Article

Introduction

Maize (Zea mays L.), a vital cereal crop, holds a prominent position in global agriculture due to its diverse uses in food, feed, and industrial applications [1]. As a highly heterozygous and cross-pollinated species, maize exhibits remarkable genetic variability, making it an ideal candidate for studying key genetic phenomena such as heterosis (hybrid vigor) and inbreeding depression. Understanding these concepts is crucial for improving maize productivity and ensuring food security in the face of growing global demands [2]. Heterosis, the superior performance of hybrid offspring compared to their parents, has been a cornerstone of hybrid maize breeding programs. This phenomenon enhances various agronomic traits, particularly grain yield, plant vigor, and stress tolerance. The genetic basis of heterosis can be attributed to dominance, overdominance, and epistatic interactions among genes, which collectively influence the hybrid’s performance [3]. The exploitation of heterosis through hybrid breeding has revolutionized maize production worldwide, enabling the development of high-yielding and climate-resilient hybrids, inbreeding depression, characterized by the reduction in fitness and productivity due to the fixation of deleterious alleles, poses significant challenges during the development of pure lines for hybrid breeding. Prolonged inbreeding leads to increased homozygosity, which can negatively impact traits such as grain yield, plant height, and resistance to biotic and abiotic stresses [4]. Managing inbreeding depression is a critical step in maize breeding, as it directly affects the efficiency of hybrid seed production and the overall success of breeding programs. The interplay between heterosis and inbreeding depression underscores the complexity of maize breeding. While heterosis exploits genetic diversity to enhance hybrid performance, inbreeding is essential for developing pure parental lines with desirable traits. Striking a balance between these opposing processes is vital for sustainable genetic improvement in maize [4]. Advances in molecular genetics, genomic selection, and modern breeding techniques have provided new opportunities to study and harness these phenomena more effectively.

Maize, as a staple food crop in many regions, is also a model organism for genetic and genomic research. Studies on heterosis and inbreeding depression in maize have not only contributed to the crop’s improvement but also provided valuable insights into the genetic mechanisms underlying these phenomena. For instance, research on the genetic basis of heterosis has revealed the role of quantitative trait loci (QTLs) and the importance of genetic distance between parental lines in determining hybrid performance [5]. Similarly, investigations into inbreeding depression have highlighted the need for maintaining genetic diversity within breeding populations to prevent the accumulation of deleterious alleles [6]. Globally, hybrid maize varieties have played a crucial role in addressing food security challenges, particularly in regions with high population growth and limited arable land. Countries such as the United States, China, and India have witnessed significant yield improvements through the adoption of hybrid maize. However, the success of hybrid breeding programs depends on the availability of well-characterized inbred lines and efficient strategies to mitigate inbreeding depression.

The development of molecular tools and biotechnological approaches has further enhanced our understanding of heterosis and inbreeding depression. Techniques such as marker-assisted selection (MAS), genomic selection, and CRISPR-based genome editing have enabled breeders to identify and manipulate key genes associated with these phenomena. For example, genomic selection allows for the prediction of hybrid performance based on genomic data, reducing the time and resources required for breeding. Similarly, the use of doubled haploid (DH) technology has accelerated the production of homozygous inbred lines, addressing the challenges associated with inbreeding depression [7]. In addition to genetic and molecular approaches, breeding strategies such as reciprocal recurrent selection (RRS) and heterotic group classification have been instrumental in optimizing heterosis and minimizing inbreeding depression [8]. These strategies focus on maintaining genetic diversity within breeding populations while maximizing the combining ability of parental lines. The integration of conventional and modern breeding methods has paved the way for the development of superior maize hybrids with enhanced yield and agronomic traits [9], heterosis and inbreeding depression are fundamental concepts in maize breeding with significant implications for crop improvement. The study of these phenomena has not only advanced our understanding of maize genetics but also provided practical solutions to address the challenges of global food security. By leveraging the genetic potential of maize through innovative breeding strategies, researchers and breeders can ensure sustainable productivity and resilience in this vital crop.

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Heterosis in Maize

Genetic Basis

The genetic basis of heterosis in maize arises from the interaction of alleles at multiple loci, driven by three primary hypotheses: dominance, overdominance, and epistasis. The dominance hypothesis posits that hybrid vigor results from the complementation of dominant alleles, which mask the deleterious effects of recessive alleles. This genetic complementation enhances the overall performance of hybrid offspring. On the other hand, the overdominance hypothesis suggests that specific heterozygous allele combinations at certain loci confer a fitness advantage over either homozygote, resulting in superior hybrid performance. Additionally, epistasis—the interactions among multiple loci—further contributes to heterosis by enhancing trait expression beyond the additive effects of individual genes [10]. These genetic mechanisms collectively enable hybrid maize to exhibit superior yield and adaptability compared to their parental lines.

Yield and Agronomic Traits

Heterosis in maize significantly impacts various yield-related traits, including grain yield, biomass, and kernel size. Hybrid maize varieties typically outperform their parental lines by 15-50% in grain yield, making them a cornerstone of modern maize production. Furthermore, heterosis influences other agronomic traits such as plant height, ear length, and resistance to biotic and abiotic stresses. The extent of heterosis observed in hybrids depends on the genetic distance between parental lines, with greater diversity often resulting in higher levels of heterosis [11]. These enhancements contribute to the widespread adoption of hybrid maize in diverse agro-climatic regions, addressing challenges such as resource constraints and climate variability.

Exploitation in Breeding

The efficient exploitation of heterosis in maize breeding relies on the strategic selection of inbred lines with complementary traits. Advances in molecular markers and genomic selection have revolutionized the identification of parental lines with high combining ability. By employing these modern tools, breeders can predict hybrid performance with greater accuracy, reducing the time and resources required for hybrid development. Additionally, the use of biotechnological approaches such as doubled haploid (DH) technology has streamlined the production of homozygous inbred lines, addressing challenges associated with inbreeding depression [12]. These innovations have paved the way for the development of superior maize hybrids that combine high yield potential with resilience to environmental stresses.

Inbreeding Depression in Maize

Genetic Basis

Inbreeding depression in maize results primarily from increased homozygosity, which leads to the expression of deleterious recessive alleles that are otherwise masked in heterozygous states. As homozygosity increases, genetic diversity decreases, reducing the associated fitness advantages [13]. This loss of genetic diversity directly impacts the plant’s ability to adapt and thrive under variable conditions, further exacerbating the negative effects of inbreeding.

Impact on Traits

The effects of inbreeding depression are most pronounced in yield-related and fitness traits. Prolonged inbreeding often results in reduced grain yield, smaller ears, diminished plant vigor, and weakened resistance to biotic and abiotic stresses. Quantitative traits, such as yield, are particularly vulnerable, as they are controlled by multiple loci, each potentially harboring recessive deleterious alleles [14]. The magnitude of inbreeding depression can vary significantly across different traits, underscoring the importance of maintaining genetic diversity in breeding programs.

Mitigation Strategies

Addressing inbreeding depression in maize requires a multifaceted approach. Key strategies include:

1. Recurrent Selection: This involves the cyclic selection and recombination of superior individuals within a population [15]. By maintaining genetic diversity while improving specific traits, recurrent selection helps mitigate the adverse effects of inbreeding.
2. Haploid Breeding: Doubled haploid (DH) technology accelerates the production of homozygous inbred lines, reducing the time required for breeding while minimizing inbreeding depression [16]. This approach ensures the development of pure lines with desired traits.
3. Marker-Assisted Breeding: The use of molecular markers allows for the identification and exclusion of deleterious alleles during the selection process [17]. Marker-assisted breeding enables breeders to focus on favorable genetic combinations, reducing the risk of inbreeding depression.

By integrating these strategies, maize breeders can effectively balance the challenges of inbreeding depression with the benefits of hybrid breeding, ensuring the sustainable development of high-performing maize varieties.

Interplay Between Heterosis and Inbreeding Depression

While heterosis and inbreeding depression represent opposing phenomena, they are interconnected in breeding programs. Hybrid breeding leverages heterosis for superior performance, while inbreeding is necessary to develop parental lines [18]. Balancing these processes is critical for achieving long-term genetic gains in maize.

Conclusion

Understanding the genetic principles underlying heterosis and inbreeding depression is pivotal for advancing maize breeding programs. The exploitation of heterosis through hybrid development has been a cornerstone in enhancing maize productivity globally. However, the challenges posed by inbreeding depression necessitate the integration of innovative breeding strategies such as marker-assisted selection, genomic selection, and doubled haploid technology. By addressing these challenges, maize breeders can develop resilient, high-yielding cultivars to meet the growing demands for food security. The continued focus on genetic research and the application of advanced biotechnological tools will ensure sustainable improvements in maize yield and agronomic traits, contributing significantly to agricultural sustainability.

References

1. Kiani, T. T., Hussain, M., & Rahman, H. (2015). Heterosis and inbreeding depression for grain yield variables in indigenous maize germplasm. Sarhad Journal of Agriculture, 31(4), 217-223.
2. Botelho, F. B. S., Bruzi, A. T., Lima, I. P., Rodrigues, C. S., & Botelho, R. D. C. (2016). Inbreeding depression in single, three way and double-cross hybrids of maize. Genetics and Molecular Research, 15(3), 1-7.
3. Duvick, D. N. (2005). The contribution of breeding to yield advances in maize (Zea mays L.). Advances in agronomy, 86, 83-145.
4. Carr, D. E., & Dudash, M. R. (2003). Recent approaches into the genetic basis of inbreeding depression in plants. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 358(1434), 1071-1084.
5. Lariepe, A., Mangin, B., Jasson, S., Combes, V., Dumas, F., Jamin, P., .& Moreau, L. (2012). The genetic basis of heterosis: multiparental quantitative trait loci mapping reveals contrasted levels of apparent overdominance among traits of agronomical interest in maize (Zea mays L.). Genetics, 190(2), 795-811.
6. Yang, J., Mezmouk, S., Baumgarten, A., Buckler, E. S., Guill, K. E., McMullen, M. D., & Ross-Ibarra, J. (2017). Incomplete dominance of deleterious alleles contributes substantially to trait variation and heterosis in maize. PLoS genetics, 13(9), e1007019.
7. Li, Z., Coffey, L., Garfin, J., Miller, N. D., White, M. R., Spalding, E. P., & Hirsch, C. N. (2018). Genotype-by-environment interactions affecting heterosis in maize. PloS one, 13(1), e0191321.
8. Hochholdinger, F., & Baldauf, J. A. (2018). Heterosis in plants. Current Biology, 28(18), R1089-R1092.
9. Charlesworth, D., & Charlesworth, B. (1987). Inbreeding depression and its evolutionary consequences. Annual review of ecology and systematics, 237-268.
10. Luo, L. J., Li, Z. K., Mei, H. W., Shu, Q. Y., Tabien, R., Zhong, D. B., & Paterson, A. (2001). Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. II. Grain yield components. Genetics, 158(4), 1755-1771.
11. Al-Naggar, A. M. M., Atta, M. M. M., Ahmed, M. A., & Younis, A. S. M. (2016). Useful heterosis and combining ability in maize (Zea mays L.) agronomic and yield characters under well watering and water stress at flowering. Journal of Advances in Biology and Biotechnology, 8, 1-18.
12. Cané, M. A. (2011). Heterosis in maize (Zea mays, L.): characterization of heterotic quantitative trait loci (QTL) for agronomic traits in near isogenic lines (NILs) and their testcrosses.
13. Schnable, P. S., & Springer, N. M. (2013). Progress toward understanding heterosis in crop plants. Annual review of plant biology, 64(1), 71-88.
14. Hu, S., Wang, C., Sanchez, D. L., Lipka, A. E., Liu, P., Yin, Y., & Lübberstedt, T. (2017). Gibberellins promote brassinosteroids action and both increase heterosis for plant height in maize (Zea mays L.). Frontiers in plant science, 8, 1039.
15. Wannows, A. A. M. Y., Sabbouh, Y., & Al-Ahmad, S. (2015). Generation mean analysis technique for determining genetic parameters for some quantitative traits in two maize hybrids (Zea mays L.). Jordan Journal of Agricultural Sciences, 11(1).
16. Wallace, J. G., Larsson, S. J., & Buckler, E. S. (2014). Entering the second century of maize quantitative genetics. Heredity, 112(1), 30-38.
17. Ogunniyan, D. J., & Olakojo, S. A. (2014). Genetic variation, heritability, genetic advance and agronomic character association of yellow elite inbred lines of maize (Zea mays L.). Nigerian Journal of Genetics, 28(2), 24-28.
18. Troyer, A. F. (2006). Adaptedness and heterosis in corn and mule hybrids. Crop science, 46(2), 528-543.