Abstract:The evidence of increased crossing over rate in tomato hybrids infected with TAV (Tomato aspermy virus), PVX (Potato virus X), TMV (Tobacco mosaic virus), TMV+PVX indicates the recombinogenic effect of viral infection. Cytological studies of the early diakinesis in healthy and virus-infected tomato revealed significant changes in chiasma number and position. The most significant changes were established for bivalents with two interstitial chiasmata and with one terminal and one interstitial. The data obtained indicate redistribution of the chiasmata position and induction of additional exchanges. The virus-induced recombination is segment-specific and depends on the host plant genotype, virus infection and the interaction between them.

Since the first reports by Kostoff (1933), studies of effects of viral infection on host plant genetic material have advanced considerably. It has been established that viral infection can contribute to chromosome breaks (Nagar et al. 1995(Nagar et al. , 2002Bass et al. 2000), activation of transposing elements (Dellaporta et al. 1984;Johns et al. 1985;Kalendar et al. 2000;Ikeda et al. 2001) and bring about modifications in genome regulation and structure in the host plant (Madlung and Comai 2004;Boyko et al. 2007).The contribution of viral infection to the enhancement of host-plant genetic variability has been recognized. Moreover, a number of studies elucidate the role of viral infection in evolution of organisms, that is, viral infection acting as a factor of selection favoring new genotypes both in plants and animals (Anderson and May 1982;Alexandrov and Golubovsky 1983;Kondrashov 1988;Kelley 1994;Yamauchi 1999;Busch et al. 2004). In the progeny of infected plants some phenomena have been observed, such as aberrant ratio (Nelson 1981) or genetic instability (Sandfaer 1979;Mottinger et al. 1984). Later on, deviations in inheritance of quantitative and qualitative traits as a result of virus infection action have been established in agricultural crops, such as bean (Sarrafi and Ecochard 1985), wheat (Burdun et al. 1984) and sorghum (Mock et al. 1985;Stokes et al. 1988). The above evidence suggests that variability generated in this manner can be used in crop improvement programs.Various abiotic factors are commonly used for increasing variability in tomato (Singh 1981;Gavazzi et al. 1987), and there is only fragmentary evidence for use of viral infection for inducing variability in tomato (Chiriac and Bujoreanu 1996). Traditionally, combining two or more valuable traits in a genotype is achieved by crossing parental forms possessing the traits to be combined. However, attaining the desired result in tomato takes a long time due to linked inheritance of valuable traits, which tend to be negatively correlated (Tanksley et al. 1992). The opportunity Role of Viral Infection in Inducing Variability 477 for shortening the breeding time and increasing the frequency of individual plants combining valuable traits can be realized through the use of factors increasing recombination frequency and spectrum (Schuermann et al. 2005). Recently, viral infection has been found to be involved in the enhancement of mitotic (Kovalchuk et al. 2003) and meiotic (Chiriac et al. 2006) recombination, and their effects could persist in the subsequent untreated generation.In view of the above, the objectives of the present study were as follows:1. Evaluation of the effect of viral infection on the inheritance of quantitative traits (fruit weight, earliness, soluble solids content) in the progeny of plants infected with tobacco mosaic virus (TMV) and potato virus X (PVX); 2. Elucidating a possible relationship between chiasma frequency in the infected plants and variability of traits in the progeny; and 3. Identifying the genotypic partic...