**5. Effect of maize domestication on genetic diversity (Domestication Bottleneck)**

Both domestication and artificial selection during crop improvement led to selecting only desirable/beneficial traits, resulting in the reduced genetic diversity of the unselected genes. During the process of domestication, nearly all crop species experience "Domestication Syndrome" or "Domestication Bottleneck" [22, 56]. These effects happen in two stages; i) initial bottleneck effect, when a subset of crop wild species population brought under cultivation and ii) subsequent reduction in the genetic diversity through selective breeding for the desirable traits during crop improvement is improvement bottleneck. Among crop species, maize experiences a relatively mild genetic bottleneck, as domesticated maize retains around ~81% of the genetic diversity of teosinte [15]. Approximately 2–4% of genes were the target during the initial domestication and crop improvement stage [14, 15]. It is established that genetic diversity generally declines with the domestication of teosinte to the landraces. Subsequently, modern plant breeding reduces the genetic diversity of modern-day maize inbred lines relative to the landraces (**Figure 2**). Therefore such genes strongly influenced by domestication or improvement are enriched in modern improved varieties in the subset of genes that show low nucleotide diversity [14]. Yamasaki et al. [57] proposed a model containing three types of genes: 'neutral genes that demonstrate diversity reduction by general bottleneck effects, domestication genes in which diversity by selection between the teosintes and landraces is significantly reduced, and improvement genes in which diversity by selection between landraces and inbreds is significantly reduced (**Figure 2**).'

*Landraces - Traditional Variety and Natural Breed*

2. Glume hardness *teosinte glume* 

1. Plant

3. Paired and

4. Distichous and polystichous ear

5. Disarticulating rachides and nondisarticulating rachises

*QTLs/gene their chromosome location and phenotype.*

**Table 1.**

single spikelets of maize

architecture

morphological traits. Further research leads to the cloning of some of these QTLs, enabling identifying differences in the morphological traits between maize and teosinte. Plant architecture grassy tillers1 (*gt1*) encodes the Homeodomain leucine zipper transcription factor [40]. Plant architecture is impacted by the enhanced expression of BTB/POZ ankyrin repeat protein and Homeodomain leucine zipper transcription factor encoded by *tru1 and grassy tillers1* (*gt1*), respectively [33, 41]. The increased expression of transcription factors encoded by *gt1* (*grassy tillers1)* and *tru1* (*tassels replace upper ears1*) encodes Homeodomain leucine zipper transcription factor BTB/POZ ankyrin repeat protein [33, 40, 41]. *Tassels replace the upper ears1* (*tru1*) confers a sexual conversion of the terminal lateral inflorescence in teosinte to ear (pistillate) in maize from tassel (staminate). Other genes for seed filling *ZmSWEET4c* [42], *UB3*, *ids1/Ts6* for kernel row number [43], shattering *ZmSh1–1, ZmSh1–5.1 + ZmSh1–5.2* [37] and for inflorescence architecture *ra1* [44], were cloned, key domesticated genes of maize. Most of these domesticated genes were the transcription factors that were unregulated during domestication. A maizeteosinte-derived BC2S3 population, the QTLs *UPA1* (*Upright Plant Architecture1*) and *UPA2*, which confer on upright plant architecture, were identified. The teosinte allele at *UPA2*, which reduces leaf angle, was lost during maize domestication [45]. More compact plants and improved yields under high planting densities could be developed by incorporating this allele into modern maize hybrids [45, 46].

**SN. Trait Gene/QTLs Chromosome Phenotype Reference**

1 Number

4 Inhibits

7 3

2 10

> 1 5

of basal branches or tillers, Limited number of large ears

secondary sexual traits in the female flower, preventing glumes from hardening

High number of kernels in each row of the ear of modern maize parents

Multiple ear ranks along the inflorescence meristem

Shattering, ear size

[28, 33]

[9, 28, 34]

[35, 36]

[9, 28, 34]

[33, 37–39]

*teosinte branched1 (tb1), grassy tillers 1 (gt1)*

*architecture1 (tga1)*

*ramosa1, ramosa2, ramosa3, ramosa7*

> *Zea floricaula leafy2(zfl2), Zea floricaula leafy1(zfl1)*

*ZmSh1–1, ZmSh1– 5.1 + ZmSh1–5.2, Zga1*

**36**

#### **Figure 2.**

*Domestication and plant breeding effects on genetic diversity of maize genes [redrawn from Yamasaki et al. [57]]: Shapes with different color represents different genes and shaded area depicts the bottleneck effect.*

The genes that experienced artificial selection during domestication and crop improvement have significantly reduced genetic diversity in the modern germplasm and cannot contribute to the agro morphological traits. Therefore the selected genes are difficult to identify in the genetic screens and may not be useful in traditional breeding programmes. If we need to utilize the selected genes fully, the new variation must be reintroduced from teosintes. Additionally, for the 98% of genes, which do not experience selection during domestication and crop improvement, there are huge genetic variations in the diverse inbred lines that could be utilized by identifying the genetic loci/QTLs and improvement through plant breeding.

#### **6. Wild relatives of maize**

Wild relatives of crops are the species of wild plants that are genetically linked to cultivated crops. Unattended by humans, they continue to grow in the wild, developing traits that farmers and breeders can cross with domesticated crops to produce new varieties, such as drought tolerance or pest resistance. The *Zea* genus of grass consists of seven genera with different chromosome numbers divided into two groups: viz. old-world and new world groups. *Chionachne, Coix, Polytoca, Sclerachne* and *Trilobachne* originated in Southeast Asia and belonged to the old-world group. The new world group consists of *Zea* and *Tripsacum* and originated in Mexico and Central America. *Zea mays ssp. mays* is the only species of economic importance and other species referred to as teosintes.

#### **7. Landraces of maize**

A landrace is defined as 'dynamic population(s) of a cultivated plant that has a historical origin, distinct identity, and lacks formal crop improvement and often

**39**

*Wild Progenitor and Landraces Led Genetic Gain in the Modern-Day Maize (*Zea mays *L.)*

being genetically diverse, locally adapted, and associated with traditional farming systems' [58]. Compared to other crops, maize has tremendous genetic diversity, which offers potential for crop improvement for biotic/abiotic stresses, nutritional quality and grain yield. In landraces, the diversity/genetic variations lie withinpopulation rather than among populations. Worldwide, the landraces have been characterized both morphologically and molecularly. The genetic variability present in the available landraces has been utilized to improve agro morphological traits, biotic/abiotic stresses and specialty traits. In crop centers of origin and diversity, often biotic and abiotic conditions vary across the landscape, creating the possibility of local adaptation of crops. Local landraces perform better than non-local ones under local conditions. Some of the examples of their utilization are given below:

The conservation of landraces is fundamental to safeguarding crop diversity, food security, and sustainable production. 'Jala' is a particular maize landrace from the region in and around the Jala Valley of Mexico that produces the largest ear and tallest plant of all maize landraces in the world. Changing socio-economic and environmental conditions in the Jala Valley could lead to the genetic erosion of the ancestral 'Jala' landrace, leading to global consequences [59]. In southwest China,

Maize landrace accessions constitute an invaluable gene pool of unexplored alleles that can be harnessed to mitigate the challenges of the narrowing genetic base, declined genetic gains, and reduced resilience to abiotic stress in modern varieties developed from repeated recycling of few superior breeding lines. Some landraces of Mexico origin that imparts abiotic stress tolerance are Bolita, Breve de Padilla, Conica, Conica Nortena, Chalqueno × Ancho de Tehuacan cross (alkalinity tolerant), La Posta Sequia, Nal Tel, Oloton (acid soil tolerant) and Tuxpeno (drought tolerant) [60]. At CIMMYT, the production of inbred lines, droughttolerant population-1 (DTP-1) and drought-tolerant population-2 (DTP-2) is exploited for imparting drought tolerance. Some of the inbred lines derived from 'La Posta Sequia' were reported to have drought and heat tolerance [61]. Maize landraces L25, L14, L1, and L3, are reported as the most valuable source of drought tolerance [62]. A higher transcript accumulation in shoot tissues of *ZmATG* genes reported in landrace 'Argentino Amarelo' under the osmotic stress conditions compared to landrace 'Taquarão' [63]. Nelimor et al. [64] identified extra-early maize landraces that express tolerance to drought and heat stress. Root system architecture plays a crucial role in water and nutrient acquisition in maize. *ZmCKX5* (cytokinin oxidase/dehydrogenase) was resequenced in maize landraces

Four-row Wax landrace, with four rows of kernels on the cob.

and revealed its importance in developing the maize root system [65].

Maize crops encounter a lot of diseases due to their wide distribution. Among fungal diseases, Turcicum leaf blight (TLB) and Maydis leaf blight (MLB) results in the decline In maize production throughout the world. A subpopulation Tuxpeno Crema derived from the landrace Tuxpeno known to possess resistance to the foliar diseases [66]. Palomero Toluqueno, a landrace of popcorn reported to have resistance to the maize weevil [67], few Carrebian landraces possess resistance to larger grain borer [68]. Two Kenyan maize landraces (Jowi and Nyamula) and one Latin

*DOI: http://dx.doi.org/10.5772/intechopen.96865*

**7.1 Agromorphological traits**

**7.2 Tolerance to abiotic stresses**

**7.3 Resistance to biotic stresses**

*Wild Progenitor and Landraces Led Genetic Gain in the Modern-Day Maize (*Zea mays *L.) DOI: http://dx.doi.org/10.5772/intechopen.96865*

being genetically diverse, locally adapted, and associated with traditional farming systems' [58]. Compared to other crops, maize has tremendous genetic diversity, which offers potential for crop improvement for biotic/abiotic stresses, nutritional quality and grain yield. In landraces, the diversity/genetic variations lie withinpopulation rather than among populations. Worldwide, the landraces have been characterized both morphologically and molecularly. The genetic variability present in the available landraces has been utilized to improve agro morphological traits, biotic/abiotic stresses and specialty traits. In crop centers of origin and diversity, often biotic and abiotic conditions vary across the landscape, creating the possibility of local adaptation of crops. Local landraces perform better than non-local ones under local conditions. Some of the examples of their utilization are given below:

#### **7.1 Agromorphological traits**

*Landraces - Traditional Variety and Natural Breed*

The genes that experienced artificial selection during domestication and crop improvement have significantly reduced genetic diversity in the modern germplasm and cannot contribute to the agro morphological traits. Therefore the selected genes are difficult to identify in the genetic screens and may not be useful in traditional breeding programmes. If we need to utilize the selected genes fully, the new variation must be reintroduced from teosintes. Additionally, for the 98% of genes, which do not experience selection during domestication and crop improvement, there are huge genetic variations in the diverse inbred lines that could be utilized by identify-

*Domestication and plant breeding effects on genetic diversity of maize genes [redrawn from Yamasaki et al. [57]]: Shapes with different color represents different genes and shaded area depicts the bottleneck effect.*

Wild relatives of crops are the species of wild plants that are genetically linked to cultivated crops. Unattended by humans, they continue to grow in the wild, developing traits that farmers and breeders can cross with domesticated crops to produce new varieties, such as drought tolerance or pest resistance. The *Zea* genus of grass consists of seven genera with different chromosome numbers divided into two groups: viz. old-world and new world groups. *Chionachne, Coix, Polytoca, Sclerachne* and *Trilobachne* originated in Southeast Asia and belonged to the old-world group. The new world group consists of *Zea* and *Tripsacum* and originated in Mexico and Central America. *Zea mays ssp. mays* is the only species of economic importance and

A landrace is defined as 'dynamic population(s) of a cultivated plant that has a historical origin, distinct identity, and lacks formal crop improvement and often

ing the genetic loci/QTLs and improvement through plant breeding.

**6. Wild relatives of maize**

**Figure 2.**

other species referred to as teosintes.

**7. Landraces of maize**

**38**

The conservation of landraces is fundamental to safeguarding crop diversity, food security, and sustainable production. 'Jala' is a particular maize landrace from the region in and around the Jala Valley of Mexico that produces the largest ear and tallest plant of all maize landraces in the world. Changing socio-economic and environmental conditions in the Jala Valley could lead to the genetic erosion of the ancestral 'Jala' landrace, leading to global consequences [59]. In southwest China, Four-row Wax landrace, with four rows of kernels on the cob.

#### **7.2 Tolerance to abiotic stresses**

Maize landrace accessions constitute an invaluable gene pool of unexplored alleles that can be harnessed to mitigate the challenges of the narrowing genetic base, declined genetic gains, and reduced resilience to abiotic stress in modern varieties developed from repeated recycling of few superior breeding lines. Some landraces of Mexico origin that imparts abiotic stress tolerance are Bolita, Breve de Padilla, Conica, Conica Nortena, Chalqueno × Ancho de Tehuacan cross (alkalinity tolerant), La Posta Sequia, Nal Tel, Oloton (acid soil tolerant) and Tuxpeno (drought tolerant) [60]. At CIMMYT, the production of inbred lines, droughttolerant population-1 (DTP-1) and drought-tolerant population-2 (DTP-2) is exploited for imparting drought tolerance. Some of the inbred lines derived from 'La Posta Sequia' were reported to have drought and heat tolerance [61]. Maize landraces L25, L14, L1, and L3, are reported as the most valuable source of drought tolerance [62]. A higher transcript accumulation in shoot tissues of *ZmATG* genes reported in landrace 'Argentino Amarelo' under the osmotic stress conditions compared to landrace 'Taquarão' [63]. Nelimor et al. [64] identified extra-early maize landraces that express tolerance to drought and heat stress. Root system architecture plays a crucial role in water and nutrient acquisition in maize. *ZmCKX5* (cytokinin oxidase/dehydrogenase) was resequenced in maize landraces and revealed its importance in developing the maize root system [65].

#### **7.3 Resistance to biotic stresses**

Maize crops encounter a lot of diseases due to their wide distribution. Among fungal diseases, Turcicum leaf blight (TLB) and Maydis leaf blight (MLB) results in the decline In maize production throughout the world. A subpopulation Tuxpeno Crema derived from the landrace Tuxpeno known to possess resistance to the foliar diseases [66]. Palomero Toluqueno, a landrace of popcorn reported to have resistance to the maize weevil [67], few Carrebian landraces possess resistance to larger grain borer [68]. Two Kenyan maize landraces (Jowi and Nyamula) and one Latin

American landrace (Cuba 91) shown a lower number of eggs and egg batches deposition of *C. partellus* due to production of herbivore-induced plant volatiles (HIPVs) [69, 70]. The fall armyworm *Spodoptera frugiperda* J. E. Smith (Lepidoptera: Noctuidae) is one of the most damaging maize production pests in tropical areas. The maize landraces 'Chimbo' and 'Elotillo' had the lowest leaf damage, calculated by the area under the severity progress curve [71]. The maize landrace 'Pérola' from Brazil showed resistance to fall armyworm in the winter and summer seasons [72].

### **7.4 Enhancement of specialty traits**

A northeastern Indian landrace, 'Murlimakkai,' was utilized to develop Baby Corn composite VL Baby Corn [60]. Several landraces, viz., Azul, Bolita, Tlacoya, Pepitilla and Oaxaqueno, were very popular and utilized for tortilla quality. Mexican popcorn landrace 'Palomero,' utilized to understand the landrace structure and improvement in the popping quality. Landraces had significantly higher values than checks for oil content, oleic acid, MUFA and tocopherol contents. Genetic analyses suggest that the kernel quality traits could be successfully manipulated using the investigated plant material [73].

#### **7.5 Unlocking the genetic variability present in the landraces**

Using landraces for broadening the genetic base of elite maize germplasm is hampered by heterogeneity and high genetic load. Production of DH line libraries can help to overcome these problems. Landraces of maize (*Zea mays* L.) represent a vast reservoir of genetic diversity untapped by breeders. Genetic heterogeneity and a high genetic load hamper their use in hybrid breeding. Production of doubled haploid line libraries (DHL) by the in vivo haploid induction method promises to overcome these problems. Böhm et al. [74] developed doubled haploid lines from European flint landraces and reported considerable breeding progress. This reveals that there is tremendous potential of landraces for broadening the narrow genetic base of elite germplasm. DH technology's use demonstrated broadening the flint heterotic pool's narrow genetic base [75]. Altogether, the DH technology also provides new opportunities for characterizing and utilizing the genetic diversity present in gene bank accessions of maize [76].
