RAPD genetic homogeneity and high levels of bacterial wilt tolerance in Solanum torvum Sw. (Solanaceae) accessions from Reunion Island

Solanum torvum Sw. is a wild species closely related to eggplant (S. melongena L.), native to India. Although invasive in some areas of introduction, this species is of particular interest as it has been identified as a potential source of resistance to bacterial wilt disease for cultivated susceptible Solanaceae species. Little is known about the levels of resistance and genetic diversity in this species, particularly in areas of introduction such as Reunion Island, where it is used as rootstock for grafting of susceptible tomato and eggplant varieties. Eight accessions were collected in diverse geographical locations on Reunion Island for further characterisation. Regardless of the accession, no wilted plants were detected four weeks after inoculation with three different Ralstonia solanacearum strains (phylotypes) from Reunion Island. Latent bacterial colonisation in symptomless plants was however demonstrated, mainly in roots and/or lower stem, showing that S. torvurn can be considered as resistant to the race 1 biovar 1 strain, and tolerant to the race 1 biovar 3 and race 3 biovar 2 strains. The homogeneity between accessions regarding high bacterial wilt tolerance was confirmed by the genetic homogeneity (0% polymorphism) revealed using 168 RAPD markers. Similarly, only 1.2% polymorphism was revealed by the genetic study of seven accessions of S. torvum from Java used as reference. All the Java accessions, but one, were identical to Reunion accessions. The level of polymorphism between S. torvum accessions is assumed to be similar to the level of genetic differences existing between vegetatively propagated clones, therefore reduced to minimal mutational events. This reduced genetic variation is typical of what is expected for an autogamous species introduced on an island. Some differences in latent bacterial colonisation scores for some of the Reunion accessions were however noticed. These results are essential for further genetic and epigenetic studies on bacterial wilt resistance in S. torvum. (C) 2004 Elsevier Ireland Ltd. All rights reserved.