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 a new production opportunity for the poor resources 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 price 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 (Uguru and Umukoro, 2005; 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 workers 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, 1981; 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 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 QTL for fruit size in a single population would produce large fruited 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:
- 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.
- To establish the number, magnitude of effects and the interaction of the QTL in the determination of fruit size.
- 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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