Crop Breeding and Applied Biotechnology 6: 261-268, 2006 261 Crop Breeding and Applied Biotechnology 6: 261-268, 2006 Brazilian Society of Plant Breeding. Printed in Brazil ABSTRACT  Studies involving the firm tomato (Lycopersicon esculentum) mutant have shown that pleiotropy or genetic linkage are responsible for modifications in morphological and postharvest traits. The objective of this report was to evaluate the hypothesis of pleiotropy or genetic linkage linked to morphologic traits and to verify the effect of QTL on fruit firmness. Plants of mutant firm and L. cheesmani were intercrossed; the F 2 and F 3 generations were analyzed for segregation of morphological traits and firmness, and the RAPD technique was used for the F 2 population. Results showed that the recessive pleiotropic gene is responsible for the morphological traits, but environmental and/or genetic factors affect the penetrance and expressivity of the mutation. By the RAPD analysis, a QTL was detected in the group represented by the markers AS-08 622 , AQ-16 747 and l-2 that explains 29.77% of the variation to fruit firmness. Key words: pleitropic mutant, penetrance, expressivity, RAPD markers. Genetic control of modified genomic region in a firm ripening tomato (Lycopersicon esculentum Mill.) mutant Adilson Ricken Schuelter *1 , Josiani Marochio 1 , Cristina Soares de Souza 1 , Claudia Cristina Olsen Philippsen 1 , Michele Cristina Heck 1 , SØrgio Dias Lannes 2 , Ivan Schuster 3 , Fernando Luiz Finger 2 , Isabel Regina Prazeres de Souza 4 1 Departamento de CiŒncias Biolgicas. Universidade Paranaense. Avenida Parigot de Souza, 3636. Jardim Prada. 85.903-170. Toledo, PR, Brasil. *E-mail: adilson@unipar.br 2 Departamento de Fitotecnia. Universidade Federal de Viosa, Avenida P.H. Rolfs s/n. 36.571-000, Viosa, MG, Brasil 3 Cooperativa Central de Pesquisa Agrcola. Rodovia BR 467 km 98, 38.600-000, Cascavel, PR, Brasil 4 Nœcleo de Biologia Aplicada, Embrapa Milho e Sorgo, C. P. 151, 35.701-970, Sete Lagoas, MG, Brasil INTRODUCTION Tomato is economically the second most important vegetable, produced worldwide at around 115.95 million tons per year according to Camargo et al. (2006). In spite of the large volume, many factors are responsible for field losses, due to the susceptibility of the tomato plant to pathogens and pests, as well as during the posharvest stage, in particular due to inadequate fruit handling. From a total of 3.49 million tons of tomato produced in Brazil (Camargo et al. (2006), 14.92% fresh tomato is lost during postharvest due to physical damage and 60% due to inadequate packing (Vilela and Luengo 2002). Postharvest practices have been developed to make storage easier, extend the shelf-life and minimize physical damages through handling. One of the most meaningful advances has however been achieved by using natural mutants as source of genes and techniques of genetic engineering for the development of commercial long shelf life and transgenic varieties with fruits with longer postharvest conservation (Gray et al. 1994). Several genes have been identified in the Lycopersicon esculentum species, among them those that modify different aspects of fruit ripening. The pleiotropic mutants never-ripe (Nr), non-ripening (nor), Received 06 May 2006 Accepted 05 August 2006