The Impact of Polyploidization on the Evolution of Weed Species: Historical Understanding and Current Limitations

Whole genome duplication via polyploidization is a major driver of diversification within angiosperms and it appears to confer the most benefit during times of rapid environmental change. Polyploidization offers expanded access to novel phenotypes that facilitate invasion of new environments and increased resistance to stress. These new phenotypes can arise almost immediately through the novel interactions among or between transcription factors of the duplicated genomes leading to transgressive traits, and general heterosis, or they can occur more slowly through processes like neofunctionalization, and subfunctionalization. These processes are characterized by the changes within homologs of the duplicated genomes, homoeologs. It has been proposed that redundant homoeologs are released from selective constraints and serve as an additional source of adaptive genetic variation, particularly in neo and meso-polyploids. Current practices in weed management create rapid environmental change through the use of chemicals, practices that are meant to cause the extirpation of the designated weed, and represent a strong recurrent selective event—a scenario that should favor polyploidy species. Here we ask the question, “Do polyploids make better weeds?” It is our conclusion that such a question is impossible to answer at this time due to the lack of resources and understanding in weed genomics. The growing contingent of research in weed genomics, however, driven by herbicide resistance evolution is rapidly improving our understanding of weed molecular biology and will aid in improving understanding of the impacts of ploidy levels on weed evolution and adaptation in the future.

Ten Ways That Weed Evolution Defies Human Management Efforts Amidst a Changing Climate

The ability of weeds to evolve is key to their success, and the relationship between weeds and humans is marked by co-evolution going back to the agricultural revolution, with weeds evolving to counter human management actions. In recent years, climate change has emerged as yet another selection pressure imposed on weeds by humans, and weeds are likewise very capable of adapting to this latest stress of human origin. This review summarizes 10 ways this adaptation occurs: (1) general-purpose genotypes, (2) life history strategies, (3) ability to evolve rapidly, (4) epigenetic capacity, (5) hybridization, (6) herbicide resistance, (7) herbicide tolerance, (8) cropping systems vulnerability, (9) co-evolution of weeds with human management, and (10) the ability of weeds to ride the climate storm humans have generated. As pioneer species ecologically, these 10 ways enable weeds to adapt to the numerous impacts of climate change, including warming temperatures, elevated CO2, frequent droughts and extreme weather events. We conclude that although these 10 ways present formidable challenges for weed management, the novelty arising from weed evolution could be used creatively to prospect for genetic material to be used in crop improvement, and to develop a more holistic means of managing agroecosystems.

Genetically Engineered Herbicide-Resistant Crops and Herbicide-Resistant Weed Evolution in the United States

Genetically engineered (GE) herbicide-resistant crops have been widely adopted by farmers in the United States and other countries around the world, and these crops have caused significant changes in herbicide use patterns. GE crops have been blamed for increased problems with herbicide-resistant weeds (colloquially called by the misnomer “superweeds”); however, there has been no rigorous analysis of herbicide use or herbicide-resistant weed evolution to quantify the impact of GE crops on herbicide resistance. Here, I analyze data from the International Survey of Herbicide Resistant Weeds and the USDA and demonstrate that adoption of GE corn varieties did not reduce herbicide diversity, and therefore likely did not increase selection pressure for herbicide-resistant weeds in that crop. Adoption of GE herbicide-resistant varieties substantially reduced herbicide diversity in cotton and soybean. Increased glyphosate use in cotton and soybean largely displaced herbicides that are more likely to select for herbicide-resistant weeds, which at least partially mitigated the impact of reduced herbicide diversity. The overall rate of newly confirmed herbicide-resistant weed species to all herbicide sites of action (SOAs) has slowed in the United States since 2005. Although the number of glyphosate-resistant weeds has increased since 1998, the evolution of new glyphosate-resistant weed species as a function of area sprayed has remained relatively low compared with several other commonly used herbicide SOAs.

Slowing weed evolution with integrated weed management

Harker, K. N. 2013. Slowing weed evolution with integrated weed management. Can. J. Plant Sci. 93: 759–764. For millennia, weeds have slowly evolved in response to ever-changing environments and crop production practices. Weeds are now evolving much more quickly due to consistently repeated cropping systems and intense herbicide selection pressures. Weed resistance to herbicides now threatens cropping system sustainability in several industrialized nations. Integrated weed management (IWM) provides opportunities to reduce selection pressure for weed resistance while maintaining current crop yields. Combining optimal IWM tactics that discourage weeds by minimizing disturbance (no till, direct-seeding), adopting diverse crop rotations, and attempting to preclude resource acquisition by weeds are encouraged. New research knowledge on practical IWM systems is available, but despite current and looming threats of major weed resistance, most crop producers will require greater incentives than those currently available to more-fully adopt IWM systems in the near future.