EXPLOITATION OF HETEROSIS IN SWEETPOTATO (Ipomoea batatas L. (Lam)) VIA PROGENY TESTING AND USE OF MOLECULAR MARKERS

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 (Goul√£o 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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Item Type: Ghanaian Postgraduate Material  |  Attribute: 108 pages  |  Chapters: 1-5
Format: MS Word  |  Price: GH50  |  Delivery: Within 30Mins.
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