An integrated molecular and conventional breeding scheme for enhancing genetic gain in maize in Africa

Open Access Journal; Published online: 06 Nov 2019Maize production in West and Central Africa (WCA) is constrained by a wide range of interacting stresses that keep productivity below potential yields. Among the many problems afflicting maize production in WCA, drought, foliar diseases, and parasitic weeds are the most critical. Several decades of efforts devoted to the genetic improvement of maize have resulted in remarkable genetic gain, leading to increased yields of maize on farmers’ fields. The revolution unfolding in the areas of genomics, bioinformatics, and phenomics is generating innovative tools, resources, and technologies for transforming crop breeding programs. It is envisaged that such tools will be integrated within maize breeding programs, thereby advancing these programs and addressing current and future challenges. Accordingly, the maize improvement program within International Institute of Tropical Agriculture (IITA) is undergoing a process of modernization through the introduction of innovative tools and new schemes that are expected to enhance genetic gains and impact on smallholder farmers in the region. Genomic tools enable genetic dissections of complex traits and promote an understanding of the physiological basis of key agronomic and nutritional quality traits. Marker-aided selection and genome-wide selection schemes are being implemented to accelerate genetic gain relating to yield, resilience, and nutritional quality. Therefore, strategies that effectively combine genotypic information with data from field phenotyping and laboratory-based analysis are currently being optimized. Molecular breeding, guided by methodically defined product profiles tailored to different agroecological zones and conditions of climate change, supported by state-of-the-art decision-making tools, is pivotal for the advancement of modern, genomics-aided maize improvement programs. Accelerated genetic gain, in turn, catalyzes a faster variety replacement rate. It is critical to forge and strengthen partnerships for enhancing the impacts of breeding products on farmers’ livelihood. IITA has well-established channels for delivering its research products/technologies to partner organizations for further testing, multiplication, and dissemination across various countries within the subregion. Capacity building of national agricultural research system (NARS) will facilitate the smooth transfer of technologies and best practices from IITA and its partners

Assessing the Genetic Diversity in Crops with Molecular Markers: Theory and Experimental Results with CIMMYT Wheat and Maize Elite Germplasm and Genetic Resources

Genetic diversity is a valuable natural resource and plays a key role in future breeding progress. Germplasm collections as a source of genetic diversity must be well-characterized for an efficient management and effective exploitation. The advent of PCR-based molecular markers such as sim-ple sequence repeats (SSRs) has created an opportunity for fine-scale genetic characterization of germplasm collections. The objective of this research was to optimize the utilization of genetic re-sources conserved at the International Wheat and Maize Improvement Center (CIMMYT), with the aid of DNA markers. Choice of suitable dissimilarity measures is important to facilitate the interpretation of findings from DNA marker studies on a theoretically sound basis. The objective of a theoretical study was to examine 10 dissimilarity coefficients widely used in germplasm surveys, with special focus on applications in plant breeding and seed banks. The distance and Euclidean properties of the dissimi-larity coefficients were investigated as well as the underlying genetic models. Application areas for different coefficients were suggested on the basis of the theoretical findings. It has been claimed that plant breeding reduces genetic diversity in elite germplasm, which could seriously jeopardize the continued ability to improve crops. The objectives of the presented ex-perimental study with wheat were to examine the loss of genetic diversity during (i) domestication of the species, (ii) change from traditional landrace cultivars (LC) to modern breeding varieties, and (iii) intensive selection over 50 years of international breeding. A sample of 253 CIMMYT or CIMMYT-related modern wheat cultivars, LC, and Triticum tauschii accessions were characterized with up to 90 SSR markers covering the entire wheat genome. A loss of genetic diversity was ob-served from T. tauschii to LC and from LC to the elite breeding germplasm. Wheat genetic diver-sity was narrowed from 1950 to 1989, but was enhanced from 1990 to 1997. The results indicate that breeders averted the narrowing of the wheat germplasm base and subsequently increased the genetic diversity through the introgression of novel materials. The LC and T. tauschii contain nu-merous unique alleles that were absent in modern wheat cultivars. Consequently, both LC and T. tauschii represent useful sources for broadening the genetic base of elite wheat breeding germ-plasm. In the 1980's, CIMMYT generated more than 100 maize populations and pools but little is known about the genetic diversity of this germplasm. The objective of the study with 23 CIMMYT maize populations was to characterize their population genetic structure with SSRs. The populations adapted to tropical, subtropical intermediate-maturity, subtropical early-maturity, and temperate mega-environments (ME) were fingerprinted with 83 SSR markers. Estimates of genetic differen-tiation between populations revealed that most of the molecular variation was found within the populations. Principal coordinate analysis based on allele frequencies of the populations revealed that populations adapted to the same ME clustered together and, thus, supported clearly the ME structure. Novel strategies were suggested to optimize the conservation of the genetic diversity within and among the populations. Heterotic groups and patterns are of fundamental importance in hybrid breeding. The objective of the presented study with a subset of 20 out of the 23 maize populations was to investigate the rela-tionship between heterosis and genetic distance determined with SSR markers. The published data of three diallels and one factorial trial evaluated for grain yield were re-analyzed to calculate het-erosis in population hybrids. Correlations of squared modified Rogers distance and heterosis were mostly positive and significant, but adaption problems caused deviations in some cases. For popu-lations adapted to the target regions, genetic distance can be used as a further criterion in the search for promising heterotic patterns and groups. For intermediate- and early-maturity subtropical germ-plasm, two heterotic groups were suggested, consisting of a flint and dent composite. For the tropi-cal germplasm, it was possible to assign population (Pop29) to the established heterotic group A and propose new heterotic groups (Pop25, Pop43). Our experimental results corroborate that SSRs are a powerful tool to (i) detect relationships among different germplasm, (ii) assess the level of genetic diversity present in germplasm pools and its flux over time, and (iii) search for promising heterotic groups for hybrid breeding in complementa-tion to field trials.Die genetische Diversität ist für den zukünftigen Züchtungsfortschritt von zentraler Bedeutung. In Genbanken ist ein bedeutender Anteil der Diversität von Nahrungspflanzen konserviert. Eine optimale Erhaltung und bestmögliche Nutzung dieser genetischen Vielfalt bedarf einer fundierten Charakterisierung der vorhandenen Genotypen. DNA Marker stellen hierzu ein vielversprechendes Werkzeug dar. Die vorliegende Arbeit befasst sich daher mit dem Einsatz von Markertechnologie zur Nutzbarmachung genetischer Ressourcen des Internationalen Mais- und Weizenforschungszentrums (CIMMYT) für die Pflanzenzüchtung. Die Wahl eines geeigneten Ähnlichkeitskoeffizienten spielt bei der Interpretation von Ergebnissen aus DNA-Markerstudien eine entscheidende Rolle.In einer theoretischen Untersuchung wurden zehn häufig in Diversitätsanalysen benutzte Ähnlichkeitskoeffizienten im Hinblick auf ihre Eignung für Pflanzenzüchtungs- und Genbankstudien untersucht. Die den Ähnlichkeitskoeffizienten zugrundeliegenden mathematischen und genetischen Konzepte wurden detailliert diskutiert. Auf der Grundlage dieser Ergebnisse konnten für die unterschiedlichen Koeffizienten Anwendungsgebiete vorgeschlagen werden. Eine populäre Hypothese ist, dass Pflanzenzüchtung die genetische Diversität im Elitezuchtmaterial reduziert und somit den zukünftigen Zuchtfortschritt gefährdet. Ziel einer experimentellen Arbeit mitWeizen war, einen möglichen Diversitätsverlust zu untersuchen während (i) der Domestikation dieser Art, (ii) dem übergang von traditionellen Landsorten (LC) zu modernen Weizensorten (MWC) und (iii) 50 Jahren intensiver Selektion durch internationale Weizenzüchtung. Eine Stichprobe von 253 CIMMYT oder CIMMYT verwandten MWC, LC und Triticum Tauschii Akzessionen wurde mit 90 SSRs genotypisiert. Ein drastischer genetischer Diversitätverlust wurde beim Vergleich von T. tauschii mit den LR und LR mit den MWC beobachtet. Die genetische Vielfalt von MWC nahm von 1950 bis 1989 ab, stieg aber von 1990 bis 1997 wieder an. Die Befunde deuten darauf hin, dass die Weizenzüchter am CIMMYT die Gefahr einer Einengung der genetischen Basis erkannten und erfolgreich die genetische Diversität im Zuchtmaterial durch Introgression neuer Genressourcen erweiterten. Zahlreiche Allele waren in LC oder in T. tauschii vorhanden, die jedoch in MWC nicht gefunden wurden. Folglich stellen sowohl LC als auch T. tauschii eine wertvolle Ressource zur Erweiterung der genetischen Basis des Elitezuchtmaterials bei Weizen dar. In den 80'er Jahren wurden am CIMMYT über 100 Maispopulationen etabliert. Allerdings ist wenig über die genetische Diversität dieses Pflanzenmaterials bekannt. Eine Untersuchung von 23 Maispopulationen zielte auf die Charakterisierung ihrer populationsgenetischen Struktur mit SSR Marker Daten ab. Insgesamt 672 Genotypen der Maispopulationen, adaptiert an tropische, subtropische und gemäßigte Anbauzonen (ME), wurden mittels 83 SSR Markern molekularbiologisch charakterisiert. Der größte Teil der genetischen Varianz wurde innerhalb der Maispopulationen detektiert und der geringere Teil zwischen den Populationen. Eine Hauptkoordinatenanalyse, basierend auf den Populationsallelfrequenzen, ergab eine Gruppierung von Populationen, die an die gleichen Umweltbedingungen adaptiert sind und stützt somit die Einteilung in ME. Es konnten alternative Strategien vorgeschlagen werden, um den Erhalt der genetischen Diversität zwischen und innerhalb der Populationen zu verbessern. Heterotische Gruppen sind von grundlegender Bedeutung in der Hybridzüchtung. Eine Studie mit 20 der 23 Maispopulationen sollte die Beziehung zwischen Heterosis und genetischen Distanzen auf der Grundlage von SSR Markern untersuchen. Publizierte Ergebnisse für den Kornertrag von vier Experimenten mit diallelen bzw. faktoriellen Populationskreuzungen wurden reanalysiert und der Heterosiszuwachs der Populationshybriden berechnet. Die Korrelationen zwischen genetischen Distanzen und Heterosiszuwachs waren meist positiv und signifikant. Allerdings verursachten Adaptionsprobleme in einigen Fällen Abweichungen. Bei Populationen, die an die Zielumwelten angepasst sind, können genetische Distanzen zur Etablierung heterotischer Gruppen benutzt werden. Im subtropisch adaptierten Material wurden zwei heterotische Gruppen, bestehend aus einer Dent- und Flint-Mischpopulation, vorgeschlagen. Bei den tropischen Populationen konnte Population Pop29 in die bereits etablierte heterotische Gruppe A eingeordnet und zwei neue heterotische Gruppen (Pop25, Pop43) vorgeschlagen werden. Nach den Ergebnissen dieser Studie sind SSR Analysen eine geeignete Methode, um (i) Verwandtschaftsbeziehungen aufzudecken, (ii) den zeitlichen Trend und die vorhandene genetische Diversität in Populationen zu untersuchen und (iii) vielversprechende heterotische Gruppen in Kombination mit Feldversuchen zu etablieren

Analysis of genetic diversity among tropical and subtropical maize inbred lines using SSR markers

Genetic diversity of 24 tropical and subtropical elite maize lines was assessed at molecular level employ-ing 42 Simple Sequence Repeats. A total of 107 alleles with an average of 2.55 alleles per locus were detected. The Polymorphism Information Content (PIC) values of 42 SSR loci ranged from 0.08 (UMC1428) to 0.68 (UMC2189 and UMC2332) with the overall calculated PIC mean value of 0.44, whereas the Discrimination Rate (DR) value for SSR markers ranged from 0.09 (UMC2089) to 0.42 (UMC1311) with the average DR value of 0.26. Pair-wise genet-ic similarity (GS) values, calculated by Jaccard’s coefficients, ranged between 0.25 and 0.78 with a mean genetic similarity of 0.63, indicating the existence of adequate amount of genetic divergence among the genotypes selected for the study. The cluster dendrogram separated all the inbred lines into six main clusters with sub clusters based on genetic similarity. Factorial analysis also confirmed a nearly similar pattern for grouping these inbred lines as pre-sented by cluster dendrogram. In this study, SSR markers were found to be powerful tool for detection of genetic diversity in maize inbred lines. These findings could provide information for effective utilization of these materials for development of maize hybrids as well as for genetic improvement of inbred lines

Theoretical and experimental investigations on the exploitation of heterosis in hybrid breeding

Hybrid breeding has played a key role in the improvement of the productivity of many crops. Genetic variability, an essential prerequisite in plant breeding, is expected to decrease in heterotic groups as a result of continuous breeding efforts. The consequences of the narrowing of genetic variability are a decrease in selection gain and an increase in the susceptibility of cultivars. Thus, establishment of heterotic groups and broadening the genetic base of established heterotic groups are very important research topics in hybrid breeding. Our objectives were to (1) evaluate heterosis in winter triticale and identify heterotic groups based on field and SSR marker data, (2) investigate by field evaluations and SSR markers the heterotic relationships between the Central European heterotic groups in maize and rye and exotic germplasm from the US and Eastern Europe, respectively, and (3) monitor temporal changes over 30 years in the magnitude of variances due to general combining ability (GCA) and specific combining ability (SCA) in an applied hybrid maize breeding program. Triticale has low heterosis and, therefore, no hybrid cultivars have been developed, although an effective CMS system is available. Twenty-one lines and their 210 diallel crosses were field-evaluated for grain yield at five locations in Germany. 95 SSR markers were used to analyze the parents for identifying two diverse subgroups. Hybrid performance, midparent heterosis, and estimates of variance due to GCA and SCA were determined in a diallel, a 10 × 11 factorial, and the remaining two sub-diallels with 10 and 11 parents. In addition, we applied an enumeration algorithm, which explored the entire sample space to identify diverse heterotic groups and optimize different criteria in this context. The ratio of variance due to GCA to variance due to SCA was higher for factorials between groups of parents than in diallels and subdiallels within groups. The analyses indicated a more favorable ratio in situations with genetically distinct populations compared to situations with genetically less distinct populations. Application of the enumeration algorithm improved all criteria. F1 performance followed by heterosis were the most important criteria for development and enrichment of heterotic groups. Introgression of new germplasm to broaden the genetic base of heterotic groups is required to ensure continued genetic gains in hybrid breeding. In maize, we evaluated 19 inbreds belonging to two Central European heterotic groups and US heterotic groups and their factorial crosses in F1 and F2 generations for grain yield and dry matter concentration. The parental inbreds were additionally fingerprinted with 266 SSR markers. Multi-environment evaluation was performed in three mega-environments: Central Europe, US Cornbelt, and Southeast Europe. We found higher genetic diversity in the exotic germplasm than in the Central European heterotic groups. Based on F1 performance and heterosis, we conclude that non-Stiff Stalk germplasm should be introgressed into the Flint group and the Stiff Stalk germplasm into the Dent group. In rye, we evaluated testcrosses of 610 S0 clones belonging to the two Central European heterotic groups and five East European open-pollinated varieties (OPVs). S0 clones were fingerprinted with 30 SSR markers. We found higher genetic diversity in the OPVs compared to the Central European heterotic groups. The Carsten group had a narrow genetic base and should, therefore, be the primary target for genetic broadening. Nevertheless, all five OPVs were genetically closer to Petkus than Carsten. Two OPVs were identified as good candidates for introgression into Petkus and one into Carsten. We suggest to use selected clones of these populations for introgression. Continuous selection is expected to narrow the genetic base of heterotic groups over time. We studied the nature and magnitude of genetic variability in the breeding materials of the maize program of the University of Hohenheim, which is based on two heterotic groups with continuous enrichment by other germplasm. The data generated in multilocation field trials based on inter-group factorial designs conducted from 1975 to 2004 for grain yield and dry matter concentration were analyzed. There was neither a decrease in the magnitude of genetic variance nor a change in the predominance of variance due to GCA over variance due to SCA. Consequently, for avoiding the adverse effects of selection on genetic variation and for ensuring medium and long-term selection gains, heterotic groups should not be treated as closed populations, but should be continuously enriched by introgression of new germplasm.Für die Verbesserung vieler Kulturpflanzen spielt die Hybridzüchtung eine Schlüsselrolle. Genetische Varianz ist eine wichtige Voraussetzung für Pflanzenzüchtung, die bedingt durch den kontinuierlichen Züchtungsfortschritt in heterotischen Gruppen, abnimmt. Konsequenzen sind sinkender Selektionsgewinn und steigende Anfälligkeit der Sorten. Deshalb sind die Etablierung und die Erweiterung heterotischer Gruppen sehr wichtig für die Hybridzüchtung. Die Ziele sind: (1) Bewertung der Heterosis in Inzuchtlinien von Wintertriticale und Bestimmung heterotischer Gruppen basierend auf Feld- und SSR-Markerdaten, (2) Untersuchung der heterotischen Beziehungen zwischen den mitteleuropäischen heterotischen Gruppen bei Mais und Roggen und exotischem Genmaterial aus den USA und Osteuropa basierend auf Ergebnissen von Feldexperimenten und SSR-Markern, sowie (3) Untersuchungen der Veränderung des Ausmaßes an Varianz der allgemeinen (GCA) und der spezifischen Kombinationsfähigkeit (SCA) in einem Hybridmaiszuchtprogramm über 30 Jahre. Triticale hat eine niedrige Heterosis, weshalb keine Hybridsorte entwickelt wurde, obwohl ein effektives CMS System vorhanden ist. In unserer Studie wurden 21 Linien und ihre 210 diallelen Kreuzungen auf Kornertrag in Feldexperimenten bewertet. Die Eltern wurden mit 95 SSR-Markern analysiert, um verschiedene Untergruppen zu identifizieren. Hybridleistung, Heterosis und Schätzwerte für GCA- und SCA-Varianz wurden in einem Diallel, einem 10 x 11 faktoriellen Kreuzungsschema und den verbleibenden zwei Unterdiallelen bestimmt. Zusätzlich wurde ein Aufzählungsalgorithmus verwendet, der alle Kombinationen bestehend aus zwei disjunkten Untermengen von Linien untersucht, um heterotisches Genmaterial zu identifizieren und verschiedene Kriterien in diesem Kontext zu optimieren. Das Verhältnis der GCA- zur SCA-Varianz war für die faktoriellen Kreuzungen größer als in den diallelen Kreuzungen und Unterdiallelen. Die Ergebnisse zeigten, dass in Situationen mit genetisch verschieden Populationen ein günstigeres Verhältnis der GCA- zur SCA-Varianz erreicht werden kann als mit Populationen, die nicht genetisch verschieden sind. Mit dem Aufzählungsalgorithmus konnte eine Verbesserung aller Kriterien erreicht werden (F1 Leistung gefolgt von Heterosis sind die wichtigstenKriterien für die Entwicklung und Erweiterung der heterotischen Gruppen). Die Introgression neuen Genmaterials zur Verbreiterung der genetischen Basis von heterotischen Gruppen ist erforderlich, um einen kontinuierlichen Selektionsfortschritt in der Hybridzüchtung sicherzustellen. In einer Maisstudie untersuchten wir 19 Inzuchtlinien aus zwei mitteleuropäischen und drei amerikanischen heterotischen Gruppen. Die faktoriellen Kreuzungen in den Generationen F1 und F2 wurden für die Eigenschaften Kornertrag und Korntrockensubstanzgehalt geprüft. Die Eltern wurden mit 266 SSR-Markern genotypisiert. Die Prüfglieder wurden in mehreren Umwelten in drei Mega-Umwelten getestet: Mitteleuropa, US Cornbelt und Südosteuropa. In dem exotischen Genmaterial wurde eine höhere genetische Diversität gefunden als in den mitteleuropäischen heterotischen Gruppen. Aufgrund der F1 Leistung und Heterosis konnte der Schluss gezogen werden, dass eine Einlagerung von non-Stiff Stalk Genmaterial in Flint und Stiff Stalk Genmaterial in Dent empfehlenswert ist. In einer Roggenstudie wurden Testkreuzungen von 610 S0 Klonen, die den beiden mitteleuropäischen heterotischen Gruppen und fünf osteuropäischen offenabblühenden Populationenssorten (OPVs) angehörten, bewertet. Die S0 Klone wurden mit 30 SSR-Markern genotypisiert. In den OPVs lag eine höhere genetische Diversität vor als in den mitteleuropäischen heterotischen Gruppen. Carsten hatte eine engere genetische Basis und sollte deshalb vorrangig erweitert werden. Trotzdem waren alle fünf OPVs genetisch näher an Petkus. Zwei Populationen wurden als gute Kandidaten für die Erweiterung der genetischen Diversität von Petkus und eine für die Erweiterung von Carsten eingestuft. Dabei könnten einzelne Klone dieser Populationen für die Introgression verwenden werden. Es wird erwartet, dass Selektion über längere Zeit die genetische Basis der heterotischen Gruppen einengt. Wir untersuchten die Art und das Ausmaß genetischer Varianz im Maiszuchtmaterial der Universität Hohenheim, welches auf zwei heterotischen Gruppen basiert, die kontinuierlich mit anderem Genmaterial erweitert wurden. Es wurden Leistungsprüfungsergebnisse aus den Jahren 1975 bis 2004 für Kornertrag und Korntrockensubstanzgehalt aus mehrortig geprüften faktoriellen Kreuzungen zwischen Flint- und Dent-Linien ausgewertet. Es konnte weder eine Reduktion der genetischen Varianz noch eine Änderung im Verhältnis der GCA- zur SCA-Varianz festgestellt werden. Daraus folgt, dass heterotische Gruppen nicht als geschlossene Populationen zu betrachten sind, sondern kontinuierlich durch Introgression von neuem Genmaterial erweitert werden sollten

Performance based grouping of adapted and exotic drought tolerant maize (Zea mays L) inbred lines under stressed and nonstressed conditions

Knowledge of the heterotic responses of elite adapted and exotic maize inbred lines can facilitate their utilization for population improvement and hybrid development. In the present study, a line x tester mating design was used to determine the combining ability of 20 elite drought-tolerant maize inbred lines developed at CIMMYT and IITA and to classify them into heterotic groups under diverse growing conditions. The 20 lines were crossed each with two inbred line testers representing the tropical and temperate heterotic pattern established in West and Cental Africa (WCA), to generate 40 testcrosses. A trial comprising the 40 testcrosses along with the cross between the two testers and three hybrid checks were evaluated at two environments in the dry season and at six environments in the rainy season. GCA effects were more important than SCA effects in controlling grain yield in both seasons. Two exotic lines in the dry season and four exotic lines in the rainy season had significantly positive GCA effects. Only EXL22 was identified as a superior line in the two seasons. Only two adapted lines had significantly positive GCA effects in either or both seasons while three adapted lines consistently had significantly negative GCA effects in both seasons. Hybrid between EXL22 and tester 9071 showed broad adaptation to all test environments. The two testers separated some of the lines into two main heterotic groups. the lines in each heterotic group and the good combiners will be utilized for developing populations for extracting new improved inbred lines

Genetic diversity of Swiss maize ( Zea mays L. ssp. mays ) assessed with individuals and bulks on agarose gels

About 65years ago, more than 150 Swiss maize landraces (Zea mays L. ssp. mays) of the flint type were collected and conserved ex situ. Due to the climatically and culturally diverse environment of the Alps, a considerable genetic diversity of this material was assumed. To prove this, an efficient method was required to carry out genetic profiling of all the accessions in the Swiss Gene Bank. Simple sequence repeat marker (SSR) profiling in combination with the visualization of the polymerase chain reaction (PCR) products on agarose gels was chosen. Here a set of 19 different landrace accessions was analyzed to: (i) investigate their genetic diversity, (ii) investigate and display the population structure and (iii) determine whether DNA bulks rather than single plants can be used for such analyses. Four repeated samples of one accession were found to be much closer to one another than to the rest of accessions. Furthermore, specific alleles were identified for several accessions. The PCR products of the bulked DNA samples represented only a small part of the variation revealed by the analysis of individuals. Loci with four base repeat motifs performed better in the analysis of bulks than loci with other repeat motifs. The correlation between genetic distance matrices, based on the analysis of individuals and bulks, respectively, was significant. Thus, the single plant approach allowed for sufficient differentiation of accessions, and DNA bulks visualized on agarose gels led to correlated genetic distances although a limited number of alleles were detected. Although the limited resolution of agarose gels likely causes some bias, profiling of larger sets with the individual plant approach appears feasible and more informative compared to the bulk analysis we conducte

Molecular breeding for resilience in maize - A review

Abiotic and biotic constraints have widespread yield reducing effects on maize and should receive high priority for maize breeding research. Molecular Breeding offers opportunities for plant breeders to develop cultivars with resilience to such diseases with precision and in less time duration. The term molecular breeding is used to describe several modern breeding strategies, including marker-assisted selection, marker-assisted backcrossing, marker-assisted recurrent selection and genomic selection. Recent advances in maize breeding research have made it possible to identify and map precisely many genes associated with DNA markers which include genes governing resistance to biotic stresses and genes responsible for tolerance to abiotic stresses. Marker assisted selection (MAS) allows monitoring the presence, absence of these genes in breeding populations whereas marker assisted backcross breeding effectively integrates major genes or quantitative trait loci (QTL) with large effect into widely grown adapted varieties. For complex traits where multiple QTLs control the expression, marker assisted recurrent selection (MARS) and genomic selection (GS) are employed to increase precision and to reduce cost of phenotyping and time duration. The biparental mapping populations used in QTL studies in MAS do not readily translate to breeding applications and the statistical methods used to identify target loci and implement MAS have been inadequate for improving polygenic traits controlled by many loci of small effect. Application of GS to breeding populations using high marker densities is emerging as a solution to both of these deficiencies. Hence, molecular breeding approaches offers ample opportunities for developing stress resilient and high-yielding maize cultivars

Application of Microsatellites in Genetic Diversity Analysis and Heterotic Grouping of Sorghum and Maize

Sorghum and maize are major cereal crops worldwide and key food security crops in Sub-Saharan Africa. The difference in the mating systems, maize as predominantly a cross-fertilizer and sorghum as a self-fertilizer is reflected in differences in visible phenotypic and genotypic variations. The reproductive differences dictate the level of genetic variation present in the two crops. Conventionally, a heterotic group assignment is made based on phenotypic values estimated through combining ability and heterosis analyses. However, phenotypic evaluation methods have their limitation due to the influence of the environment and may not reflect the heterotic pattern of the lines accurately. Therefore, more effective and complementary methods have been proposed for heterotic grouping of candidate lines. Estimation of molecular-based genetic distance has proven to be a useful tool to describe existing heterotic groups, to identify new heterotic groups, and to assign inbreds into heterotic groups. Among the molecular markers, microsatellites markers have proved to be a powerful tool for analyzing genetic diversity and for classifying inbred lines into heterotic groups. Therefore, the aim of this chapter was to elucidate the use of microsatellite markers in genetic diversity analysis and heterotic grouping of sorghum and maize