Population and quantitative genetic analyses of life‐history trait adaptations in Amaranthus palmeri S. Watson

Abstract

Amaranthus palmeri S. Watson (commonly known as Palmer amaranth) is a dioecious species that has become one of the most problematic weeds in the USA, not only because of its high reproduction rate and ability to compete with crops but also because of its rapid expansion of herbicide-resistant biotypes throughout the country in a relatively short period of time. This last aspect is particularly puzzling because A. palmeri is native from the south-western USA, and it was not considered a frequent or problematic weed until the end of the 20th century (Sauer, 1957;Ward et al., 2013). In the last 30 years, there has been an increasing number of reports of this weed species invading agricultural fields throughout the Southern and Midwestern USA (Butts et al., 2016;Webster and Coble, 1997).The increase in the evolution of herbicide-resistant biotypes has been associated with the success of this weed as an invader of agricultural fields (Butts et al., 2016;Ward et al., 2013). But just as importantly, there are multiple factors within a cropping system that should be considered to understand weed species survival and increasing colonisation of agricultural systems. Amaranthus palmeri populations have high levels of genetic di-versity but most is found within populations and very little across populations (Chandi et al., 2013). In other words, due to high rates of gene flow, populations of A. palmeri tend to be similar, at least when considering neutral markers. This information has important practical implications because if most populations are similar, then solutions to manage a particular population should be effective to manage other populations. However, selective forces in agricultural fields can be high enough to overcome gene flow and fix alleles or traits that favour the survival of the weed in a particular cropping system (Karn and Jasieniuk, 2017). The most common and documented case of this situation is herbicide resistance, but other traits with less conspicuous phenotypes

Rapid evolution of competitive ability in giant foxtail (Setaria faberi) over 34 years

“…Also, field commercial doses were high enough to ensure the same selection pressure in all yearlines. Therefore, we propose that the uniformity in the agricultural landscape resulting from the widespread adoption of glyphosateresistant crops created conditions favoring the evolution of more competitive biotypes (Bravo et al 2017;Leon and van der Laat 2021;Owen 2008). Landscape fragmentation and diversity favor genetic diversity and dispersal-related traits in human-disturbed landscapes.…”

In-field assessment of EPSPS amplification on fitness cost in mixed glyphosate-resistant and glyphosate-sensitive populations of Palmer amaranth (Amaranthus palmeri)

“…Another important consideration is that in a species such as A. palmeri with high genetic variability and obligate outcrossing, the genetic architectures in which the resistance allele(s) operate are also variable within populations. Furthermore, other traits that affect fitness are also under selection, and they could additively (or synergistically/antagonistically) increase or mitigate the fitness change caused by the resistance allele(s) Leon and van der Laat 2021). In fact, Bravo et al (2017Bravo et al ( , 2018 documented in commercial farms the existence of A. palmeri populations in which GR biotypes were taller and produced more biomass than GS biotypes.…”

Weak population differentiation and high diversity in Salsola tragus in the inland Pacific Northwest, USA

“…Weed managers should expect that future herbicide‐resistance traits will likely spread across the region with similar rapidity, and be prepared to adjust herbicide programs and other relevant management inputs with equal speed, regardless of their proximity to the point of initial origin. Low across‐range differentiation, usually in combination with high levels of genetic diversity, has also been suggested as a shared trait among problematic and adaptable weeds, 79 with examples in a range of arable 67,69,80–87 and invasive 88,89 weeds. This pattern appears to be emerging with particular regularity in outcrossing dicot weeds with widespread herbicide resistance, including Bassia scoparia , 67,85,86 Amaranthus palmeri S. Watson, 83,85 Amaranthus tuberculatus (Moq.)…”

“…Low across‐range differentiation, usually in combination with high levels of genetic diversity, has also been suggested as a shared trait among problematic and adaptable weeds, 79 with examples in a range of arable 67,69,80–87 and invasive 88,89 weeds. This pattern appears to be emerging with particular regularity in outcrossing dicot weeds with widespread herbicide resistance, including Bassia scoparia , 67,85,86 Amaranthus palmeri S. Watson, 83,85 Amaranthus tuberculatus (Moq.) J.D.…”