Shifts in outcrossing rates and changes to floral traits are associated with the evolution of herbicide resistance in the common morning glory.

Human-mediated selection can strongly influence the evolutionary response of natural organisms within ecological timescales. But what traits allow for, or even facilitate, adaptation to the strong selection humans impose on natural systems? Using a combination of laboratory and greenhouse studies of 32 natural populations of the common agricultural weed, Ipomoea purpurea, we show that herbicide-resistant populations self-fertilise more than susceptible populations. We likewise show that anther-stigma distance, a floral trait associated with self-fertilisation in this species, exhibits a nonlinear relationship with resistance such that the most and least resistant populations exhibit lower anther-stigma separation compared to populations with moderate levels of resistance. Overall, our results extend the general finding that plant mating can be impacted by human-mediated agents of selection to that of the extreme selection of the agricultural system. This work highlights the influence of human-mediated selection on rapid responses of natural populations that can lead to unexpected long-term evolutionary consequences.

A resurrection experiment finds evidence of both reduced genetic diversity and potential adaptive evolution in the agricultural weed Ipomoea purpurea.

Despite the negative economic and ecological impact of weeds, relatively little is known about the evolutionary mechanisms that influence their persistence in agricultural fields. Here, we use a resurrection approach to examine the potential for genotypic and phenotypic evolution in Ipomoea purpurea, an agricultural weed that is resistant to glyphosate, the most widely used herbicide in current-day agriculture. We found striking reductions in allelic diversity between cohorts sampled nine years apart (2003 vs. 2012), suggesting that populations of this species sampled from agricultural fields have experienced genetic bottleneck events that have led to lower neutral genetic diversity. Heterozygosity excess tests indicate that these bottlenecks may have occurred prior to 2003. A greenhouse assay of individuals sampled from the field as seed found that populations of this species, on average, exhibited modest increases in herbicide resistance over time. However, populations differed significantly between sampling years for resistance: some populations maintained high resistance between the sampling years whereas others exhibited increased or decreased resistance. Our results show that populations of this noxious weed, capable of adapting to strong selection imparted by herbicide application, may lose genetic variation as a result of this or other environmental factors. We probably uncovered only modest increases in resistance on average between sampling cohorts due to a strong and previously identified fitness cost of resistance in this species, along with the potential that nonresistant migrants germinate from the seed bank.

How weeds emerge: a taxonomic and trait-based examination using United States data.

Weeds can cause great economic and ecological harm to ecosystems. Despite their importance, comparisons of the taxonomy and traits of successful weeds often focus on a few specific comparisons - for example, introduced versus native weeds. We used publicly available inventories of US plant species to make comprehensive comparisons of the factors that underlie weediness. We quantitatively examined taxonomy to determine if certain genera are overrepresented by introduced, weedy or herbicide-resistant species, and we compared phenotypic traits of weeds to those of nonweeds, whether introduced or native. We uncovered genera that have more weeds and introduced species than expected by chance and plant families that have more herbicide-resistant species than expected by chance. Certain traits, generally related to fast reproduction, were more likely to be associated with weedy plants regardless of species' origins. We also found stress tolerance traits associated with either native or introduced weeds compared with native or introduced nonweeds. Weeds and introduced species have significantly smaller genomes than nonweeds and native species. These results support trends for weedy plants reported from other floras, suggest that native and introduced weeds have different stress adaptations, and provide a comprehensive survey of trends across weeds within the USA.

Microsatellite loci for Gossypium davidsonii (Malvaceae) and other D-genome, Sonoran Desert endemic cotton species.

PREMISE OF THE STUDY: Microsatellite primers previously developed for domesticated cotton (Gossypium hirsutum; tetraploid) were screened for their utility in investigating genetic structure and gene flow within G. davidsonii and five other wild, Mexican, D-genome cotton species (all diploid). METHODS AND RESULTS: We screened 50 microsatellite primer pairs from the Cotton Marker Database, identifying 10 loci as polymorphic within G. davidsonii. In genotyping approximately 200 individuals from four populations, we found that the number of alleles per locus ranged from seven to 17, and mean observed and expected heterozygosity ranged from 0.145 to 0.492 and from 0.436 to 0.734, respectively. We genotyped six to 20 individuals from each of the remaining species, finding these 10 loci to cross-amplify in all cases and in most cases to be polymorphic. CONCLUSIONS: These markers may be useful for further investigation of population genetics of G. davidsonii and other wild D-genome cotton species.

Fitness costs of herbicide resistance across natural populations of the common morning glory, Ipomoea purpurea.

Although fitness costs associated with plant defensive traits are widely expected, they are not universally detected, calling into question their generality. Here, we examine the potential for life-history trade-offs associated with herbicide resistance by examining seed germination, root growth, and above-ground growth across 43 naturally occurring populations of Ipomoea purpurea that vary in their resistance to RoundUp®, the most commonly used herbicide worldwide. We find evidence for life-history trade-offs associated with all three traits; highly resistant populations had lower germination, shorter roots, and smaller above-ground size. A visual exploration of the data indicated that the type of trade-off may differ among populations. Our results demonstrate that costs of adaptation may be present at stages other than simply the production of progeny in this agricultural weed. Additionally, the cumulative effect of costs at multiple life cycle stages can result in severe consequences to fitness when adapting to novel environments.

The geographic mosaic of herbicide resistance evolution in the common morning glory, Ipomoea purpurea: Evidence for resistance hotspots and low genetic differentiation across the landscape.

Strong human-mediated selection via herbicide application in agroecosystems has repeatedly led to the evolution of resistance in weedy plants. Although resistance can occur among separate populations of a species across the landscape, the spatial scale of resistance in many weeds is often left unexamined. We assessed the potential that resistance to the herbicide glyphosate in the agricultural weed Ipomoea purpurea has evolved independently multiple times across its North American range. We examined both adaptive and neutral genetic variations in 44 populations of I. purpurea by pairing a replicated dose-response greenhouse experiment with SSR genotyping of experimental individuals. We uncovered a mosaic pattern of resistance across the landscape, with some populations exhibiting high-survival postherbicide and other populations showing high death. SSR genotyping revealed little evidence of isolation by distance and very little neutral genetic structure associated with geography. An approximate Bayesian computation (ABC) analysis uncovered evidence for migration and admixture among populations before the widespread use of glyphosate rather than the very recent contemporary gene flow. The pattern of adaptive and neutral genetic variations indicates that resistance in this mixed-mating weed species appears to have evolved in independent hotspots rather than through transmission of resistance alleles across the landscape.

Permanent genetic resources added to molecular ecology resources database 1 October 2012-30 November 2012.

This article documents the addition of 153 microsatellite marker loci to the Molecular Ecology Resources Database. Loci were developed for the following species: Brassica oleracea, Brycon amazonicus, Dimorphandra wilsonii, Eupallasella percnurus, Helleborus foetidus, Ipomoea purpurea, Phrynops geoffroanus, Prochilodus argenteus, Pyura sp., Sylvia atricapilla, Teratosphaeria suttonii, Trialeurodes vaporariorum and Trypanosoma brucei. These loci were cross-tested on the following species: Dimorphandra coccicinea, Dimorphandra cuprea, Dimorphandra gardneriana, Dimorphandra jorgei, Dimorphandra macrostachya, Dimorphandra mollis, Dimorphandra parviflora and Dimorphandra pennigera.