Studies on the Genetic Pathway and Selection Response for Increased Fruit Size and Yield in Tomato (Solanum Species) Using a Modified Three-Way Cross

ABSTRACT

Modified three-way crosses involving advanced interspecific tomato hybrids (W x R), (R x W) and (W x T) at F12 generation and commercially cultivated tomato varieties namely Supersteak (S), Beef (Florida) (BF) and Plumb (Rio grande) (PR) were made to generate F1 hybrids. The F1 hybrids were crossed to both the pollen and seed parents to produce the backcrosses and were also allowed to random mate to produce the F2 and F3 populations. The parents, F1s, F2s, F3s, and backcrosses were evaluated under rainfed conditions. Floral trait and genomic analyses using Single Nucleotide Polymorphisms (SNPs) markers of the quantitative trait loci (QTL) underlying fruit size in tomato were also performed. The Analysis of Variance (ANOVA) on agronomic, yield, floral and fruit traits showed significant differences (P < 0.05) among the tomato genotypes. The cross, S x (W x R), was the most promising three-way hybrid that can be exploited for increased tomato fruit size and yield in the humid tropics. Mean fruit weight had significant and positive correlation with all the floral traits with the exception of flower and style lengths. The number of locules per fruit had the highest correlation value (r = 0.984**) with fruit size. Path coefficient analysis revealed that the number of locules per fruit had the highest positive direct effect (p = 0.8086) on fruit size. This was closely followed by ovary diameter (p = 0.7942) and stigma diameter (p = 0.7685). On the other hand, style length had the highest negative direct effect (p = - 0.9147) on fruit size. The fruit shape index showed significant positive correlation with the ovary shape index (r = 0.835**) and seed shape index (r = 0.718**). However, fruit shape index was negatively and significantly correlated with ovary diameter (r = -0.601*), fruit diameter (r = -0.576*) and seed diameter (r = -0.519*). The population structure analysis revealed that the tomato genotypes studied had three sub-populations. The association mapping using 25 SNPs markers detected nine markers with significant association with mean fruit weight, fruit length, fruit diameter, number of locules per fruit and fruit shape index. The SNP marker, Solyc11-17, exhibited significant association with both fruit diameter, number of locules per fruit and fruit shape index. The variation in fruit diameter explained by the marker, Solyc11-17, was higher than the variations in number of locules per fruit and fruit shape index (141.5%, 23% and 18.3%, respectively). All the nine markers detected are recommended for fruit size improvement breeding programe in tomato.

TABLE OF CONTENTS

TITLE PAGE.............................................................................................................................. i

CERTIFICATION ..................................................................................................................... ii

DEDICATION.......................................................................................................................... iii

ACKNOWLEDGEMENTS....................................................................................................... iv

TABLE OF CONTENTS .......................................................................................................... vi

LIST OF FIGURES ....................................................................................................................x

LIST OF PLATES .................................................................................................................... xi

LIST OF APPENDICES .......................................................................................................... xii

PUBLICATIONS.................................................................................................................... xiii

ABSTRACT ............................................................................................................................xiv

INTRODUCTION ......................................................................................................................1

LITERATURE REVIEW............................................................................................................5

Botanical description and production...............................................................................5

Tomato production constraints in Southern-eastern Nigeria .............................................6

High temperature, relative humidity and rainfall..............................................................6

Genetic variation and improvement in tomato..................................................................7

Fruit size variation in tomato ...........................................................................................8

Genetic markers and mapping........................................................................................11

MATERIALS AND METHODS...............................................................................................17

RESULTS.................................................................................................................................30

DISCUSSION.........................................................................................................................141

CONCLUSION.......................................................................................................................156

REFERENCES .......................................................................................................................157

APPENDICES ........................................................................................................................170