Fitness Effects of Transgenic Disease Resistance in Sunflowers

Abstract

Fears about transgene escape have focused attention on the potential for hybridization between crops and their wild relatives. Al-though transgenes will often escape from cul-tivation (1), their rate of spread will be main-ly governed by their fitness effects, not the migration rate (2). Thus, only highly advan-tageous transgenes will spread rapidly enough to have a substantial ecological im-pact. Therefore, research on the risks associ-ated with transgene escape should focus on the fitness effects of the gene(s) in question. Here, we examined the fitness effects of a transgene conferring resistance to white mold (Sclerotinia sclerotiorum) in sunflower (He-lianthus annuus). Unfortunately, attempts to breed white mold resistance have met with little success, and chemical control methods are costly and often ineffective. Therefore, attention has turned to genetic modification. An oxalate oxidase (OxOx) transgene has now been used to enhance white mold resis-tance in cultivated sunflower [supporting on-line material (SOM) text], presumably by degrading oxalic acid, which contributes to white mold pathogenicity (3). Most of the sunflower acreage in the United States occurs within the range of wild sunflow-er, and many fields flower coincidentally with neighboring wild populations (4). Where they come into contact, cultivated and wild sun-flower often hybridize (5), making transgene escape a virtual cer-tainty. Combined with the efficacy of the OxOx transgene, this raises the possi-bility of hybridization giving rise to a more invasive wild sun-flower. To simulate the early stages of es-cape, we back-crossed the OxOx transgene into wild sunflower and grew the resulting plants in containment cages at field sites in Indi-