ABSTRACT
Genetic
diversity and heterosis (mid and better parent heterosis) was assessed in a
West African breeding population by using 15 simple sequence repeat markers in
28 parents exclusively and other 10 parents with their 30 progenies in
sweetpotato. The polymorphism information content (PIC) of the SSR primers used
revealed that all of them were polymorphic except IB-297 and J116 A with PIC
less than 50% for the 10 parents and their progenies and 6 primers (IB-248,
IBS-10, IBS-18, IBR-12, IBR-16 and IBR-19) out of 15 were polymorphic for the
28 parents. These results mean that most of the primers used in this work can
be used for parent and progeny diversity study in sweetptatoto. The progenies
were produced from biparental crosses, and heterosis was estimated in some
pre-harvest and harvest traits, harvest index and some quality traits. Parents
PG12086-18, Nanungungungu and Apomuden occur most in the crosses. Some of the
crosses exhibited high heterosis. Correlation among total yield and
pre-harvest, harvest and quality traits revealed highly significant positive
correlations between total yield and marketable roots, root size, number of
marketable roots and harvest index. Vine vigour was also correlated
significantly and positively with the weight of marketable roots, root size,
number of plants harvested, percentage dry matter, iron and starch contents.
Virus severity was significantly correlated negatively with the number of
plants at maturity, percentage dry matter, iron content, percentage protein and
starch content but not significant with total yield, weight of marketable
roots, root size and number of marketable roots. The weevil damage was
significantly correlated negatively with total yield, weight of marketable
roots, root size, number of marketable roots and harvest index, and Percentage
dry matter correlated significantly and positively with iron content,
percentage protein, starch and zinc contents. The diversity study showed that
the parents could be grouped into five clusters. The progenies from distantly
related parent such as Nanungungungu x Bohye, Nanungungungu x
Faara, Nanungungungu x Hi-starch, Faara x Nanungungungu, Nanungungungu x
PG12086-18, Apomuden x Sauti and CIP440390 x PG12086-18 exhibited high
heterosis for total yield, harvest index and the quality traits; and
Nanungungungu x Otoo, Sauti x Nanungungungu, Apomuden x Hi-starch, PG12086-18 x
Apomuden, Apomuden x Faara and Apomuden x PG12086-18 exhibited high heterosis
for some yield related traits and quality traits. This study showed that all
the progenies were not close to their parents due to the high somatic
transformation in sweetpotato which is a source of genetic variation among
genotypes.
TABLE OF CONTENTS
ABSTRACT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF ABBREVIATIONS
CHAPTER ONE
1.0 INTRODUCTION
CHAPTER TWO
2.0 LITERATURE REVIEW
2.1 Origin and evolution
2.2 Biology and morphology
2.3 Importance of sweetpotato
2.3.1 In human diet and
animal feeding
2.3.2 In health
2.3.3 In industries
2.4. Major pest and diseases
of sweetpotato
2.5. Pest and disease
management in sweetpotato
2.5.1 Integrated pest
management
2.5.2 Sweetpotato virus
disease management
2.6 Heterosis
2.6.1 Definitions
2.6.2 Genetic basis
2.7 Importance of heterosis
in crop plants
2.8 Heterosis in sweetpotato
2.9 Molecular
characterization
CHAPTER THREE
3.0 MATERIALS AND METHODS
3.1. Experimental site
3.2. Germplasm source
3.3. Field experiment
3.3.1. Nursery preparation
3.3.2 Land preparation and
planting
3.3.3. Soil sampling and
analysis
3.3.3.1 Determination of the
soil pH
3.3.3.2 The total nitrogen
3.3.3.3 Determination of soil
organic carbon
3.3.3.4. The percent organic
matter
3.3.3.5. Available phosphorus
3.3.3.6. Determination of
exchangeable base: potassium (K+)
3.3.4 Harvesting
3.3.5 Quality traits
3.4. Molecular work
3.4.1. DNA extraction
3.4.2. Simple sequence repeat
(SSR) amplification
3.4.2.1 Polymerase Chain
Reaction
3.4.2.2 MetaPhor Agarose Gel
Electrophoresis (MAGE)
3.4.2.3 Polyacrylamide Gel Electrophoresis
(PAGE)
3.4.3 Data analysis
3.4.3.1 Quantitative and
qualitative data
3.4.3.2 Molecular data
CHAPTER FOUR
4.0 RESULTS
4.1. Chemical properties of
the experimental site
4.2 ANOVA for pre-harvest and
harvest traits; and harvest index
4.3 Heterosis estimates,
means of parents and progenies for pre-harvest and harvest traits, harvest
index and quality traits
4.3.1 Total yield
4.3.2 Harvest index
4.3.3 Vine vigour
4.3.4 Number of plants with
roots
4.3.5 Vine weight
4.3.6 Weight of marketable roots
4.3.7 Number of marketable
roots
4.3.8 Root size
4.3.9 Percentage dry matter
4.3.10 Percentage protein
4.3.11 Starch content
4.3.12 Iron content
4.3.13 Zinc content
4.4 Correlation among
pre-harvest, harvest and quality traits
4.5 Diversity studies among
sweetpotato genotypes with Microsatellites or Simple Sequence Repeats (SSR)
markers
4.5.1: Genetic information by
the SSR markers in the parents
4.5.2 Genetic information by
the SSR markers in the ten parents and their progenies
4.5.3 Factor analysis
4.5.3.1 Parents
4.5.3.2 Parents and progenies
4.5.4 Cluster analysis
4.5.4.1 Parents
4.5.4.2 Parents and progenies
CHAPTER FIVE
5.0 DISCUSSION
5.1 Importance of soil pH and
some nutrient in sweetpotato growth
5.2 Evaluation of some yield
parameters and heterosis in sweetpotato
5.2.1 Severity of weevils and
virus infestation in sweetpotato
5.2.2 Exploitation of
heterosis by progeny testing in sweetpotato
5.2.3 Heterosis in the total
root yield and for harvest index of yield in sweetpotato
5.3 Evaluation of some
quality parameters and heterosis in sweetpotato
5.4 Diversity among the
sweetpotato genotypes using simple sequence repeat makers
5.5 Diversity study using
factor and cluster analysis
CHAPTER SIX
6.0 CONCLUSION AND
RECOMMANDATONS
6.1 Conclusion
6.2 Recommandations
REFERENCES
CHAPTER ONE
1.0 INTRODUCTION
Sweetpotato (Ipomoea batatas [L.]
Lam) is a herbaceous dicotyledonous plant grown at latitudes ranging from 48°N
to 40°S of the equator and altitudes from 0 to 3000 m above sea level (Woolfe,
1992; Vaeasey et al., 2008; Low et al., 2009; Troung et al., 2011). It belongs
to the family Convolvulaceae, genus Ipomoea. This genus contains about 600 to
700 species (Vaeasey et al., 2008; Cao et al., 2009). Sweetpotato needs
temperatures from 12 to 350 C for better growth and root production (Kuo, 1991;
Woolfe, 1992). An annual rainfall of 600 to 1600 mm is required for its growth
(Low et al., 2009) and also a soil pH of 5.5 to 6.5 (Woolfe, 1992). It is
usually considered that sweetpotato is of South or Central America origin
according to Huaman, (1999).
This crop is one of the most
economically important crops in the world. It is the seventh most important
food crop in the world after rice, wheat, maize, potato, barley and cassava
and, the third most important root and tuber crop in the world after potato and
cassava (Belehu, 2003). The world production of sweetpotato was about
175,900,000 tons. China produced 75% of the global production and Africa
produced about 14% of the world production led by Nigeria (3,300,000 tons in
2011) and Uganda (2,554,000 tons in 2011) (FAOSTAT, 2012). Annual production of
sweetpotato in Africa has increased from 12.9 million tons in 2006 to 14.2
million tons in 2010 according to FAOSTAT, (2010).
In developing countries,
sweetpotato is an important source of carbohydrate, vitamins A and C, fiber,
iron, potassium and protein (Woolfe, 1992). The crop has flexibility in West
Africa and is used in numerous food provisions in place of rice, cassava, yam,
plantain and other basic foods (Ellis et al., 2001; Meludu et al., 2003;
Zuraida, 2003).
Sweetpotato
is very important in African agriculture for the prevention of food insecurity
and malnutrition.
The Food and Agriculture
Organization estimated that West, East, Central and Southern Africa had annual
production of 4.2, 7.2, 1.2, and 0.5 million tons respectively in 2006. This
proves that in-spite of its economic reputation the production of sweetpotato
in Africa was very low because of lack of funds for its breeding. Also it is
basically produced by smallholders especially the women for home consumption
and to generate little incomes. It is often starred as orphan crop by many
people as little breeding consideration is directed towards its improvement.
According to FAOSTAT, (2012)
sweetpotato production in Ghana was low. Production was about 1.8 t/ha in 2011
compared to Nigeria 2.9 t/ha, Mali 18.8 t/ha and Burkina Faso 19.03 t/ha in the
sub region. This low production can be attributed to various constraints,
particularly viruses, weevils, lack of processing technology, poor availability
of quality planting materials and inadequacy of improved cultivars with high
and stable yield (Fuglie, 2007). It can also be because of limited market
demand, with production mainly significant in the Northern and Coastal parts of
Ghana where it is both a food security and cash crop (Otoo et al., 2000; Otoo et
al., 2001).
Sweetpotato breeding has received
less attention from plant breeders than many other crops, partly because of the
genetic complexity of the crop: it is a polyploid. It is a hexaploid
cross-pollinating crop with 2n=6x=90 (Austin and Huaman, 1996), and does not
readily flower in some environments, it is self-incompatible and incompatible
in some cross combinations. To boost productivity, new varieties of high
yielding potentials must be created, which will incorporate both quantitative
and qualitative traits and also resistance to viruses and weevils. With good
crop management practices and improved varieties, high yield can be achieved in
large areas such as China who produce
about 75% of the world production (FAOSTAT, 2012). However, in the past two
decades, the yield of sweetpotato in Sub-Saharan Africa (SSA) were very low
compared to the West pacific (China, Japan and Korea) and USA. The production
per year was approximately 1.4, 2.1 and 1.2% for West pacific, USA and SSA
respectively (FAOSTAT, 2011).
To improve yield in sweetpotato,
breeding for new varieties with high and stable yield is very necessary. In
this situation, the phenomenon of heterosis, which is the increase in yield or
other traits in the hybrid, must be applied in sweetpotato breeding because it
is an important way of increasing yield rapidly and improving quality by
creating improved varieties (hybrids) from crosses between genetically diverse
parents.
However, little is known about the
use of heterosis to increase yield, resistance to stress and tuber quality in
sweetpotato.The understanding and the use of heterosis in sweetpotato breeding
will help identify better progenies that will produce high yield and perform
well in termes of qualitative traits and resistance to stress. Applying
heterosis in sweetpotato can help to solve the ever growing population demand
in sweetpotato. According to The breeding program in Ghana under the West
Africa Agricultural Productivity Program (WAAPP) in collaboration with the
Sweetpotato Action Security and Health in Africa (SASHA), one of the objectives
is to develop high and stable yielding varieties. Therefore, heterosis can be
applied in sweetpotato breeding to increase yield and boost Africa countries‟
economy and allow improvement of lives of several million families' lives by
2020 according to Sweetpotato for Profit and Health Initiative: SPHI, (Wasonga et
al., (2014).
Molecular markers have been used to
study the genetic diversity in sweetpotato because they cover a large part of
the genome and there is no environmental effect (Goulão and Oliveira,
2001). Many studies have shown that
Simple Sequence Repeat (SSR) markers 3
are more
variable and provide an efficient means to recognize differences between
genotypes (Powell et al., 1996). Therefore, heterosis will be exploited through
development of mutual heterotic gene pools for the improvement of qualitative
traits such as protein, beta-carotene, starch, dry matter, sugars and minerals
(iron, zinc).
The main objective of this study
was to develop heterotic sweetpotato gene pools for West Africa from regionally
adapted advanced and released parents. The specific objectives were to:
identify progenies in the Ghanaian
breeding program that exhibit heterosis increments from biparental crosses;
determine the phenotypic
correlation among traits;
allocate the parents to separate
gene pools based on molecular assessment of genetic distance
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