ABSTRACT
Soybean is a very important leguminous seed crop; known for its highly valued protein and oil owing to its use in food, feed and industrial applications. The cultivation of soybean could be of great impact in curbing food insecurity and its improvement could be of great benefit to humanity. Genetic variability is the basic requirement for crop improvement as it provides wider scope for selection. Selection of traits through morphological and molecular means increases the frequency for favourable alleles which can be explored in breeding programme for any crop. Therefore, study on genetic variability among soybean was undertaken to evaluate the genetic variation among some accessions of soybean for effective selection, utilization and improvement. Twenty accessions of soybeans were collected from the gene bank of International Institute of Tropical Agriculture (IITA), Ibadan. The accessions were evaluated for agro- morphological parameters using randomized complete block design (RCBD) with three replicate. Soybean accessions were further evaluated for genetic diversity using simple sequence repeat (SSR) marker, pollen parameters was also used to characterize the accessions. There are significant (P < 0.05) wide ranges of variability observed in all the morphological characters assessed, with specific accessions being favoured by different trait(s). Accessions TGX–2016-4E recorded the highest plant height (46.17cm), TGX–1485-1D recorded highest pod production (216.47) and TGX-2027-1E recorded the highest seed yield per plant (1167.67). Wide variability was also observed for qualitative traits such as leaflet size, leaflet shape, pubescence, pubescence density, pubescence colour, pubescence type, cotyledon colour, corolla colour, seed shattering and mature pod colour. The phenotypic coefficient of variation was higher than the genotypic coefficient of variation for all the traits. Moderate to high broad sense heritability ranging from 32.99 to 95.37 was obtained and the genetic advance as percentage mean ranges from 0.16%- 58.89%. Most of the accessions exhibit high pollen viability up to 84.60%; except TGX-2010-11F which had 26.70%. Molecular diversity of 11 selected genotypes from the initial twenty accessions using simple sequence repeat (SSR) DNA marker generated 150-320 base pair with six primers. Genetic similarity among the genotypes varies from 0.17-4.09 with an average gene diversity of 0.28. Clustered dendrogram of the 11 genotypes revealed two major clade (TGX- 1987-62F and TGX-2011- 6F). The high genetic variability obtained for both morphological and molecular characterizations coupled with high heritability and genetic advance in most parameters studied indicate that the genotypes could be selected for those heritable traits and used as tool for crop improvement.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background to the Study
Soybean [Glycine max (L) Merrill] is a member of Papilionancae family and believed to have originated in North Eastern China and distributed in Asia, USA, Brazil and Argentina (Chandrawat et al., 2017). The crop has a fairly wide range of climatic condition for adaptation and mostly cultivated on rain fed land (Chandrawat et al., 2017). Soybean is aptly called as “Golden bean” or “Miracle crop” of the 20th century and is one of the most important oil seed crop in the world (Adoloju et al., 2009) followed by cotton, sunflower canola, palm oil and peanut. It is also very important crop for rotation with cereals like maize and sorghum because of biological nitrogen fixation that is important in improving soil fertility (Abush et al., 2017) and is considered a strategic crop in fighting the world’s food shortage and malnutrition problems and most food aids to displaced and malnourished people are fortified with soybean (Abush et al., 2017).
Soybean is grown in many parts of the world and is a primary source of vegetable oil and protein used in food, feed and industrial applications (Endres, 2001). Soybean was domesticated in the Eleventh (11th) century BC around northeast China (Hymowitz and Shurtleff, 2005). It may have been introduced to Africa in the Nineteenth (19th) century by Chinese traders along the east coast of Africa (Giller and Dashiell, 2006). Reports indicate that soybean was cultivated in Tanzania in 1907 and Malawi in 1909 (Giller and Dashiell, 2006). African countries with the largest area of production are Nigeria (650,000 ha), South Africa (245,000 ha), Uganda (147,000 ha), Malawi (79,480 ha), and Zimbabwe (69,900 ha)(Ishaq and Ehirim, 2014).
Nigeria is the largest producer of soybeans in Africa (Agada, 2015). Soybeans were first introduced into Nigeria in 1908 (Fennel, 1966) but the first successful cultivation was in 1937 with the Malayan variety which was found suitable for commercial production in Benue State (Oyekan, 1985). The producing areas of Central Nigeria have been responsible for a large proportion of the domestic requirement for this cheap source of plant protein. Today, soybean has made a successful incursion into diet of many Nigerians, particularly children and nursing mothers.
Nutritional value of soybean lies in protein (40-42 %) and oil content (18-22 %) thereby containing twice the protein of meat or poultry and contain all eight essential amino acids needed for childhood development (Malik et al., 2006). In Nigeria markets, soybean cost about one-fifth as much as other forms of protein, including diary and fish and are easier to store and transport. They also fix atmospheric nitrogen which reduces the need for farmers to purchase fertilizers. Soybean is also medicinal and is extremely useful for treatment of malnutrition, particularly among children and fight against disease such as heart disease, cancer diabetes, high blood pressure, stroke, ulcer as well as loss of body mass among people living with HIV/AIDS (Agada, 2015). Concerted efforts are greatly needed towards improvement of this crop due to its high demand lately. Unfortunately, many factors, both biotic and abiotic, heavily hinder the production of this crop in Nigeria as a whole. Thus, there is need to further assess the available land races in order to exploit some hidden genetic variability for improvement of the crop in the future.
Genetic variability is the basic requirement for crop improvement as this provides wider scope for selection (Pushpa and Ketoswara, 2013). Thus effectiveness of selection is dependent upon the nature, extent and magnitude of genetic variability present in material and extent to which it is heritable. The success of any crop improvement depends on the nature and magnitude of genetic variability present in the crop along with in-depth understanding of the underlying gene action and genetic architecture of traits related to yield. The knowledge of nature and magnitude of genetic variability is of immerse value for planning efficient breeding programme to improve the yield potentials of genotypes (Pushpa and Ketoswara, 2013). The role of genetic diversity for crop improvement programmes has been emphasized by Sujata and Basavaraja (2011). Genetic divergence among parents is essential since the crossing programme involving diverse parents is likely to produce high heterotic effects and also more variability could be expected in the segregating generations.
1.2 Statement of the Research Problem
Despite the economic importance of soybean as source of oil and other pulses, the crop has been facing some challenges of agronomic inferiority particularly among accessions compared to commercial cultivars. Inadequate knowledge of the scope, nature of the alleles and their relationship with those already introgressed into commercial cultivars has been reported to be limiting its genetic diversity (Kaudzu et al., 2017). In Nigeria, the genetic diversity of this crop has not been fully studied and poorly understood, as well as improper documentation of research finding for exploitation in its breeding programme, thus narrowing its genetic base. Major focus on genetic diversity on soybean has been on Random Amplified Polymorphic DNA and Restriction Fragment Length Polymorphism, limited works have been done using Simple Sequence Repeat marker to determine genetic
variability among the accessions. Proper attention is yet to be given to pollen characteristics, such as pollen viability, as important trait in the characterization of soybean genotypes.
1.3 Justification for the Study
Soybean is a very important leguminous seed crop; known for its highly valued protein and oil owing to its use in food, feed and industrial application. It enriches the soil by fixing nitrogen in symbiosis with bacteria and is ranked number one in the world among the major oil crops (Rajkumar et al., 2010). The cultivation of soybean will be of great impact in curbing food insecurity and its improvement will be of great benefit to humanity. Before planning any breeding programme assessment of genetic variability is a necessary step (Bhagasara et al., 2017). Soybean exhibits genetic variability for agro morphological traits and yield components. Morphological traits can be used to assess phenotypic variation in growing environment and are also used as tool for the indirect analysis of genetic variability and diversity (Ravindra et al., 2017). Thus for soybean to assume this prominence, the genetic amelioration work will pay a key role. The development of superior varieties is based on presence and extent of genetic variability (Rajkumar et al., 2010).
Genetic parameter estimates have been recognized as veritable tool for selecting important traits in various crops. Suliystyo and Majeya (2018) opined that any character with high heritability value and genetic advance as percentage of mean (GAM) is a good character to be selected. Pollen Viability has over time been recongnised as a source of high yield and gene for hybridization. Thus according to Olaoye et al. (2014), high viability of pollen usually results in high yield.
Molecular characterization of the crop will provide better understanding of the selective impacts of breeding practices as well as broaden the genetic base of the crop for its maximum utilization (Tantaswat et al., 2011). Among different DNA markers, Restriction fragment length polymorphism (RFLPs), Random Amplified polymorphic DNA (RAPDs), Amplified fragment length polymorphism (AFLPs), Single Nucleotide polymorphism (SNPs) and simple sequence repeat (SSRs) have been used extensively in soybean. SSR have been shown to produce the highest polymorphism compared to RFLPs, AFLPs and RAPDs and have much greater ability to identify unique alleles in elite and parental soybean germplasm than other markers (Tantaswat et al., 2011). Thus it is on these premises that this research is set in order to achieve the goal, which will eventually lead to improvement of the crop in the future.
1.4 Aims and Objective of Study
The aim of this study is to evaluate the genetic variation among accessions of soybean for effective selection, utilization and improvement.
The objectives were to:
i. Determine the morphological variation among some accessions of soybean.
ii. Identify appropriate traits of soybean for effective selection.
iii. Determine the variation in some pollen parameter among the accessions iv. determine the genetic diversity among some accession of soybean using SSR Molecular markers.
This material content is developed to serve as a GUIDE for students to conduct academic research
GENETIC VARIABILITY STUDIES OF SOME NIGERIAN SOYBEAN (GLYCINE MAX (L) MERRIL) ACCESSIONS
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