Abstract
There is increasing interest in documenting adaptation of weedy plant species to agricultural ecosystems, beyond the evolution of herbicide resistance. Waterhemp (Amaranthus tuberculatus) is a native plant of the Midwestern U.S. that began infesting agricultural fields in the 20th century within the central portion of its range. We hypothesized that the vegetative growth and reproductive traits of waterhemp from this heavily infested central region provide differential fitness benefits in agricultural environments. We collected seeds from across the species’ native range, representing regions with varying degrees of waterhemp infestation, and planted them together in common garden soybean plots. A 2010 common garden experiment was conducted within the range of agriculturally weedy waterhemp (in Missouri), and a 2011 common garden experiment was conducted outside of this range (in Ohio). Days to flowering and flowering plant height, mature plant size data (height, number of branches, and length of the longest branch), and above-ground biomass were measured to estimate relative fitness. In both common garden locations, plants from regions where waterhemp occurs as an agricultural weed — including those from the heavily infested Mississippi Valley region (Iowa, Illinois, and Missouri) and the less severely infested Plains region (Nebraska, Kansas, and Oklahoma) — had higher relative performance in almost all fitness-related measures than plants from the Northeast region (Ohio, Michigan, and Ontario), which had little to no agriculturally weedy waterhemp at the time of our study. Further analysis revealed that fewer days to flowering in the Northeast populations can be largely accounted for by latitude of origin, suggesting a strong genetic influence on this reproductive trait. These findings suggest intraspecific variation in agricultural adaptation in a native U.S. weed, and support the use of agricultural weeds to study adaptation.
Highlights
Agricultural weeds can be viewed as invasive plants of heavily human-modified environments, namely crop fields and rangelands [1]
Agricultural weeds cost an estimated $33 billion annually in the U.S alone [14]; given their economic impact, it is surprising that adaptive evolution of weeds in traits other than herbicide resistance has not been more frequently studied
In the fall of 2009 and 2010, seeds were collected from populations across the range of A. tuberculatus s.l. for use in common garden experiments in two locations: Missouri and Ohio
Summary
Agricultural weeds can be viewed as invasive plants of heavily human-modified environments, namely crop fields and rangelands [1]. Studies of adaptation in agricultural weeds have focused primarily on the evolution of herbicide resistance (reviewed in [9,10,11,12]). Agricultural weeds cost an estimated $33 billion annually in the U.S alone [14]; given their economic impact, it is surprising that adaptive evolution of weeds in traits other than herbicide resistance has not been more frequently studied. There has recently been increased interest in developing agricultural weeds as model systems for understanding rapid adaptation to human-altered habitats, for example in sunflowers [15, 16], morning glory [17, 18], radish [19, 20], and weedy rice [21]
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