The potential for introgression of cultivated radish (Raphanus sativus) alleles into wild radish (Raphanus raphanistrum) populations

Hybridization between cultivated plants and their wild relatives has been documented for many species including sunflower, rice, sugar beet, and radish. The persistence of crop derived traits has rarely been examined for more than two generations, and even more rarely under natural field conditions. The purpose of this study was to determine the potential for persistence of traits from cultivated radish (Raphanus sativus) in populations of wild radish (R. raphanistrum). Four artificial populations were created in northern Michigan, and each initially consisted of 100 F 1 hybrids and 100 wild radishes planted in 1996. Three indicators of crop allele persistence were monitored over several generations: pollen viability, white flower color frequency (a dominant trait), and crop specific allozyme allele frequency.;Pollen fertility was low (<70%) in F1 hybrids, but fertility increased significantly in subsequent years which indicates that fertility barriers to hybridization are minimal. White flower color frequency and crop-specific allozyme allele frequency initially decreased, but increased in frequency or remained stable in subsequent years. Our results demonstrate that wild populations are apt to respond differently following hybridization, but that introgression of genes from R. sativus to R. raphanistrum is likely. To further examine the relative fitness of hybrids compared to wild plants under field conditions, in 1999 I planted wild, F1, and BC1 radish seeds in a fallow field in Michigan under two growing conditions: with and without competition. Germination and mortality were significantly different among cross types (BC1 > F1 > wild). F 1 plants produced significantly fewer flowers and fruits than wild or BC1 plants, and all cross types differed significantly in seed production (wild > BC1 > F1). Although measures of performance were generally lower with competition, this treatment significantly affected only seed production. F1 fitness was markedly lower than the other two cross types; however, BC1s and wilds had similar fitness in the presence of competitors. These results indicate that even early generation hybrids are capable of ecologically significant levels of reproduction under natural conditions. This study provides further evidence that hybridization between crop and wild radish could lead to long-term introgression of crop alleles into wild radish populations.