Evolutionary and social consequences of introgression of nontransgenic herbicide resistance from rice to weedy rice in Brazil

Abstract

Several studies have expressed concerns about the effects of gene flow from transgenic herbicide-resistant crops to their wild relatives, but no major problems have been observed. This review describes a case study in which what has been feared in transgenics regarding gene flow has actually changed biodiversity and people's lives. Nontransgenic imidazolinone-resistant rice (IMI-rice) cultivars increased the rice grain yield by 50% in southern Brazil. This increase was beneficial for life quality of the farmers and also improved the regional economy. However, weedy rice resistant to imidazolinone herbicides started to evolve three years after the first use of IMI-rice cultivars. Population genetic studies indicate that the herbicide-resistant weedy rice was mainly originated from gene flow from resistant cultivars and distributed by seed migration. The problems related with herbicide-resistant weedy rice increased the production costs of rice that forced farmers to sell or rent their land. Gene flow from cultivated rice to weedy rice has proven to be a large agricultural, economic, and social constraint in the use of herbicide-resistant technologies in rice. This problem must be taken into account for the development of new transgenic or nontransgenic rice technologies.

Keywords: ClearfieldTM; acetolactate synthase; fitness; gene flow; herbicide resistance; imazethapyr; outcrossing; red rice.

Figure 1

Progress of grain yields and per hectare costs of production of irrigate rice in Rio Grande do Sul state (Brazil), 2002/03 to 2014/15. Source: Adapted from CONAB (2015).

Figure 2

Illustrations of the diagnostic methods for herbicide resistance identification using whole‐plant herbicide spraying (A), seed soaking (B), seedling floating (C), tillering floating (D), enzyme activity (E), and molecular markers (F).

Figure 3

Time to germination reach 50% (GR 50%) based on the logistic model at temperatures of 20 and 25°C of rice cultivars carrying different ALS alleles. Source: Adapted from Goulart et al. 2012a .

Figure 4

Illustration of the structure analysis of weedy rice populations. Each population comprises 20 plants represented by a vertical bar assigned to six potential genetic backgrounds (six different colors). The analysis was performed with the software structure as described in Goulart et al. (2014).