Nnungu, Stephen Issa

Stephen Nnungu PhD ScholarGenetic Pathway and Selection Response to an Increased Fruit Size in Tomato (Solanum Species)


Tomatoes (Solanum lycopersicum) are an important horticultural crop and the second-most consumed vegetable after potatoes. As a major agricultural enterprise, tomato production employs people on farms and in processing industries, and provides smallholder farmers with a higher income per hectare than most staple crops. Tomatoes also play a key role in human health as a source of micronutrients and vitamins A and C. They furthermore contain lycopene, an anti-oxidant known to reduce the incidence of heart and age-related diseases and cancer. Tomatoes are grown in most countries worldwide. Production in tropical and subtropical areas, however, is limited by high temperature and humidity. High humidityand rainfall limit the yield and quality of crops, as they cause a proliferation in tomato diseases, such as damping off, fusarium wilt, bacterial wilt, leaf curl and leaf spot. The selection of cultivars or the breeding of tomato lines tolerant to high humidity is therefore necessary to increase tomato production in high-rainfall zones.

The wild tomato, Solanum pimpinellifolium, is tolerant to high humidity, disease and abiotic stress and is capable of producing up to 743 tiny fruits per plant. The fruits are very small, however, and are generally unacceptable to urban and local markets. Crosses between the commercially acceptable but poorly adapted cultivars, including Roma VF, Tropica and Nsukkalocal, and the wild tomato variety have produced promising genotypes endowed with prolific fruiting potentials. Successive evaluations of the progenies at different filial generations, from F1 to F12, have shown reliable evidence of increased fruit yield, (particularly in terms of the number of fruit) and increased disease resistance. The average fruit size, however, is still not acceptable to the local market. 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 the tomatoes that are associated with fruit development, size, shape, colour, ripening and organoleptic quality and yield. The inheritance studies of tomato fruits have concentrated the genes for profused fruiting in tomatoes under rainfall conditions. The main challenge would be to enlarge fruit size to exploit the prolific fruiting in the interspecific hybrids to advantage. This would necessitate further crosses between the hybrids with exotic breeds with large in-bred tomato lines and a selection from the segregating population.

This study therefore aims at generating novel tomato lines with acceptable fruit size for the humid zones of tropical and subtropical regions. The experiments expected to be done are the development and evaluation of populations, crosses between advanced tomato hybrids and large fruited in-bred tomatoes, fruit-development analysis and molecular-marker analysis.

Supervisor: Prof Michael. I. Uguru, Department of Science, Faculty of Agriculture, University of Nigeria.

Disciplinary history: BSc Molecular Biology & Biotechnology, University of Dar Es Salaam, Tanzania; MSc Botany, University of Dar Es Salaam, Tanzania

Faculty/department of registration: Faculty of Agriculture, Department of Crop Science, University Of Nigeria, Nsukka (2013)