Fitness correlates of crop transgene flow into weedy populations: a case study of weedy rice in China and other examples

Abstract

Whether transgene flow from crops to cross-compatible weedy relatives will result in negative environmental consequences has been the topic of discussion for decades. An important component of environmental risk assessment depends on whether an introgressed transgene is associated with a fitness change in weedy populations. Several crop-weed pairs have received experimental attention. Perhaps, the most worrisome example is transgene flow from genetically engineered cultivated rice, a staple for billions globally, to its conspecific weed, weedy rice. China's cultivated/weedy rice system is one of the best experimentally studied systems under field conditions for assessing how the presence of transgenes alters the weed's fitness and the likely impacts of that fitness change. Here, we present the cultivated/weedy rice system as a case study on the consequences of introgressed transgenes in unmanaged populations. The experimental work on this system reveals considerable variation in fitness outcomes - increased, decreased, and none - based on the transgenic trait, its introgressed genomic background, and the environment. A review of similar research from a sample of other crop-wild pairs suggests such variation is the rule. We conclude such variation in fitness correlates supports the case-by-case method of biosafety regulation is sound.

Keywords: Oryza sativa; conspecific weed; evolutionary potential; fitness; herbicide resistance; insect resistance; introgression; red rice.

Figure 1

A schematic illustration demonstrating the evolutionary relationships among the populations of a domesticated species (Crop), its wild progenitor (Wild), and a weedy taxon conspecific with the crop (Weedy). Broken arrows indicate historic primary pathways of evolution: Domestication of the crop species (1) from the wild progenitor in the past, and subsequent evolution of the weed populations from natural hybridization (2) between the crop and wild progenitor, and directly through de‐domestication (3) from the crop. Two‐headed arrows indicate continuing sporadic gene flow among the crop, weedy, and wild plants that can result in dynamic evolution of the populations (adapted from Ellstrand 2003a).

Figure 2

A schematic illustration showing the establishment of crop‐weed rice hybrid lineages with or without a transgene for the comparison of fitness effects between the transgene‐present and transgene‐absent individuals in different generations. ++, homozygous transgenic hybrid lineages/segregants; +−, heterozygous transgenic hybrid lineages/segregants; − −, nontransgenic hybrid lineages/segregants.