**2. Traditional approaches in maize crop improvement**

Based on recent molecular analysis, it is now believed that the process of maize domestication commenced in the Central Balsas River Valley approximately 8700 long ago in southwestern Mexico. This domestication process occurred rapidly, originating from *Zea mays* ssp. *parviglumis* wild precursor, a subspecies of teosinte. This information is supported by studies conducted by Liu et al. [13], Piperno et al. [14], and Ranere et al. [15]. At the International Institute of Tropical Agriculture (IITA), maize underwent enhancements in various quantitative traits through classical or conventional methods. These improvements encompassed traits such as Striga resistance, nitrogen utilization efficiency, drought tolerance, resilience to stem borers, mitigation of aflatoxin accumulation, yield potential, and enhancement of nutritional quality [16, 17]. In traditional maize breeding, the approach entails the development of new plant cultivars by adhering to the principles of natural inheritance. This involves the selection of plants based on their exceptional performance in specific traits or characteristics, as discussed by Lamichhane and Thapa [18]. Conventional breeding methods have been employed for the two, self-pollinated and cross-pollinated plants for quite shortly. One example is the concept of pure line selection, which was introduced by Johannsen [19], as documented by Poehlman [20]. This method involves the creation of pure lines through the self-pollination of a single superior homozygous parental genotype. Following several years of conducting multi-locational trials, typically spanning approximately 6–7 years and involving the comparison with established check varieties, superior genotypes are officially introduced as new maize varieties. Pure-line selection is less effective due to low heritability caused by environmental effects, as genetic makeup closely resembles parental genotypes [21]. Mass selection, akin to pure-line selection, relies on highly heritable traits for plant choice [22]. Mass selection can be executed in two ways; the first is single-parental,

where one kind of gamete is controlled, and the second one is bi-parental, where a couple of gametes, female and male are controlled. The chosen individuals are then planted in the crop land and harvested when they reach maturity. After harvest, seeds are mixed and sowed for the next generation. In the next year, crop plants grown from mixed seeds are justified with a check variety for variance. Selected plants are released as new varieties after multi-location trials. Backcross breeding introduces desirable traits from one plant into another without affecting other traits by crossing with a homozygous parent [23]. In this method, donor parents possess the desired trait, and recurrent parents receive these selected genes. After five to six generations of repeated backcrossing with the recurrent parent, the backcrossed progeny should inherit approximately 98% of the recurrent parent's genome [24]. In backcross breeding, the newly formed variety typically inherits a majority of its genes from the recurrent parent, with only a few coming from the donor parent, as noted by Singh [25]. Another method is recurrent selection, a term introduced by Hull [26], primarily applied in maize breeding but later extended to other cereal crops, as discussed by Ramya et al. [27]. This process entails the continued selection of favorable traits over multiple generations, to increase their prevalence through crosses between high-performing individuals from the heterozygous recurrent parent and inbred individuals, as discussed by Bangarwa [28]. Hybridization is another method for creating hybrids with desirable traits by mating genetically distant parents within the same species (Intraspecific hybridization) or between different species (Interspecific hybridization). It involves combining characteristics from different parents to produce genetically superior offspring, whether through natural or artificial means [21].

In a remarkable long-term study conducted with conventional breeding techniques, researchers at the Illinois Agricultural Experiment Station successfully enhanced the oil concentration in maize. They started with a base of approximately 5% oil content and, throughout 100 generations, developed high oil-producing maize lines, which now boast an impressive 20% oil content [29]. However, conventional breeding methods do have their limitations. For instance, identical parents do not produce variation due to the lack of segregation of gametes in conventional breeding [30]. Additionally, this process is often time-consuming, typically spanning over a decade or more before a new cultivar is ready for release, as noted by Bharti and Chimata [31]. Moreover, conventional breeding heavily relies on the cultivars phenotypic expressions to identify superior ones. Hence, the chosen cultivars may not consistently be without errors, given that phenotypes are significantly affected by genotype-environment interactions [32]. The selection process involves choosing individuals for breeding based on their differences in desired features, which are usually measurable or observable traits [33]. It's worth noting that conventional breeding is an applied science that heavily depends on the observations, skills, and experiences of breeders for judgment, as highlighted by Allard [34].
