ABSTRACT

A diallel set consisting of p2 combinations (p parents ½ P(p-1) F2 hybrids, and the backcrosses among eight rice breeding lines were grown in replicated trails. The eight rice breeding lines were selected from an initial thirty one rice breeding lines comprising eighteen medium maturing rice lines and thirteen early maturing rice lines on the basis of their maturity (early to medium ), plant height (semi – dwarf to intermediate ), tillerring ability (low to high) and spikelet fertility (partly sterile to highly fertile). The diallel study was aimed at elucidating the genetic system underlying the inheritance of quantitative traits; number of days to 50% flowering, plants height number of tillers/plant, number of tillers/m2, number of spikelets/ panicle, number of fertile spikelets/ panicle, 1000-grain weight and grain yield. The results of the diallel analysis show that much of the genetic variation of each of the twelve traits was due to additive effect. Dominance effect was significant in all the traits. Indirect evidence indicated that gene interaction played a negligible role. There was overdominance for number of panicles/m2, number of spikelets /panicles, number of fertile spikelets/panicle and for number of days of maturity. The positive gene effects in the parents were unequally distributed. However, the dominant alleles exceeded the respective alleles for number of fertile spikelets/ panicle and for number of days to maturity and the recessive alleles exceeded the dominant alleles in all the other traits. Heritability estimates were generally high in all the traits for the lines. High percentage co-efficient of variation (c.v %) was observed for parameters such as number of spikelets / panicle. The intercept of the slope was less than zero indicating over dominance for four of the parent while the other parents have partial dominance. The point distribution on the Wr/Vr graph indicates that the differences in parents were genetic in origin. The regression coefficient of the points did not differ significantly this implicating additive gene effects. Significant GCA and SCA effect were observed for most of the parameters. The results indicated that the breeding line (WAB 450 – 1-B 163 – 41) was the best general combiner for most of the characteristics estimated except for days to 50% flowering, number of tiller/ plant,

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number of tiller/m2, number of days to maturity, number of panicles/hill and number of panicle/m2. A reciprocal recurrent selection method is recommended for further improvement of the selected rice lines especially for grain yield and yield components

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INTRODUCTION

The cultivated rice (Oryza sativa L.) belongs to the tribe Oryzea under the sub-family Pooidea in the grass family Poaceae (Gramineae). Bio-systematics have divided the genus into two cultivated species, Oryza sativa L . and Oryza glabberrima Steud and more than twenty wild species distributed throughout the tropics and subtropics (Lu, 1999).

Oryza species have already been attracting enormous attention from scientists, world wide because of their economic importance. Many studies on taxonomy, phylogeny and genetic relationships of the Oryza species have been conducted (Chang, 1985, Morishinma et al. 1992, Wang et al. 1992, Lu et al. 1998). Diversity in Oryza is tremendous, which is reflected in the different genomes and genomic combinations in the genus and in the significant morphological variations within and between species.

Rice has been cultivated in South - Eastern Asia since ancient times where it is one of the oldest of food crops, but is now grown in all regions of the world where conditions are suitable for its cultivation (Purseglove, 1975). Rice is the stable food of about half of the human race (Bray, 1986) and of the total area of 100 million hectares, over 90% is grown in Southern and Eastern Asia, which are considered major centres of the world’s population. It is the most important tropical cereal, and on a world basis, production is only slightly below that of wheat (Purseglove, 1975).

The crop is often the major source of calories and the principal food of many millions of people and is very rich in vitamin B1, thiamine. According to Courtois (1988),

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one of the unique characteristics of rice is its adaptability to a wide range of eco-systems especially regarding its moisture needs.

Khush (1984), stated that rice is grown over a wide range of environmental conditions divided into five main categories namely ; (1) Irrigated lowland (nearly 52.8% of worlds rice area, (2) rain fed lowland (about 22.6% of world’s rice area), (3) Rain fed upland (about 13.0% of world’s rice area), (4) Deep water ( about 8.25 of world’s rice area) and (5) Tidal wet-lands (about 3.4% of world’s rice area).

In terms of importance as a food crop, rice provides more calories per hectare than any other cereal crop. For example, at average world’s yield, a hectare of rice could sustain 5.7 persons a year compared to 5.3 for maize and 4.1 for wheat. The total caloric output of all world food is equal to 3119 cal/person/day at the farm gate, with rice accounting for 55.2Kcal/person per day or 18% of the total (De Datta, 1981).

In Nigeria, the demand for rice is increasingly higher than its production due to taste and rapid population growth. In 1981, the International Rice Research Institute (IRRI), reported that due to increased urbanization and ease of cooking and storage, rice is progressively replacing other cereals and traditional food crops in West Africa. It stressed further that most African countries are increasingly resorting to importation which became costlier due to trade imbalance.

In plant breeding, the objective in self fertililizing crops is to concentrate different useful genes (yield, resistance to disease, food quality, etc.) in the same genotype. Success in breeding for quantitative traits depends upon the “gene action” involved for the traits concerned.

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Hybridization or crossings of individuals produces hybrids which combine the characters from both parents. Occasionally, the recombination of genetic factors following crossing do result in the production of new desirable characters hidden in either of the parents.

Heterosis is a common phenomenon in nature representing higher vigour in F1

plants than in parental lines and is caused by the specific gene combinations derived from different parents. It has features that completely differ from any other trait, existing only in heterozygosity mainly in the F1. For this reason, the genes referred to in heterosis

should be in gene combinations formed by hybridizing different parental lines. Progress in breeding for higher yield in Oryza species therefore depends on the ability to identify and assess the genes and combination most likely to produce higher heterotic effect and one of the major method by which this can be achieved is through the use of diallel crosses.

Diallel crossing procedures are normally used to obtain information concerning the inheritance of quantity traits in self-pollinated crops. Also, diallel crossing experiments is useful for analyzing inheritance pattern of genes regulating quantitative characters and their dominance relationship. The method is an effective way normally used to examine the system of genes involved in the control of a quantitative trait.

The making of all possible single crosses among a set of inbred lines has been a popular scheme for diallel analysis for over 35 years (Sprague and Tatum, 1942). Statistical analysis of diallel crosses and genetic interpretation of such analysis has been the subject of many research papers since 1954 (Baker, 1978).

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The diallel crossing method, introduced by Griffing (1956 a, b), Jinks and Hayman (1953) and Hayman (1954 a, b) and later modified by Pooni et al. (1984) and Wright (1985) is often used by breeders to obtain useful information on the quantitative characters of different parental lines crossed in all possible ways. Although, the method has been questioned (Baker, 1978), It has been used successfully in many crop varieties (Ehdai and Ghaderi 1972, Ghaderi et al. 1973, Sarrafi et al., 1978). Windham and Williams (1991) have used quantitative genetic methods to investigate the reaction of maize diallel – crosses for nematode (Meloidoggne incognita) resistance in field trials. Hobbs and Mahon (1985) have used this method to study the genetic environment dependent and interaction components of photosynthesis in pears ( Pyrus communis). In a second study Hobbs and Mahon, (1985), investigated using diallel crosses, the transmission components of photosynthesis in pears. In another study, Hobbs and Mahon, (1985), investigated, using diallel crosses, the transmission of chlorophyll contents, ribulose – 1,5 – biphosphate – carboxylase activity and split-opening resistance in pears. Diallel analysis enables the breeder to estimate the general and specific combining abilities of the inbred lines (Obi , 1990). When a particular line shows higher than average performance judged on the basis of all their hybrids and in this case its general combining ability (GCA) is considered to be good. The GCA estimate when significant statistically shows that additive gene action is operating. On the other hand, when one parent displays above average performance based on the yield of a particular offspring its specific combing ability (SCA) is judged high. The SCA estimate when statistically significant shows preponderance of dominant and epistatic gene actions. The result of

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diallel analysis enables the breeder to propose and design an appropriate improvement programme for the crop.

The diallel crossing method in rice is used principally to determine the general and specific combining abilities of the quantitative properties (Chan et al. 1990, Hoang and Tran, 1991). The method is also very useful for analyzing inheritance pattern of genes regulating quantitative characters and their dominance relationship.

Several workers have emphasized the importance of indirect selection for yield though the use of component characters governed predominantly by genes with additive action and showing strong correlation with yield (Falconer, 1989).

An understanding of the inheritance pattern of the traits in the rice crop is necessary in organizing a systematic breeding programme for the crop. In the study of quantitative genetics, the breeder uses the observations made on the population to predict the outcome of any particular breeding method. The breeder also, determines how these observable properties are being influenced by the properties of the genes concerned and by the various non-genetic circumstances. Flores et al. (1986) have noted that in population improvement, it is important to determine the extent of genetic variation for a trait to be improved. The basic idea in the study of variation is its partitioning into components attributable to different causes and the relative magnitude of these components determines the genetic properties of the population (Falconer, 1989).

The observed variation in a population, otherwise known as phenotypic variation is made up of the genotypic variation and environmental variation. The interaction of these components may be included if such is assumed to exist. The partitioning of variation into these components enables the breeder to estimate the relative importance of

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the various determinants of the total genetic constitution of the crop and that of the environment on the crop and that of the environment on the trait. This led to the concept of “heritability” which specifies the proportion of the total variability that is due to genetic causes or simply, the ratio of the genetic variance to the total variances. However, heritability as measured above can lead to an over estimation if some basic assumptions such as the existence of additive, dominance and epistatic components of the genetic variance are neglected.

When heritability is expressed as the proportion of the genetic variance to the total variance, we obtain an estimate of heritability in a “broad sense” and expresses the extent to which individual phenotypes are determined by the genotype. Low to moderate “broad sense” heritability, therefore suggests large environmental effects, while higher values may be attributed to greater difference between the parents involved (Osman et al. 1983). But when heritability is expressed as the proportion of the additive genetic variance to the total variance, it is defined as heritability in a “narrow sense” and measures the extent to which phenotypes are determined by the genes transmitted from the parents. A high estimate of “narrow sense” heritability indicates that selection can be effectively made on phenotypic basis by mass selection, as a result of low environmental influence on the character concerned (Sandu and Phul, 1984).

The adoption of any breeding methodology for improvement of a crop will depend upon the contribution of these components of total variation. It therefore, becomes crucial to estimate the nature and magnitude of genetic variation, and also relative importance of genotype – environment ( g x e) interaction if any, for different quantitative characters in various populations, to predict the out come of breeding and