancements in maize breeding are accessible to all farmers.

Addressing Ethical and Regulatory Issues: Conduct comprehensive studies on the ethical, environmental, and health implications of genetically modified maize, ensuring robust regulatory frameworks are in place to address public concerns.

The future of maize breeding and agriculture will depend on the successful integration of conventional breeding and genetic engineering. This hybrid approach can lead to the development of maize varieties that are higher yielding, more nutritious, and better adapted to environmental stresses. By combining the broad genetic base and adaptability provided by conventional breeding with the precision and speed of genetic engineering, breeders can address the challenges posed by climate change, population growth, and food security.

Furthermore, sustainable maize breeding practices will contribute to environmental conservation by reducing the need for chemical inputs and enhancing the resilience of maize crops to biotic and abiotic stresses. Policymakers and stakeholders must support research initiatives, invest in new technologies, and create policies that promote the equitable distribution of advanced maize varieties.

Acknowledgments

We would like to express our gratitude to the two anonymous peer reviewers.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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