Weedy Sunflower Research Articles

Reakcija populacija Xanthium strumarium L. i Helianthus annuus L. na nikosulfuron

Weedy sunflower, Helianthus annuus L. is an economically very important, invasive species widely distributed in many regions of the world, especially in the areas where sunflower hybrids tolerant to some als-inhibiting herbicides are intensively cultivated. The discovery and development of a new technology in cultivated sunflower increased the emergence of resistant biotypes due to high selection pressure and spontaneous hybridisation (transfer of genes responsible for tolerance from crop to wild species). This study examined the responses of two populations of weedy sunflower (HELAN 1 and HELAN 2) to nicosulfuron and imazamox in bioassay under greenhouse conditions. The experiment included five treatments with different application rates of imazamox (12, 24, 48, 96 and 192 g ha-1) and nicosulfuron (10, 20, 40, 80 and 160 g ha-1) that were applied over plants at their growth stage of 2-3 pairs of developed leaves. In addition to the visual assessment of plant damage, which was performed 7, 14, 21 and 28 days after the herbicide application, the following parameters were also measured: fresh and dry weight. The obtained results indicate changes in the sensitivity of the examined weedy sunflower populations to als inhibitors. Reduced sensitivity of the HELAN 1 to imazamox and HELAN 2 to nicosulfuron suggests a possible development of resistant populations of weedy sunflower in the Surčin area.

Distribucija i kvantitativna zastupljenost korovskog suncokreta Helianthus annuus L. u Srbiji

The Helianthus annuus L. species which appears as a weedy sunflower differs from the sunflower crop in height, pronounced branching, phenotypic variability, presence of anthocyan pigment, numerous small heads, relatively small and morphologically variable seeds (shape, color, motlling), dormancy and seed brittleness, etc. Weedy sunflover (WS) has been spreading for more than two decades on crop and non-cropland in the area with intensive sunflower crop production in Serbia, where it causes large yield losses in many crops (row crops and cereals). Bearing this in mind, as well as the fact that there have been confirmed cases of reduced herbicide efficacy on WS, the aim of this study was to determine the distribution and define the population size of WS in the whole territory of Serbia. During a three year period, in the region with intensive sunflower crop production (Banat, Bačka, Srem, Stig), the present WS populations were monitored and the quantitative and qualitative traits of their populations were assessed. A scale from 1-4 was used to estimate the number of plants in the population (1 - species cover up to 5% of the surveyed area, 2 - species cover 10-15% of the surveyed area, 3 - species cover 25-50% of the surveyed area, and 4 - species cover over 50% of the surveyed area). Over 200 populations of WS have been recorded in the whole territory of Serbia, and this is shown on a UTM map of 10x10 km scale. The largest WS populations (5 to 15 ha), with the highest number of plants (> 100,000) and average density (about 5 m-2 plants) were recorded in the site of Padinska Skela (in crop and non-croplands), Zbeg (predominantly on non-cropland) and Surčin-Galovica (predominantly in cropland). Based of the phenotypic diversity/discrimination traits, the population from Stari Žednik was the most similar to sunflower crop [with the central head and apical branching, the lowest number of heads compared with other WS populations (18.5 plants-1, while some populations had up to 59.4 heads plant-1) and the largest head size (8.1 cm in diameter)]. These results should be a guide for the farmers when planning crop planting, to undertake the best measures to control WS within the concept of antiresistance strategy, given the confirmed cases of reduced susceptibility of this weed to herbicides, as well as the obligation to eliminate it on non-cropland, as its main corridors of expansion are areas along roads and canals, field crop margines, lost cropland, etc.

Shattercane (Sorghum bicolor (L.) Moench Subsp. Drummondii) and Weedy Sunflower (Helianthus annuus L.)—Crop Wild Relatives (CWRs) as Weeds in Agriculture

Shattercane (Sorghum bicolor (L.) Moench subsp. drummondii) and weedy sunflower (Helianthus annuus L.) are two examples of crop wild relatives (CWRs) that have become troublesome weeds in agriculture. Shattercane is a race belonging to a different subspecies than domesticated sorghum (Sorghum bicolor (L.) Moench subsp. bicolor). Weedy sunflower populations are natural hybrids between wild and domesticated sunflower (Helianthus annuus L.). Both species have key weedy characteristics, such as early seed shattering and seed dormancy, which play an important role in their success as agricultural weeds. They are widely reported as important agricultural weeds in the United States and have invaded various agricultural areas in Europe. Shattercane is very competitive to sorghum, maize (Zea mays L.), and soybean (Glycine max (L.) Merr.). Weedy sunflower causes severe yield losses in sunflower, maize, soybean, pulse crops, and industrial crops. Herbicide resistance was confirmed in populations of both species. The simultaneous presence of crops and their wild relatives in the field leads to crop–wild gene flow. Hybrids are fertile and competitive. Hybridization between herbicide-tolerant crops and wild populations creates herbicide-resistant hybrid populations. Crop rotation, false seedbed, cover crops, and competitive crop genotypes can suppress shattercane and weedy sunflower. Preventative measures are essential to avoid their spread on new agricultural lands. The development of effective weed management strategies is also essential to prevent hybridization between sorghum, sunflower, and their wild relatives and to mitigate its consequences.

Cover Image, Volume 76, Issue 11

The cover image is based on the Research Article First case of glyphosate resistance in weedy sunflower (Helianthus annuus) by Vijay Singh et al., https://doi.org/10.1002/ps.5917. image

First case of glyphosate resistance in weedy sunflower (Helianthus annuus).

Weedy sunflower (Helianthus annuus L.) is a troublesome weed in row-crop production fields in South Texas. Populations with suspected resistance to glyphosate were evaluated with 1X and 4X rates (X = 868 g ae ha-1 ) of the herbicide, followed by a dose-response assay of the most resistant population. Molecular studies were conducted to determine if target-site mechanisms were responsible for resistance in these populations. Additionally, field experiments were conducted at two locations (Somerville and Granger, TX) to evaluate the effectiveness of different tank-mix combinations in controlling naturally infesting glyphosate-resistant (GR) weedy sunflower populations in GR corn. In a study conducted in the growth chamber, seven of the 11 tested populations survived up to the 4X rate of glyphosate. The most-resistant population (TX15-11) was 29-fold more resistant to glyphosate, compared to the susceptible standard. In resistant populations, 5-21 more copies of the EPSPS gene were observed compared to the susceptible standard. In the field studies, tank-mix applications of glyphosate + halosulfuron-methyl, glyphosate + prosulfuron, glyphosate + a premix of halosulfuron-methyl and dicamba or glyphosate + a premix of diflufenzopyr and dicamba effectively controlled GR weedy sunflower populations. Glyphosate-resistance was observed in 81% of the putative resistant weedy sunflower populations tested in this study. Resistance in these populations was conferred primarily by amplification of the EPSPS gene. Effective control of GR weedy sunflower can be achieved by tank-mixes tested in the current study, which provides acceptable levels of crop safety. © 2020 Society of Chemical Industry.

Uporedna mikro-morfloška i anatomska analiza listova gajenog i hibridne forme korovskog suncokreta (Helianthus annuus)

There are various opinions in the literature on the importance of morpho-anatomical leaf features, such as the number of stomata, the presence of trichomes, cuticle thickness, cell size, leaf thickness, etc. for the susceptibility of plants to foliar herbicides. Knowing that differences in the plant's sensitivity to herbicides may be due to differences in leaf sufrace and inner structure of leaves, a detailed micromorphological and anatomical analysis was performed on the leaves of tolerant and susceptible sunflower forms, both in cultivated hybrids and populations of weedy sunflower. It is interesting that while no significant differences in the number of trichomes and stomata between the resistant and susceptible weedy sunflower populations were observed, in comercial sunflower genotypes these numbers were significantly different. Namely, the number of stomata in the sensitive comercial sunflower hybrid was significantly higher than in the tolerant ones, but the tolerant hybrids had significantly more leaf trichomes than the sensitive one. Consequently, it is possible that the higher density of trichomes makes it impossible for the herbicides to reach the surface of the leaf epidermis, resulting in a reduced uptake. It has also been shown that plants with different levels of herbicide susceptibility also differ in various anatomical parameters.

The response of weedy sunflower (Helianthus annuus L.) to nicosulfuron: An examination of vegetative parameters and acetolactate synthase activity

Genetic and morphological interpopulation variability of weed species is often responsible for variable responses to herbicides. As weedy sunflower, an invasive form of Helianthus annuus L., possesses high morphological and genetic variability, very different responses of its populations to herbicides can be expected. This species is one of the dominant weed species in row crops, including maize, in many European countries in which nicosulfuron is intensively used for weed control. There are little available data about the response of this sunflower form to nicosulfuron or of the interpopulation variability of its response to other herbicides. The responses of three weedy sunflower populations to nicosulfuron were studied in field dose-response experiments, and acetolactate synthase (ALS) enzyme activity at different herbicide concentrations was determined in vitro. Interpopulation variability in the response to nicosulfuron was confirmed. Populations WS2 and WS3 were more that 20-fold and 30-fold less susceptible to nicosulfuron, respectively, than population WS1, based on fresh weight, whereas the differences were not so prominent based on other parameters, including plant height, leaf area and ALS activity, and ranged from 2 to 12-fold.

Studies on gene flow from herbicide resistant to weedy sunflower

Gene flow is a main concern associated with the use of herbicide resistant sunflower crops because it could transfer herbicide resistance traits to weedy sunflower. In order to estimate potential gene flow from imazamox and tribenuron-methyl resistant sunflower hybrids to weedy sunflower, field experiments and DNA analysis were conducted. The progeny of weedy sunflower which grown near imazamox (WS1) and tribenuron-methyl (WS2) resistant hybrid in previous experiments were used. In the field experiment, recommended rates of imazamox and tribenuron-methyl were applied to WS1 and WS2, respectively, and plants surviving were recorded. Herbicides effect on fresh weight of survived plants were also determined. The presence of mutations responsible for sunflower resistance to herbicides (imazamox and tribenuron-methyl) checked based on DNA analysis of selected survived plants. Percentage of survived plants in field experiment was recorded at maturity and depend on weedy sunflower accession (WS1or WS2) and distance of their mother plants from resistant sunflower hybrid in previous experiment and was higher for WS2 (50.25%) than for WS1 (24.50%). As DNA analysis were not confirmed the presence of the point mutations responsible for sunflower resistance to imazamox and tribenuron-methyl.

Effects of nicosulfuron on biological production of weedy sunflower (Helianthus annuus)

Field experiments were conducted to quantify the effects of nicosulfuron on biological production of weedy sunflower (Helianthus annuus). Plants of this weed species were sown in plots sized 5x4.2 m with inter-row spacing of 24 cm, and 70 cm distance between rows. Plants with two pairs of developed leaves were treated with nicosulfuron using the next rates: 0, 10, 20, 40, 60 and 80 g a.i. ha-1. Vegetative parameters (plant height, fresh weight and leaf area) were recorded five times during vegetation, including the first measurement just before herbicide application and four measurements at intervals of about two weeks. Head and seed production (number of heads per plant, head weight, head diameter, weight of seeds per head, weight of seeds per plant, number of seeds per plant) were determined after seed maturity. The application of nicosulfuron caused growth reduction of weedy sunflower in comparison with the control. All application rates of this herbicide reduced vegetative (height, fresh weight, leaf area) and some generative (number of heads per plant, weight of seeds per plant, number of seeds per plant) parameters, which decreased with increasing rates of nicosulfuron. Effects on the remaining generative test parameters (head weight, head diameter, weight of seeds per head) were not analogous to the effects on other parameters.

Effect of nicosulfuron on the populations of invasive weedy sunflower

Weedy sunflower Helianthus annuus L. (WS) can become troublesome for cultivated sunflower given their genetic similarity which allows gene flow from weed-to-crop and from crop-to-weed. Also, it causes yield losses in different spring-sown crops. Despite this, there is still no data about its response to herbicides. Field experiments were conducted to quantify the effects of nicosulfuron on WS populations. Plants of three populations (R1-WS and R2-WS = presumably resistant and S-WS = susceptible) were treated with 40 g nicosulfuron ha-1 at the 2–4 true-leaf growth stage. Vegetative parameters (plant height, fresh weight, leaf area) and relative chlorophyll content (RCC) were measured 30 days after herbicide application. Generative parameters (number of heads plant-1, head diameter, number of seeds plant-1) were measured at maturity. After harvesting, seed germination of collected seeds was studied in Petri dishes at 25°C. Nicosulfuron was found to have a significant effect on vegetative and generative production of WS plants. The effect on RCC and postharvest seed germination was not so prominent. All this effects depended on the population, which is probably a result of different history of herbicide application and possible development of resistance in the populations R1-WS and R2-WS. Die Auswirkung von Nicosulfuron auf invasive Unkraut-Sonnenblumenbestande In Anbetracht der genetischen Ahnlichkeit zwischen der Kulturpflanze und seinem zugehorigen Unkraut, die einen Genfluss in beiden Richtungen ermoglicht, ist es zu erwarten, dass auch die Unkraut-Sonnenblume, Helianthus annuus (WS) eine bedeutende Entwicklungsstorung bei der Ackersonnenblume hervorrufen und genauso Ertragsverluste bei anderen Sommerungen verursachen kann. Angesichts der fehlenden Daten bezuglich der Herbizidreaktion des obengenannten Unkrauts wurden entsprechende Felduntersuchungen durchgefuhrt, mit dem Ziel die Auswirkungen von Nicosulfuron auf WSPopulationen quantitativ zu bestimmen. Drei unterschiedliche Pflanzenbestande (R1-WS und R2-WS = vermutlich resistent, und S-WS = anfallig) wurden mit 40 g Nicosulfuron/ha, jeweils auf das zweite Blattpaare behandelt. Vegetative Parameter (Pflanzengrose, Frischmasse, Blattflache) und relativer Chlorophyllgehalt (RCC) wurden 30 Tage nach dem Herbizideinsatz gemessen. Generative Parameter (Anzahl der Pflanzenkopfe, Kopfdurchmesser, Anzahl der Pflanzensamen) wurden im reifen Zustand gemessen. Nach der Ernte, wurde noch die Keimung der gesammelten Samen in Petrischalen mit einer Temperatur von 25 °C genauer beobachtet. Die Ergebnisse deuteten darauf hin, dass Nicosulfuron eine erhebliche Auswirkung auf vegetative so wie auch auf generative Produktion von WS-Pflanzen hat. Die Auswirkung auf RCC und Keimung durch Ernte gewonnener Samen zeigte sich jedoch nicht in dem gleichen Mase auffallig. Alle Effekte standen im Zusammenhang mit den Pflanzenpopulationen, was wahrscheinlich eine Folge vom andersartigen Verlauf der fruheren Herbizidanwendungen, und vermutlicher Resistenzentwicklung in R1-WS und R2-WS Populationen ist.

Gene Flow from Herbicide-Resistant Sunflower Hybrids to Weedy Sunflower

Weedy forms of cultivated sunflower (Helianthus annuus) are invasive species widely distributed in several regions of the world and are commonly controlled by applying acetohydroxyacid synthase (AHAS)-inhibiting herbicides, such as imidazolinones (IMIs) or sulfonylurea (SUs). The widespread adoption of herbicide-resistant crops has exposed the weedy population to the high risk of crop-to-weedy gene flow. The aim of this study was to check and quantify the gene flow from IMI- and SU-resistant sunflower hybrids to weedy sunflower populations. Field experiments were conducted in 2008 at two sites in Serbia to evaluate the relationship of distance between the crop and the weedy sunflower and its impact on the percentage of gene flow. The weedy sunflower progenies were evaluated through herbicide resistance and SSR marker study. Hybridization with IMI-resistant hybrids was not confirmed. Conversly, SU-resistance trials and SSR marker studies partially confirmed the transfer of resistance within the weedy population.

Quantifying temporal isolation: a modelling approach assessing the effect of flowering time differences on crop-to-weed pollen flow in sunflower.

Flowering time divergence can be a crucial component of reproductive isolation between sympatric populations, but few studies have quantified its actual contribution to the reduction of gene flow. In this study, we aimed at estimating pollen-mediated gene flow between cultivated sunflower and a weedy conspecific sunflower population growing in the same field and at quantifying, how it is affected by the weeds' flowering time. For that purpose, we extended an existing mating model by including a temporal distance (i.e. flowering time difference between potential parents) effect on mating probabilities. Using phenological and genotypic data gathered on the crop and on a sample of the weedy population and its offspring, we estimated an average hybridization rate of approximately 10%. This rate varied strongly from 30% on average for weeds flowering at the crop flowering peak to 0% when the crop finished flowering and was affected by the local density of weeds. Our result also suggested the occurrence of other factors limiting crop-to-weed gene flow. This level of gene flow and its dependence on flowering time might influence the evolutionary fate of weedy sunflower populations sympatric to their crop relative.

Exoferality in sunflower (Helianthus annuus L.): A case study of intraspecific/interbiotype interference promoted by human activity

Helianthus annuus subsp. annuus, a biotype of the same species of the domesticated sunflower H. annuus var. macrocarpus, is an emergent noxious weed in several regions of the world. The frequent hybridization that occurs between the two taxa could explain its diffusion in agricultural fields. The invasive dynamics of a weedy biotype was studied in a recently invaded field during four successive seasons. The weedy biotype was grown in an experimental plot and was characterized phenotypically. Competition between weedy and cultivated sunflower was studied at both the experimental plot and crop field levels under a wide range of weed densities. The weedy biotype shows evidence of crop introgression, with high morphological variability and intermediate traits between the crop and wild or ruderal sunflower. After four years under a sunflower–soybean summer crop rotation, the population was reproduced in the 75% of the field. The sunflower crop yield was reduced by more than 50% with >4weedsm−2. The weedy sunflower achenes are similar in size to the crop and so can be harvested, adding over 300kgha−1 to the harvested yield in crops with >4weedsm−2. However this contribution was not enough to balance the loss (1919kgha−1) in the crop yield. Weedy sunflower has lower oil content and different fatty acid composition than the crop and therefore it could affect the oil quantity and quality of the harvested grains, by physical contamination. It was shown that weedy sunflower invades and remains in agricultural fields, causing up to 74% loss in sunflower crop yield, which emphasizes the need to prevent weedy sunflower colonization and invasion in sunflower fields. In order to prevent the introduction of weedy biotypes into non-invaded areas seed purity and thorough cleaning of agricultural machinery are of utmost importance. This is the first study of weedy sunflower density effect on sunflower yield loss.

How to escape from crop‐to‐weed gene flow: phenological variation and isolation‐by‐time within weedy sunflower populations

The evolution of crop-related weeds may be constrained by recurrent gene flow from the crop. However, flowering time variation within weedy populations may open the way for weed adaptation by allowing some weeds to escape from this constraint. We investigated this link between phenology, gene flow and adaptation in weedy sunflower populations that have recently emerged in Europe from crop-wild hybridization. We studied jointly flowering phenology and genetic diversity for 15 microsatellite loci in six cultivated sunflower (Helianthus annuus) fields infested by weedy sunflower populations. The flowering overlap of cultivated and weedy sunflowers varied between and within populations: some weedy individuals were found to be completely isolated from the crop, the frequency of these plants being higher in populations from highly infested fields. Within weedy populations, we detected a pattern of isolation-by-time: the genetic divergence between individuals was positively correlated with their divergence in flowering period. In addition, earlier weeds, which flowered synchronously with the crop, were genetically more similar than late-flowering weeds to the cultivated varieties. Overall, our results suggest that crop-to-weed gene flow occurred, but was limited by divergent phenologies. We discuss the roles of weed adaptation and population history in the generation of this partial reproductive isolation.

Les tournesols adventices : un exemple d’évolution d’une mauvaise-herbe apparentée à une espèce cultivée – Synthèse des recherches menées sur les populations adventices de tournesol en France depuis 2004

Weedy sunflowers have been officially reported in South-Western France in 2004. They display a combination of phenotypic traits of the wild and domesticated forms of the species and infest between 15 and 20% of fields of sunflower crop in this area, although at variable levels. When the infestation is strong, it affects seed yield and oil quality. Molecular studies showed that weedy sunflowers most probably resulted from accidental crop-wild hybridization during the seed production process and from the introduction of the resulting hybrids into commercial seed lots. Multiple independent introductions were at the source of the number of infested fields observed nowadays. The temporal dynamics of the infestation of a field and the detailed conditions for its success remain largely unknown. The flowering synchrony between crops and weeds is substantial, even if a variable fraction of the weeds (15-55%) flower completely outside the crop flowering period. Molecular survey of adult weedy plants and their progenies showed that crop-toweed gene flow occurs, at a rate that can reach 35 %on average, at the peak of the crop flowering period. We draw perspectives on the durability of herbicide-tolerant sunflower varieties as a solution to control these weeds, and call for more studies tackling weed management from an evolutionary lens.

Prospective Agents for the Biological Control ofTithonia rotundifolia(Mill.) S.F.Blake andTithonia diversifolia(Hemsl.) A.Gray (Asteraceae) in South Africa

Starting in 2007, two weedy sunflower species, Tithonia rotundifolia (Mill.) S.F.Blake and Tithonia diversifolia (Hemsl.) A.Gray (Asteraceae: Heliantheae), were targeted for biological control in South Africa. Surveys conducted in their native range (Mexico) revealed that there were five potential biological control agents for T. rotundifolia, and three of these are currently undergoing host-specificity and performance evaluations in South Africa. Two leaf-feeding beetles, Zygogramma signatipennis (Stål) and Zygogramma piceicollis (Stål) (Coleoptera: Chrysomelidae), are the most promising biological control agents for T. rotundifolia: preliminary host-specificity trials suggest that they are adequately host-specific. The stem-boring beetle, Lixus fimbriolatus Boheman (Coleoptera: Curculionidae), is also highly damaging to T. rotundifolia, but its host range is yet to be determined. Two other stem-boring beetles, Canidia mexicana Thomson (Coleoptera: Cerambycidae) and Rhodobaenus auctus Chevrolat (Coleoptera: Curculionidae), have also been recorded on T. rotundifolia, and these will be considered for further testing if L. fimbriolatus is found to be unsuitable for release in South Africa. Only two insect species were imported as candidate agents on T. diversifolia, the leaf-feeding butterfly Chlosyne sp. (Lepidoptera: Nymphalidae), and an unidentified stem-boring moth (Lepidoptera: Tortricidae): the latter was tested in quarantine but rejected because it attacked several sunflower cultivars. Only one pathogen, Puccinia enceliae Dietel & Holw. (Uredinales: Pucciniaceae), was found that could potentially have been used as a biological control agent against the Tithonia species, but attempts to culture this rust were unsuccessful.

Occurrence, distribution and distinctive morphological traits of weedy Helianthus annuus L. populations in Spain and France

We made a first descriptive study of weedy sunflowers infesting sunflower crop fields in one Spanish and three French regions. Overall, weedy sunflowers affected around 15% of sunflower fields. Infested fields were most often dispersed over the study areas without evident geographical clustering. In France, five weedy populations were surveyed more intensively. They were composed of a large diversity of morphotypes showing a combination of typical wild and domesticated traits in proportions that differed between populations. Yield losses reached 50% in heavily infested patches. Our results suggest that weedy sunflowers may have arisen through the hybridization of cultivated and wild sunflower, potentially during the seed production process. Such crop-wild hybrids would have been introduced recurrently into fields through the seed lots, where they evolved to locally invasive weedy populations.

Natural Variation in Gene Expression Between Wild and Weedy Populations of Helianthus annuus

The molecular genetic changes underlying the transformation of wild plants into agricultural weeds are poorly understood. Here we use a sunflower cDNA microarray to detect variation in gene expression between two wild (non-weedy) Helianthus annuus populations from Utah and Kansas and four weedy H. annuus populations collected from agricultural fields in Utah, Kansas, Indiana, and California. When grown in a common growth chamber environment, populations differed substantially in their gene expression patterns, indicating extensive genetic differentiation. Overall, 165 uni-genes, representing approximately 5% of total genes on the array, showed significant differential expression in one or more weedy populations when compared to both wild populations. This subset of genes is enriched for abiotic/biotic stimulus and stress response proteins, which may underlie niche transitions from the natural sites to agricultural fields for H. annuus. However, only a small proportion of the differentially expressed genes overlapped in multiple wild vs. weedy comparisons, indicating that most of the observed expression changes are due to local adaptation or neutral processes, as opposed to parallel genotypic adaptation to agricultural fields. These results are consistent with an earlier phylogeographic study suggesting that weedy sunflowers have evolved multiple times in different regions of the United States and further indicate that the evolution of weedy sunflowers has been accompanied by substantial gene expression divergence in different weedy populations.

Genetics and evolution of weedy Helianthus annuus populations: adaptation of an agricultural weed

Agricultural weeds are a major cost to economies throughout the world, and have evolved from numerous plant species in many different plant families. Despite their ubiquity, we do not yet know how easily or often weeds evolve from their wild ancestors or the kinds of genes underlying their evolution. Here we report on the evolution of weedy populations of the common sunflower Helianthus annuus. We analysed 106 microsatellites in 48 individuals from each of six wild and four weed populations of the species. The statistical tests lnRV and lnRH were used to test for significant reductions in genetic variability at each locus in weedy populations compared to nearby wild populations. Between 1% and 6% of genes were significant outliers with reduced variation in weedy populations, implying that a small but not insignificant fraction of the genome may be under selection and involved in adaptation of weedy sunflowers. However, there did not appear to be a substantial reduction in variation across the genome, suggesting that effective population sizes have remained very large during the recent evolution of these weedy populations. Additional analyses showed that weedy populations are more closely related to nearby wild populations than to each other, implying that weediness likely evolved multiple times within the species, although a single origin followed by gene flow with local populations cannot be ruled out. Together, our results point to the relative ease with which weedy forms of this species can evolve and persist despite the potentially high levels of geneflow with nearby wild populations.

The persistence of cultivar alleles in wild populations of sunflowers five generations after hybridization

The development of transgenic plants has heightened concern about the possible escape of genetically engineered material into the wild. Hybridization between crops and their wild relatives provides a mechanism by which this could occur. While hybridization has been documented between several crops and wild or weedy relatives, little is known about the persistence of cultivar genes in wild populations in the generations following hybridization. Wild and weedy sunflowers occur sympatrically with cultivated sunflowers throughout much of the cultivation range, and hybridization is known to occur. We surveyed two cultivar-specific RAPD markers in 2700 progeny in a naturally occurring population of wild Helianthus annuus over five generations following a single generation of hybridization with the cultivar. Moderate levels of gene flow were detected in the first generation (42% hybrids at the crop margin) and cultivar allele frequencies did not significantly decline over four subsequent generations. These results indicate that gene flow from cultivated into wild populations of sunflowers can result in the long-term establishment of cultivar alleles in wild populations. Furthermore, we conclude that neutral or favorable transgenes have the potential to escape and persist in wild sunflower populations.