ABSTRACT
Genetic diversity is a valuable resource for
improvement in crop productivity and trait performance. Maize (Zea mays
L.) is an important food security crop in Africa constrained with low yield
typically below world average. The forces of population escalation, diminishing
arable land, and climate anomalies with their attendant low yield and crop loss
arising from drought and emerging diseases indicate worsening food insecurity
in Africa. However, genetic improvement in a wide genetic base is a relevant
and logical strategy requiring availability of large reserve of alleles bearing
these traits. This strategy necessitates studies into the well-adapted
traditional African maize germplasm to identify, quantify, and explore the
basis of variation in order to reveal the historical processes that have
created and driven the level of variation for efficient exploitation. The main
objective of this research was to investigate genetic diversity of maize
originating from twelve countries, covering a wide geographical region in three
mega environments in Africa, namely, lowland, mid-altitude and highland
accessions. The accessions were tested under non-stressed environments in Ghana
by evaluation of 29 agro-morphological traits on 35 genotypes, and 16 simple
sequence repeat markers on 57 accessions held in IITA Genetic Resource Center.
Accessions showed wide variations in silk color, kernel arrangement, principal
grain color, and kernel texture except cob color. Significantly different mean
squares and large coefficient of variation indicated substantial variation
among the genotypes for all traits except anthesis-silking interval.
Variability was highest in mid-altitude followed by lowland, and was least in
the highland accessions. Being the most important traits, earliness and grain
yield varied from 49 to 66 days to anthesis and 1.7 to 6.2 Mgha-1,
respectively. Anthesis-silking interval varied from 2 to 6 days. The study
identified a single early-maturing genotype TZm-1376, with
strikingly short ASI of 2 days which also possessed high yield of 5.6 Mgha-1
in consonance to the improved check „‟Obatanpa GH‟‟ with yield of 6.3 Mgha-1..
Unusual combination of early-maturing yet high-yielding accessions were
identified in ten genotypes TZm-4, TZm-41, TZm-270, TZm-1521, TZm-275, TZm-14,
TZm-33 TZm-37, TZm-1367, and TZm-1376. Medium - maturing but high yielding
genotypes which can be incorporated into breeding programs for improvement
included TZm-1434, TZm-1356, TZm-1358 and TZm- 242. The
broad sense heritability estimates were low for all traits in the accessions
except earliness traits in the lowland genotypes. Significant (p≤ 0.01)
positive genotypic correlations of grain yield with hundred kernel weight,
kernel length, tassel length, and ear leaf width indicated that selection for
these traits will lead to simultaneous increase in grain yield. Morphological
genetic similarity measures ranged from 0.00 to 0.80 with overall mean of 0.26
indicating wide genetic variability among accessions. The African landraces
were 23 %, 29 %, and 38 % similar in the mid-altitude, lowland and highland
accessions which agreed with the level of variability revealed by the
descriptive statistical measures. A total of 70 alleles ranging from 2-10 with
a mean of 5.38 alleles per locus for the 14 SSR loci were identified. The total
number of alleles generated across the loci was 1,908 over 57 accessions. Polymorphism Information Content ranged
from 0.18 to 0.81 having an average of 0.64 and 93 % polymorphism rate. The
high average expected heterozygosity of 0.64 indicates abundance of
heterozygosity probably arising from historic admixture of two or more
divergent populations. Molecular analysis revealed average dissimilarity
coefficients of 0.70 for mid-altitude, 0.69 for lowland and 0.65 for highland
accessions with an average of 0.70 ranging from 0.00 to 1.00. These were
consistent with the low similarity values produced by the morphological analysis. The UPGMA produced four and three main clusters for
agro-morphological and SSR analysis, respectively, confirmed by the principal
components biplots. Potential good clusters for exploiting heterosis in maize
breeding programs were identified. The study has revealed wide genetic
diversity in the accessions to permit their utilization as sources of alleles
for improvement in performance and productivity of maize in Africa.
CHAPTER ONE
INTRODUCTION
Maize (Zea mays L.) is a cereal belonging to the family
Poaceae. It is believed to have originated from Central America, specifically
Mexico (Gibson and Benson, 2002). It was first introduced to Africa by the
Portuguese traders in the 16th century and has since become one of the
continent‟s staple food crops making up more than 50% of the total caloric
intake of local diets (Sinha, 2007). The diversified uses for food, feed, and
as a major industrial raw material for many products, including adhesives,
textile, paint, xylose, ethanol, biofuel and a binder for pharmaceuticals make
maize a very important crop in the global economy.
The demand for maize around the world is increasing. In
2010/11 world maize consumption was forecast to rise to 830 million metric tons
(mmt), representing 2% increase in the previous year‟s forecast while
production in the same period was 823 mmt. Over the years, demand for maize has
increased, without commensurate increase in supply. While the International
Food Policy Research Institute (IFPRI, 2003) projected a demand of 852 mmts by
2020, the actual consumption of maize was 868 mmt in 2011/2012 , with a new
projection of over 1,000 mmt by 2018/2019 (International Grain Council, 2013).
With this tendency of consumption exceeding production, as well as actual
consumption surpassing projections, the need to target improvement in maize
productivity has become more important than ever. In sub-Saharan Africa, demand
is expected to double from 27 mmt in 1995 to about 52 mmt by 2020 (Pingali and
Pandey, 2000). As demand for maize increases around the world, there has been a
commensurate increase in the acreage planted, as well as tremendous efforts by
many countries to increase productivity of maize (CIMMYT, 1994). Prior to 2001, average maize
yield of 0.9 to 1.2 t/ha was recorded for sub-Saharan Africa which is just
below a quarter of the global average of 5.5 t/ha, and about a sixth of the
average yield of 7.8 t/ha in the U.S.A. (FAOSTAT, 2006).
In recent years, impressive advancements in maize
productivity have been achieved through conventional breeding in West and
Central Africa raising the productivity from 1.2 t/ha to 3-5 t/ha. To support
this, the Food and Agricultural Organization (FAO) reported that in 2005 six
countries in Africa produced twice the amount consumed, while eight other
countries imported 5-35% and 11 countries also imported 57-100% of the maize
consumed in their respective countries (FAO, 2007). Nevertheless, maize yield
in West/Central Africa still remains below the world average.
The key constraints to crop production globally are limited
land and water resources, expanding population and abiotic and biotic stresses.
In sub-Saharan Africa, the disparity in maize productivity is exacerbated by
nutrient poor soils, drought, disease, and use of unimproved seed. The great
demand for maize of both quality and quantity requires more rapid genetic
improvement. Efforts have to be made to increase maize yield in Africa.
Genetic improvement of any crop begins with an evaluation of
the genetic diversity present in the germplasm. Genetic diversity estimates
provide valuable information for classification of germplasm for guidance in
performing crosses in crop improvement programmes. It also provides the basis
for devising strategies for conservation and sustainability. In the current
climate variability phenomena, the need for developing genotypes having less
vulnerability to drought, pest and disease resistance traits combined with high yield in a wide genetic
base is relevant. Genetic diversity information is useful for
identification of useful genes among germplasm, for inbred line development,
for assignment of inbred lines into heterotic groups, and for identification of
testers.
To date only few reports of detailed assessment of genetic
diversity among the accessions adapted to sub-Saharan Africa are documented.
Genetic diversity studies on Ethiopian maize genotypes were reported by Beyene et
al. (2006) and Legesse et al. (2007). In Ghana, Obeng-Antwi (2007) reported
genetic diversity estimates of 90 landraces. Oppong et al. (2014) worked on
genetic characterization of Ghanaian maize landraces using microsatellite
markers. Some 294 Zimbabwe, Zambia and Malawi maize genotypes were evaluated
for genetic diversity estimates (Magorokosho, 2006). Sanou et al. (1997)
determined the diversity in some West African maize genotypes by means of
isozyme diversity.
In contrast, genetic diversity among maize germplasm in North
America (James et al., 2002; Bretting et al., 1990; Smith 1986; Goodman and
Stuber, 1983 and Kahler et al., 1983.), in CIMMYT (Warburton et al., 2005;
2002; Xia et al., 2005; Carvalho et al., 2002), in Europe (Hartings et al.,
2008; Okumus, 2007), and Asia (Enoki et al., 2002; Yuan et al., 2000) have been
evaluated. As a result of this lack of information, maize breeding efforts in
sub-Saharan Africa is seriously limited. To date, old breeding materials, and
few newly developed inbred lines have been culled from CIMMYT lines, producing
maize of narrow genetic base. While landraces are known to possess many useful
alleles for crop improvement, the utilization of African landraces in breeding
programs have not been exploited. For example, the popular maize genotype,
„Obatanpa GH‟ which is in the pedigree of many of the maize cultivars produced in Ghana was
bred from CIMMYT Population 63 maize of Mexican origin (Badu-Apraku, 2006).
„Obatanpa GH‟ (Reg. no. CV-1, PI 641711), a tropically
adapted, intermediate maturing, open-pollinated cultivar was developed by the
Crops Research Institute (CRI), Kumasi, Ghana in collaboration with the
International Institute of Tropical Agriculture (IITA), the International Maize
and Wheat Improvement Center (CIMMYT), and the Sasakawa Global 2000. „Obatanpa
GH‟ is a white dent and flint endosperm Quality Protein Maize (QPM) with
elevated levels of lysine and tryptophan and was first released by CRI, Ghana
in 1992 as „Obatanpa‟ to help improve the protein nutritional status and the
health of a large population of low-income groups in sub- Saharan Africa who
depend on maize as a major component of their dietary protein intake (Sallah,
1998).
The Plant Genetic Resources Research Institute (PGRRI) in
Ghana has in store some 400 maize accessions collected in 1991. The
International Institute of Tropical Agriculture, Nigeria, also has over 800
maize accessions collected from many agroecological zones in Africa (www.iita.org, verified March 04, 2015).
Evaluation of the genetic diversity estimates within the large African maize
germplasm has the potential to reveal useful alleles for future maize breeding
programs.
The markers commonly used in genetic diversity studies
include morphological trait evaluation, isozyme and molecular markers. Accurate
estimation of genetic diversity requires the use of very efficient marker
protocols that detect fine genetic differences between the accessions. Maize
breeders in India, as in most developing countries, have differentiated
accessions mainly on the basis of major morphological characters such as plant
height, anthocyanin pigmentation of various plant parts, tassel type, tassel branching, number of days to
flowering, ear characteristics, cob colour, grain colour and grain type (Virk
and Witcombe, 1997). Although morphological descriptions are important for
ascertaining the agronomic utility of germplasm, such descriptions are not very
reliable because of complex genotype×environment interactions that require
assessment in multiple environments (Enoki, 2002; Smith and Smith, 1989), are
time-consuming, labour-intensive and require large populations (Botha and
Venter, 2000). Isozyme analysis is relatively simple and less costly compared
with molecular marker analysis; however inadequate genomic coverage, relatively
low levels of polymorphism, developmental regulation and pleiotropic effects
impose major constraints in effectively using these markers in genotype
differentiation and analysis of genetic diversity (Dubreuil et al., 1996; Smith
and Smith, 1986).
Molecular markers have proved to be more powerful tools in
genetic diversity and mapping studies. The available molecular markers include
Random Amplified Polymorphic DNA (RAPDs), Restriction Fragment Length
Polymorphism (RFLPs), Amplified Fragment Length Polymorphism (AFLP), Simple
Sequence Repeats (SSR), Sequence Tagged Sites (STS) and Single Nucleotide
Polymorphisms (SNPs)
Among the different types of PCR- based DNA markers available
for diverse applications in maize breeding, SSR markers (microsatellites) are
often preferred because they are less costly, simple to prepare, offer greater
reliability and reproducibility and more effective than the other markers
(Smith et al., 1997). The SSR markers are robust, codominant, hypervariable,
abundant, and uniformly dispersed in plant genomes. Senior and Heun (1993)
reported that SSR loci provide a high level of polymorphism in maize. Pejic et
al. (1998) and Smith et al. (1997) reported of good correlation between
SSR and RFLP diversity and pedigree-based measurements. Moreover, the
efficiency of SSRs can be increased by running multiplexed reactions under
automated electrophoresis conditions (Mitchell et al., 1997).
Genetic diversity study among the African maize inbred lines
present in the CIMMYT Centers in Ethiopia and Zimbabwe has been determined
using SSR markers (Legesse et al., 2007). Menkir et al. (2004) assessed the
genetic relationships among tropical mid-altitude inbred lines developed in
Nigeria and Cameroon, using AFLP and SSR markers. Beyenne et al. (2006)
evaluated genetic diversity of traditional Ethiopian highland maize accessions
by SSR markers. Reif et al. (2003) determined the genetic distance and
heterosis in tropical maize populations by means of SSR markers. The genetic
diversity estimates among maize accessions originating from Zambia, Zimbabwe,
and Malawi were determined by SSRs (Magorokosho, 2006).
Reif et al., (2004) determined genetic diversity within and
among CIMMYT maize populations of tropical, subtropical, and temperate
germplasm by means of SSR markers. Warburton et al. (2008) estimated genetic
diversity in CIMMYT non-temperate maize germplasm, including landraces, open
pollinated varieties, and inbred lines using SSRs. Equivalent information on
genetic diversity among West and Central African maize accessions is not
available, hence they have not formed part of maize breeding programs in
Africa.
The main objective of this research
is to estimate genetic diversity and groupings among fifty-seven maize
accessions originating from three ecological zones in Africa, viz, lowland,
mid-altitude and highland regions. The specific objectives include:
Assessment of genetic diversity by means of morphological
trait markers
Assessment of genetic diversity by means of SSR profiling
Determination of groupings within the maize collection for
the purpose of hybrid breeding
Hypothesis of current research is based on two themes
delineated as:
Over a long period of time, forces of evolution including
mutation, recombination, selection, migration, and genetic drift have
introduced allelic variation in the African maize germplasm pool
That the allelic variation can be estimated from marker
polymorphisms
Justification
The information on genetic diversity study among the maize
germplasm in Africa will be useful for identification of useful genotypes for
enhancing the performance of commercial cultivars in future breeding programs,
for application in organizing and managing the germplasm in Genetic Resource
Centers, for widening the genetic base of the gene pool, and for identification
of heterotic groups for hybrid breeding. Effective plant breeding and crop improvement
programs for food security depend on the availability of genetic diversity
information.
For more Crop & Soil Sciences Projects Click here
===================================================================Item Type: Ghanaian Topic | Size: 146 pages | Chapters: 1-5
Format: MS Word | Delivery: Within 30Mins.
===================================================================
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.