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.

INTRODUCTION

Tomato (Solanum lycopersicum. L) is an important horticultural crop worldwide, and is the second most consumed vegetable after potato (FAO, 2005). Tomato production is considered one of the main agricultural enterprises as it employs people in farms, processing industries and provides higher income per hectare to small holder farmers than most staple crops (AVRDC, 2006). It also plays a key role in human health as a source of vitamins A, C and micronutrients (Tindall, 1983; Peralta and Spooner, 2001). Tomato fruits contain lycopene, an anti-oxidant known to reduce the incidence of cancer, heart and age-related diseases (AVRDC, 2003).

Tomato production is an economically important venture in Africa, but is not profitably produced in the humid zones due to excessive precipitation and the associated high relative humidity diseases. Breeding of tomato cultivars that are high yielding with acceptable market fruit  size  and  some  level  of  tolerance  to  high  humidity  conditions  will  open  up  new production opportunities for the poor farmers in the rain forest ecologies.

An increase in the tomato-growing areas will minimize the need for long distance transportation of tomato from the drier parts to the more humid regions. This will reduce the transport induced damage or deterioration and the subsequent reduction of the market value of tomato fruits. The expansion of the production areas would also create additional employment in the sector and generate income. Increased production at a reduced cost would also benefit the industrial sector, especially those making tomato-based products because of increased availability of raw materials for processing. These benefits can be achieved if home-based cultivars with adequate adaptation to the humid environments are developed through organized breeding.

Uguru and Atugwu (2001) reported that exotic cultivars perform poorly in terms of yield and quality  under  high  humidity  conditions.  The  wild  tomato,  Solanum  pimpinellifolium, is tolerant to high humidity diseases and is capable of producing up to 743 tiny fruits per plant (Tanksley et al.,1996;  Foolad and Lin, 1999; Uguru and Atugwu, 2001). The tiny fruits are generally unacceptable in urban and local markets. Crosses between the commercially acceptable but poorly adapted cultivars including Roma VF, Tropica and Nsukka local and the wild tomato variety have produced promising genotypes endowed with prolific fruiting and reduced fruit rot. Successive evaluations of the progenies at different filial generation from F1  to F12  showed reliable evidence of increased fruit yield particularly in terms of number of fruits (Atugwu and Uguru, 2012), and increased disease resistance (Uguru and Igili, 2002). However, the average fruit size is yet to attain a level of full acceptability in the local market. The process of tomato fruit incremental pattern as monitored from F1  to F12 using  interspecific hybrids of the wild tomato and the commercially cultivated varieties showed that the rate of fruit size increment was minimal from F1  to F5, rapid from F5  to F9 and less rapid from F9  to F12 (Atugwu and Uguru, 2012). The authors were able to provide sufficient proof of the exhaustion of genetic variability and substantial decline in the effectiveness of any selection beyond these generations in the improvement of fruit size.

Other works implicated multiple loci in the inheritance of fruit size in tomato (Ibarbia and Lamberth, 1969). The authors reported as high as 10 to 20 loci for the control of fruit size in tomato  and  speculated  that  genes  behave  in  an  additive  manner  in  different  fruit developmental   pathways,   each   contributing   to   the   final   fruit   size.   For   example, developmental studies revealed that tomato size is determined by the number of ovary cells before fertilization, number of successful fertilizations, number of cell divisions that occurred within the developing fruit after fertilization and the extent of cell enlargement (Bohner and Bangerth,  1988;  Gillapsy  et  al.,  1993).  Some  of the  loci  exerted their  effects  through

modulation of the size of the carpel and number of locules, fruit length, fruit diameter and number of seeds (Nitsch, 1970). The major six fruit size quantitative trait loci (QTL), namely fw1.1,  fw1.2, fw 2.1, fw2.2, fw3.1/fw3.2 and fw11.3 are located on chromosomes 1, 2, 3 and

11 (Lippman and Tanksley, 2001). The combination and order of magnitude of these loci would determine the level of success in the improvement of fruit size in tomatoes during selection. Any selection process that is able to assemble the six major QTLs for fruit size in a single population would produce large fruit tomato variety.

Large fruit size is a desirable horticultural characteristic in tomato improvement and an important feature in crop breeding. Fruit size is quantitatively inherited and large members of QTLs have been identified in tomato that are associated with fruit development, size, shape, colour, ripening, organoleptic quality and yield (Causse et al., 2002; Van der Knaap and Tanksley, 2003). The inheritance studies of tomato fruits (Atugwu and Uguru, 2012) have concentrated the genes for profused fruiting in tomatoes under high rainfall conditions. The contending issue at present is on making additional progress in fruit size increment in order to exploit  the  prolific  fruiting  in  the  interspecific  hybrids  to  an  advantage.  This  would necessitate further crosses between the hybrids with exotic breeds with giant fruit size and selection from the segregating population. This improvement stratergy was adopted in the present study in crosses between one of the largest fruited tomato variety, Solanum lycopersicum cv, supersteak and the advanced hybrids endowed with prolific fruiting. The objectives of the present study were therefore:

1. to identify the major developmental pathway for large fruit  size in tomato as ordered by the relevant loci, using crosses between the large fruited tomato variety and the interspecific hybrids;

2. to establish the number, magnitude of effects and the interaction of the QTL in the determination of fruit size; and

3. to identify and select acceptable market fruit size tomato variety with excellent adaptation to high humidity conditions of South eastern Nigeria.

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