Literature Review Procedure

We conducted a comprehensive literature review to examine the extent of gene flow in self-compatible weed species that are predominantly self-pollinated. We selected important self-pollinated weeds from the list provided by the Weed Science Society of America (WSSA 2025). Studies reporting empirical data on the extent of gene flow and distances were collected using combination of the search words such as “self-pollinated,” “autogamous,” “self-compatible,” “gene flow,” “hybridization,” and “outcrossing” for the selected taxonomic families and species. Key variables extracted included the extent and distance of maximum gene flow, geographic location, temperature, precipitation, and relative humidity as the data were available. For subsequent analyses, the extracted data were categorized by weed species family to examine potential family-specific patterns in gene flow. However, it is critically important to note that number of studies conducted on gene flow in self-pollinated weeds is very limited.

Data analysis was conducted using JMP software (v.16; SAS Institute Inc., Cary, NC). Descriptive statistics (mean, standard deviation, range) were calculated for PMGF percent, distances, and environmental factors (temperature, precipitation, relative humidity, and elevation). To explore the relationships between gene flow extents and environmental factors, Pearson correlation analyses were performed between gene flow extents and each environmental variable. Subsequently, simple linear regression models, based on the root mean square error and R ² values, were used to evaluate the effect of each individual environmental factor on gene flow extent and distance. In each regression analysis, the environmental factor (e.g., temperature, precipitation, relative humidity, elevation) was treated as a fixed independent variable, and gene flow percent or gene flow distance was treated as the dependent variable. No random effects were modeled because the analysis was based on summarized data extracted from the literature rather than experimental replicates within hierarchical structures. All variables were checked for normality before analysis, and wherever necessary, transformations (e.g., logarithmic) were applied to improve the model fit. Significance was evaluated at P < 0.05 for all statistical tests.

Extent and Distance of Gene Flow in Self-Pollinated Weeds

PMGF Across Families. Synthesis of the average gene flow data showed that the self-pollinated weeds in the Solanaceae family had the highest gene flow, averaging 10.63% in our data, among seven taxonomic families considered in the dataset (Table 1). The second and third families with the highest gene flow were Solanaceae and Chenopodiaceae, with averages of 10.00% and 1.58%, respectively. The lowest extent of gene flow was observed in Linaceae followed by Brassicaceae and Poaceae, with average gene flow of 0.01%, 0.33% and 1.87%, respectively. Floral biology, along with factors such as mating system and pollination mechanism, contributes to variation in the extent of gene flow observed among these families (Hermanutz Reference Hermanutz1991; Marshal and Abbott Reference Marshall and Abbott1982). While Asteraceae flowers typically have capitula, an open flower structure, favoring more outcrossing as compared to Poaceace with a cleistogamous or chasmogamous flower, a closed flower structure. Consequently, the self-pollinated weed species in Astereaceae family showed gene flow to as far as 1,000 m, the longest distance found in the data set, with a mean distance of 247.43 m. Self-pollinated weeds from families such as Linaceae, Brassicaceae, and Chenopodiaceae also showed gene flow to considerable distances- 35 m, 20 m, and 15 m, respectively, although the extent of gene flow was low. However, it is highly likely that gene flow over these distances can contaminate an entire field and the nearby fields within a few years.

PMGF Across Species. Among the individual weeds, common groundsel, in the Asteraceae family, showed the highest extent of gene flow (average 23.2%) (Table 2), exhibiting 24% gene flow at 0.5 m from the pollen donor in England (Marshal and Abbott Reference Marshall and Abbott1982). In Solanaceae, eastern black nightshade showed 17% gene flow (average 10%) near the pollen source in Manitoba, Canada (Hemanutz Reference Hermanutz1991). In Arkansas in the United States, magenta spreen, a member of the Chenopodiaceae family, exhibited a maximum gene flow of 3.0% at 15 m from the donor plant (Yerka et al. Reference Yerka, de Leon and Stoltenberg2012). In Poaceae, wild oat showed 1.1% average gene flow but can show as high as 5.2% gene flow at 0 m and 0.09% at 0.6 m from the pollen donor in Saskatchewan, Canada (Murray et al. Reference Murray, Morrison and Friesen2002) (Figure 2). For wild oat and slender wild oat (Avena barbata Pott ex Link AVEBA), the rate of outcrossing has been estimated to be 1% to 12% (Grant Reference Grant1981). Jain (Reference Jain, Solbrig, Jain, Johnson and Raven1979) documented a greater outcrossing rate in wild oat than in slender wild oat and consequently larger inter-family and intra-family variance in the first species. In the Brassicaceae family, false flax [Camelina sativa (L.) Crantz] showed 0.78% gene flow at 0.2 m and 0.33% at 2.5 m in Montana (Walsh et al. Reference Walsh, Hills, Martin and Hall2015) (Table 1). Three important weed species in the Poaceae family, goosegrass [Eleusine indica (L.) Gaertn.], large crabgrass [Digitaria sanguinalis (L.) Scop.], and yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.], are reported to undergo low levels of cross-pollination; however, no research studies have yet documented the percent of PMGF (Table 2).

Table 1.

Extent of pollen mediated gene flow and distance in self-pollinated weeds at selected family level.

^a Number of species included within a family.

Figure 2.

The relation between the extent of gene flow and distance from the source in 1. Wild oat (Murray et al. Reference Murray, Morrison and Friesen2002) and 2. Foxtail millet (Wang et al. Reference Wang, Zhao, Yan, Li, Zhu, Shi, Song, Ma and Darmency2001; Wang et al. Reference Wang, Chen and Darmency1997).

Table 2.

Extent of pollen-mediated gene flow and distance in self-pollinated weeds at selected species level.^a

^a N, numberAbbreviations: N, number; SE, standard error.

^b Bisht and Mukai 2002.

^c Jones et al. 2021.

^d Kimata 2015.

Regarding pollen dispersal distance, plants in the Asteraceae family exhibit the farthest movement, with gene flow recorded up to 1,000 m in horseweed in Illinois (Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015). Plants in the Chenopodiaceae and Poaceae families have demonstrated moderate dispersal potential with maximum gene flow recorded at 96 m and 50 m, respectively (Bagavathiannan and Norsworthy Reference Bagavathiannan and Norsworthy2014; Gasquez Reference Gasquez, Jacquard, Heim and Antonovics1985; Hidayat et al. Reference Hidayat, Baker and Preston2006; Murray et al. Reference Murray, Morrison and Friesen2002; Volenberg and Stoltenberg Reference Volenberg and Stoltenberg2002; Wang et al. Reference Wang, Zhao, Yan, Li, Zhu, Shi, Song, Ma and Darmency2001; Yerka et al. Reference Yerka, de Leon and Stoltenberg2012). Although Brassicaceae plants exhibit a low extent of gene flow, some species, such as false flax, were still able to cause gene flow up to 20 m from the donor plant (Walsh et al. Reference Walsh, Hills, Martin and Hall2015). Wild oat showed a maximum gene flow distance of 0.3 m, but it was concentrated primarily adjacent to the pollen donor (5.5%), whereas foxtail millet [Setaria italica (L.) P. Beauv.] demonstrated gene flow of up to 20 m, with a 1.5% gene flow adjacent to the pollen donor (Figure 2).

Factors That Influence Gene Flow

In general, the extent of PMGF among plant populations is influenced by a combination of biological, environmental, and management-related factors. These include the mating system and reproductive biology of the species, floral structure, population size and spatial arrangement, pollination mechanisms (e.g., wind or insect-mediated), and the foraging behavior of pollinators (Beckie and Hall Reference Beckie and Hall2008; Handel Reference Handel1983; Lal et al. Reference Lal, Bhardwaj, Chahar, Dangwal, Das, Tandon, Shivanna and Koul2020; Murray et al. Reference Murray, Morrison and Friesen2002). Additionally, variables such as temperature, relative humidity, elevation, geographic location, and agricultural practices can significantly affect gene flow dynamics (Hermanutz Reference Hermanutz1991; Hulme Reference Hulme2023; Maity and Bagavathiannan Reference Maity and Bagavathiannan2021; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster and Barrett2012; Prasad et al. Reference Prasad, Boote and Allen2006; Rong et al. Reference Rong, Song, De Jong, Zhang, Sun, Xu and Lu2010; Wilkins and Thorogood Reference Wilkins and Thorogood1992).

Environmental Factors. The heat map of correlations between the extent and distance of PMGF and the environmental factors in our data set showed that except for temperature, other environmental factors exhibited weak correlations with PMGF and distances (Figure 3). Generally, none of the environmental factors alone was able to predict PMGF distance in this data set. The evaluation of the regression equation and R ² between temperature and PMGF showed that there is a strong negative relationship between temperature and PMGF extent, meaning that as temperature increases, gene flow tends to decrease (P < 0.05) (Figure 4). Qu et al. (Reference Qu, Liu, Zhuang and Qiang2022) in their survey on American sloughgrass [Beckmannia syzigachne (Steud.) Fernald] in two consecutive years (2017 and 2018) confirmed that the amount of PMGF in a herbicide resistance trait was significantly negatively correlated with temperature (correlation coefficient −0.340, P = 0.010) but it was not correlated with relative humidity. Wilkins and Thorogood (Reference Wilkins and Thorogood1992) reported that elevated temperatures (34 C) during anthesis increased self-fertilization in perennial ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] from 2% to 30%, therefore reduced outcrossing. Although self-pollinated species generally exhibit low levels of the PMGF, environmental factors can occasionally enhance outcrossing. For example, in false flax, gene flow declined sharply with increasing distance, but local wind movement allowed pollen dispersal beyond 10 m (Walsh et al. Reference Walsh, Hills, Martin and Hall2015). In common groundsel, higher plant density likely contributed to slight increase in cross-pollination among neighboring plants, while environmental factors such as seasonal conditions might also influence gene flow dynamics (Marshall and Abbott Reference Marshall and Abbott1982).

Figure 3.

Heat map on correlations of extent of gene flow and its distances with environmental factors in self-pollinated weed species selected in this analysis.

Figure 4.

Regression between the extent of gene flow across the selected weeds and local temperature.

Bagavathiannan and Norsworthy (Reference Bagavathiannan and Norsworthy2014) investigated PMGF in barnyardgrass and found that wind speed played a key role in determining the maximum distance of gene flow. In their 2010 study, PMGF was observed at up to 20 m in the northwest direction, aligning with the strongest winds recorded at 15 to 20 km/h. However, in the following year of the study (2011), the highest wind speeds shifted to the northeast at 20 to 25 km/h, and PMGF extended as far as 50 m in that direction. Despite variations in PMGF distance based on wind patterns, the overall outcrossing rates across different directions did not show significant differences (Bagavathiannan and Norsworthy Reference Bagavathiannan and Norsworthy2014). They also noted that PMGF levels were significantly and negatively correlated with air temperature; higher temperatures can reduce PMGF. Conversely, no significant relationship was found between PMGF and relative humidity. Huang et al. (Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015) highlighted the role of wind in influencing pollen dispersal distance, demonstrating that greater wind speeds led to increased pollen travel by horseweed. They also suggested that high relative humidity, in addition to the physiology of the species, could inhibit pollen movement (Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015). Similarly, Aboulaich et al. (Reference Aboulaich, Achmakh, Bouziane, Trigo, Recio, Kadiri and Kazzaz2013) explained that pollen can absorb moisture from the air, making it heavier and more likely to settle.

Reproductive Biology. Pollen production per flower can significantly influence the extent of PMGF within a plant population (Table 3). More pollen production increases the probability of pollen dispersal, thereby can enhance cross-pollination rates and facilitate PMGF between individual plants, which is crucial for maintaining genetic diversity during plant evolution (Campbell Reference Campbell1991). Conversely, fewer pollen production may limit PMGF, particularly in spatially isolated populations or those with low pollinator activity (Kwak et al. Reference Kwak, Velterop and van Andel1998). The relationship between pollen quantity and PMGF is influenced by various factors, including floral morphology, pollinator behavior, and environmental conditions (Brunet and Eckert Reference Brunet1998). Common groundsel, the weed that exhibits the highest extent of PMGF, can produce as many as 55 to 65 florets, each containing 350 to 400 pollens (Table 3) (Damgaard and Abbott Reference Damgaard and Abbott1995). Horseweed, a troublesome weed with rapidly spreading herbicide resistance, can produce as many as 1,170 to 2.1 × 10⁶ pollens per plant per day (Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015), which can suffice its faster rate of herbicide resistance spread.

Table 3.

Pollen characteristics in selected self-pollinated weed species.^a

^a A dash (–) indicates that no information was found.

Pollen size varies substantially among weed species, which may influence PMGF capacity (Table 3). Species such as velvetleaf (Abutilon theophrasti Medikus) and flax (Linum usitatissimum L.) exhibit relatively large pollen grains (51–100 µm; PalDat 2024), whereas intermediate-size grains are produced by common lambsquarters (Chenopodium album L.), mouse barley (Hordeum murinum L.), and common groundsel (26–50 µm; PalDat 2024). While false flax produces smaller pollen (approximately 21–25 µm and 10–25 µm, respectively; PalDat 2024), the pollen grains of horseweed are as small as 16 to 22 µm (Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015). Smaller pollen grains generally tend to travel farther, potentially enhancing PMGF distance. Notably, horseweed, with very small pollen grains, exhibited the greatest PMGF distance (1,000 m) in our data set. Estimated maximum PMGF of 17% observed in eastern black nightshade can be attributed to smaller (15–25 µm) pollen size. Although these factors suggest a possible relationship between smaller pollen size and greater dispersal ability, other factors such as pollination vector, pollen viability, and environmental conditions likely influence PMGF patterns.

Vector and their Behavior. Gene flow patterns are also shaped by the behavior of the vectors that facilitate the transfer of seeds and pollen both within and between populations (Nathan and Muller-Landau Reference Nathan and Muller-Landau2000; Taylor et al. Reference Taylor, Touré, Carnahan, Norris, Dolo, Traoré and Lanzaro2001; Zhang et al. Reference Zhang, Yook, Park, Lim, Kim, Song and Kim2018). Common gene flow vectors include animals, insects, and human activities (Hoyle et al. Reference Hoyle, Hayter and Cresswell2007; Pickering et al. Reference Pickering, Mount, Wichmann and Bullock2011). Animals, for instance, may disperse seeds over long distances by consuming fruits and defecating the seeds, or by carrying seeds attached to their fur, or via insects, particularly pollinators such as bees, can transfer pollen between plants, enabling cross-pollination and genetic variation (Zhang et al. Reference Zhang, Wang, Gao, Chen, Kim, Zhang and Yan2021b). Zhang et al. (Reference Zhang, Wang, Gao, Chen, Kim, Zhang and Yan2021b) conducted a 4-yr study on false flax and found that seed yield increase at higher honeybee densities (>20 bees per square meter) was primarily attributed to increased outcrossing. Their research also demonstrated that both honeybees and bumble bees could transfer false flax pollen to nearby flowers of other species such as brown mustard [Brassica juncea (L.) Czernajew] and rapeseed (B. napus L.), with pollen movement observed up to 10 to 15 m beyond the field edge. Flax exhibited minor increase in outcrossing when insect vectors such as bees were present Zhang et al. (Reference Zhang, Wang, Gao, Chen, Kim, Zhang and Yan2021b). In wild oat, gene flow was restricted to short distances, with seed dispersal and recruitment occurring primarily within a few meters under field conditions (Murray et al. Reference Murray, Morrison and Friesen2002). Furthermore, although eastern black nightshade is primarily self-pollinated, studies have shown that insect movement between closely spaced individuals can enhance occasional gene flow (Hermanutz Reference Hermanutz1991).

Human activities also contribute significantly to gene flow in weeds, as seeds and pollen can be unintentionally moved through clothing, footwear, and agricultural equipment (Pickering et al. Reference Pickering, Mount, Wichmann and Bullock2011). For example, seeds may adhere to shoes or be transported on machinery used in farming, leading to the spread of invasive species to new areas. A study by Pickering et al. (Reference Pickering, Mount, Wichmann and Bullock2011) demonstrated that humans can act as an important vector, and their clothing plays a significant role in seed dispersal, particularly in protected areas. Five weed species (biddy-biddy [Acaena novae-zelandiae Kirk], common sheep sorrel [Rumex acetosella L.[, sweet vernal grass [Anthoxanthum odoratum L.], orchardgrass [Dactylis glomerata L.], and red fescue [Festuca rubra L.]) were tested for their attachment and detachment rates on socks and trousers. Results indicated that socks retained seeds for longer distances, with some seeds still attached after 5,000 m. Theoretically, up to 2.4 million seeds could be dispersed via trousers and 1.9 million via socks in a single season, contributing to the spread of invasive species into conservation areas. Measuring long-distance dispersal presents significant challenges because the likelihood of detection decreases sharply with increasing distance from the source population (Thompson et al. Reference Thompson, Rickard, Hodkinson, Rees, Bullock, Kenward and Hails2002).

Recent advances in agricultural technology can influence gene flow patterns in self-pollinated weeds. The integration of unmanned aerial vehicles (UAVs), commonly known as drones, into agricultural practices introduces novel considerations regarding pollen dispersal in self-pollinating weed species. While drones are primarily employed for tasks such as crop monitoring and pesticide application, their rotor-induced airflow can generate significant turbulence capable of dislodging and dispersing pollen grains. This mechanical disturbance may inadvertently enhance outcrossing rates in weeds with inherently low outcrossing potential by facilitating pollen movement beyond typical self-pollination ranges. For instance, studies have demonstrated that the downwash airflow from drones that spray pesticides can influence the distribution of airborne particles, including spores, indicating the potential for increased gene flow among nearby plant populations (Qin et al. Reference Qin, Chen and He2023). Additionally, research on drone-based pollination systems has highlighted the capacity of UAVs to mimic natural pollination mechanisms, further supporting the notion that drone activity could intensify PMGF (Crazzolara et al. Reference Crazzolara, Ebner, Platis, Miranda, Bange and Junginger2019). These examples highlight that even in predominantly selfing weeds, factors such as wind, insect vectors, plant density, and seasonal environmental conditions, together with anthropogenic activities and recent farming advancements, can facilitate gene movement under favorable circumstances.

SMGF Across Species. The available evidence on SMGF in self-pollinated weed species highlights the various dispersal mechanisms involved (Table 4). Although direct data on PMGF or vegetative gene flow are limited for most self-pollinated weed species, Table 4 presents several documented cases of seed dispersal. For example, Avena sativa can disperse 14% to 18% of the seeds in places even at elevations of 1,600 to 2,200 m (Del Toro and Ribbons Reference Del Toro and Ribbons2019). A. fatua seeds can disperse 145 m, whereas cheatgrass seeds can travel 237 m at a speed of 37 cm s⁻¹. Horseweed can disperse 99% of its seed to 110 m (Dauer et al. Reference Dauer, Mortensen and Humston2006), and the speed of its seed dispersal can be as high as 444 cm s⁻¹ to travel 21 m. These findings underscore the significance of SMGF in the spread of herbicide resistance in self-pollinated weed species, particularly through both natural and agricultural dispersal mechanisms.

Table 4.

Documented cases of seed-mediated gene flow in self-pollinated weed species.

* Non-pollinating insects comprised of ants.

** Farm machinery included combine harvester.

Gene Flow Scenario in Selected Self-Pollinated Weed Species

Common Groundsel. Common groundsel is a small, annual broadleaf weed belonging to the Asteraceae family, widely distributed in nurseries, vegetable crops, and no-till systems, where it competes with crops and serves as a host for plant pathogens such as Botrytis cinerea (Dauer et al. Reference Dauer, Mortensen and Vangessel2007; Weaver and Warwick Reference Weaver and Warwick1984). It primarily reproduces through seeds and is predominantly self-pollinated, although occasional outcrossing has been reported (Abbott et al. Reference Abbott, James, Milne and Gillies2003). Common groundsel was the first documented case of herbicide resistance, with triazine-resistant populations now prevalent throughout Europe and North America (Holt and Lebaron Reference Holt and Lebaron1990).

In our analysis of the data set, common groundsel exhibited a mean of 23.2% gene flow (Campbell and Abbott Reference Campbell and Abbott1976; Marshal and Abbott Reference Marshall and Abbott1982; Müller-Schärer and Fischer Reference Müller-Schärer and Fischer2001). Marshal and Abbott (Reference Marshall and Abbott1982) reported a maximum gene flow of 24% between neighboring individuals of common groundsel within a natural population, where plants were located close together at very short distances, facilitating a high rate of gene flow. However, Müller-Schärer and Fischer (Reference Müller-Schärer and Fischer2001) found that cross-pollination in common groundsel can occur at rates up to 0.4% at short distances within 1 m, with gene flow decreasing significantly beyond this range. There are no reports of long-distance pollen dispersal for this species. Literature reviews indicate that gene flow in common groundsel generally occurs only among neighboring individuals within natural populations (Campbell and Abbott Reference Campbell and Abbott1976; Marshal and Abbott Reference Marshall and Abbott1982; Müller-Schärer and Fischer Reference Müller-Schärer and Fischer2001). Despite its limited outcrossing potential, seed dispersal plays a major role in the spread of herbicide resistance and adaptive traits in common groundsel, highlighting the need for effective management strategies. Marshal and Abbot (Reference Marshall and Abbott1982, Reference Marshall and Abbott1984a, Reference Marshall and Abbott1984b) noted that outcrossing level in common groundsel is influenced by weed physiology. Radiate plants, which have ray florets, showed significantly higher outcrossing rates (13% to 20%) during peak flowering, whereas non-radiate plants had rates below 1%.

SMGF in common groundsel can occur through multiple pathways (McHenry et al. Reference McHenry, Bushnell, Oliver and Norris1990). Each seed bears a pappus—a plume-like structure—that facilitates wind dispersal along ditches, fencerows, roadsides, and adjacent fields. Additional dispersal routes include irrigation water, contaminated crop seed lots, and farm machinery or vehicles. McHenry et al. also reported that manure from livestock fed hay containing Senecio vulgaris seeds contributed to their spread into cropland.

Eastern Black Nightshade. Eastern black nightshade is an annual weed from the Solanaceae family and native to North America, which infests most crops and causes significant challenges due to its competitive nature and toxic properties (Ogg et al. Reference Ogg, Rogers and Schilling1981). There is evidence of paraquat resistance in eastern black nightshade populations confirmed in Ontario, Canada. It shows considerable cross-pollination, primarily due to insect pollinators such as bees. In the literature we reviewed, eastern black nightshade exhibited 10% outcrossing (Hermanutz Reference Hermanutz1991). Outcrossing rates in eastern black nightshade varied between populations and were linked to floral traits such as stigma position. Hermanutz (Reference Hermanutz1991) demonstrated that the degree of stigma exertion and style position are different between populations and are correlated with outcrossing rate. Self-pollination was more likely due to close contact between the stigma and the anther (Hermanutz Reference Hermanutz1991). This ability to shift between selfing and outcrossing may have helped the species spread successfully.

Eastern black nightshade primarily relies on endozoochorous seed dispersal via frugivores: birds and mammals ingest its small berries, pass viable seeds in their droppings, and thus move them away from the parent plant rather than wind or ballistic dispersal (MSU 2023). It can produce approximately 10,000 seeds/plant (each plant produces between 50 and 100 berries, with each berry containing about 100 seeds).

Barnyardgrass. Barnyardgrass is an annual C4 grass weed that causes significant challenges in agricultural systems, particularly in the midsouthern, southern, and eastern regions, where it competes with crops such as rice (Oryza sativa L.), soybean, and cotton, leading to significant yield losses due to its aggressive growth and adaptability (Holm et al. Reference Holm, Doll, Holm, Pancho and Herberger1997; Jhala et al. Reference Jhala, Beckie, Mallory-Smith, Jasieniuk, Busi, Norsworthy and Geddes2021a; Norsworthy et al. Reference Norsworthy, Bond and Scott2013). It reproduces primarily through seeds and is predominantly self-pollinated, although some degree of outcrossing has been observed (Maun and Barrett Reference Maun and Barrett1986). Bagavathiannan and Norsworthy (Reference Bagavathiannan and Norsworthy2014) reported gene flow rates of 12% at 0 m, decreasing to 5.6% at 0.25 m and 0.01% at 50 m from the pollen donor. They also reported that the average gene flow in barnyardgrass was highly variable among directions and years (Bagavathiannan and Norsworthy Reference Bagavathiannan and Norsworthy2014; Jhala et al. Reference Jhala, Beckie, Mallory-Smith, Jasieniuk, Busi, Norsworthy and Geddes2021a).

Gravity and water movement are the main channels of short-distance seed dispersal in barnyardgrass (Zhang et al. Reference Zhang, Wang, Cao, Li and Chauhan2023). In dense local infestations, mature seeds frequently break and fall close to the parent plant (Peralta et al. Reference Peralta, Striker and Mollard2019). The seeds can also be carried by floods, runoff, and irrigation water because of their buoyancy and small weight, which helps them colonize wet and marshy areas (Bastiani et al. Reference Bastiani, Lamego, Nunes, Moura, Wickert and Oliveira2015). Across agroecosystems, barnyardgrass can persist and spread quickly due to these natural and secondary dispersal mechanisms.

Horseweed. Horseweed, also known as marestail, is an annual broadleaf weed belonging to the Asteraceae family. It is notorious for its ability to rapidly colonize disturbed areas and agricultural fields of soybean and cotton, contributing to herbicide resistance challenges (VanGessel Reference VanGessel2001; Weaver Reference Weaver2001). It primarily reproduces by producing large quantities of wind-dispersed seeds (Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015), and while it is considered predominantly self-pollinating, some level of outcrossing has been reported (Buhler and Owen Reference Buhler and Owen1997; Jhala et al. Reference Jhala, Norsworthy, Ganie, Sosnoskie, Beckie, Mallory-Smith and Stoltenberg2021b). Gene flow studies indicate that PMGF in horseweed can occur over significant distances (Jhala et al. Reference Jhala, Norsworthy, Ganie, Sosnoskie, Beckie, Mallory-Smith and Stoltenberg2021b). Davis et al. (Reference Davis, Kruger, Hallett, Tranel and Johnson2010) reported a maximum observed gene flow distance of 300 m, although most outcrossing events occurred within 100 m. In another study, horseweed showed 7.8% gene flow in the areas adjacent to pollen donors (Loux et al. Reference Loux, Stachler, Johnson, Nice, Davis and Nordby2006), whereas Huang et al. (Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015) reported that in Illinois, horseweed pollen could flow up to 1,000 m. The combination of prolific seed production, long-distance dispersal, and herbicide resistance makes persistent horseweed difficult to manage in agricultural systems.

Horseweed pollen release appears to be influenced more by the plant’s physiology than meteorological conditions (Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015; Smisek Reference Smisek1995). Correlation analysis revealed that at lower heights (<3 m) near the source field, vertical pollen transport was linked to vertical wind speed, while horizontal movement depended on horizontal wind speed. Additionally, high relative humidity inhibited pollen dispersal at greater heights (3 to 100 m) and longer distances (0 to 1,000 m) (Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015).

SMGF in horseweed primarily occurs via wind. A single mature plant can produce 100,000 to 200,000 seeds, each equipped with a pappus that enables aerial transport (Shields et al. Reference Shields, Dauer, VanGessel and Neumann2006). With wind speeds in the Planetary Boundary Layer frequently exceeding 20 m s⁻¹, seed dispersal can easily exceed 500 km in a single dispersal event. Under moderate winds (5–7 m s⁻¹), one modeling study estimated a longest-distance dispersal of ∼165 km, with ∼27% of seeds moving beyond 5 km in 1 d (Liu et al. Reference Liu, Qi and Wang2018).

Evolutionary Implications

From the perspective of evolutionary implications, when an exotic species is introduced to a new environment, it experiences a genetic bottleneck that decreases genetic diversity (Amsellem et al. Reference Amsellem, Noyer, Le Bourgeois and Hossaert-Mckey2000). When self-incompatible species are introduced to a new environment, this bottleneck can lead to a reduction in allelic variation at the self-incompatibility locus, which in turn diminishes the chances of finding cross-compatible mates in the new habitat (Caño et al. Reference Caño, Escarré, Blanco-Moreno and Sans2008; Reinartz and Les Reference Reinartz and Les1994). Contrastingly, as confirmed by Shivanna (Reference Shivanna2015), autogamy is a key adaptation in annual weed species, providing reproductive assurance and enabling their successful colonization. While some species such as shepherd’s purse [Capsella bursa-pastoris (L.) Medikus], common chickweed [Stellaria media (L.) Villars], and mouse ear-cress [Arabidopsis thaliana (L.) Heynhold] retain the potential for outcrossing, their reliance on selfing ensures survival even in the absence of pollinators (Shivanna Reference Shivanna2015). Self-pollination is often associated with genetic isolation, but this assumption is not absolute. A low rate of outcrossing can occur due to various biological and environmental factors owing to the need to create variation for better persistence and stability under dynamic niche (Goodwillie et al. Reference Goodwillie, Kalisz and Eckert2005). Almost all the self-pollinating plants exhibit both cleistogamous (closed-flower, obligate selfing) and chasmogamous (open-flower, potential outcrossing) mechanisms. Cleistogamy is often considered a strategy for reproductive assurance under unfavorable conditions, while chasmogamy allows occasional genetic exchange by outcrossing (Goodwillie et al. Reference Goodwillie, Kalisz and Eckert2005; Oakley et al. Reference Oakley, Moriuchi and Winn2007). The mechanisms of cleistogamy and chasmogamy contribute to outcrossing in an autogamous species by maintaining a mixed mating system that allows genetic exchange despite a predominant selfing strategy. This dual strategy enhances genetic diversity and helps mitigate the negative effects of inbreeding depression in weeds, which can accumulate in strictly selfing populations.

Environmental factors such as temperature, humidity, and CO₂ levels or resource allocation can modify floral traits and increase the likelihood of outcrossing. In shepherd’s purse, the proportion of cleistogamous vs. chasmogamous flowers varies depending on environmental factors that can affect outcrossing rates (Lloyd Reference Lloyd1980). For instance, elevated CO₂ has been shown to increase the proportion of chasmogamous flowers in shepherd’s purse (Jurik Reference Jurik1985). Higher temperatures can shorten floral development times, leading to incomplete self-pollination and increased reliance on external pollen (Franks and Weis Reference Franks and Weis2009). Drought stress has been reported to increase flower opening rates, promoting pollen dispersal in Arabidopsis (Heschel and Riginos Reference Heschel and Riginos2005).

Self-pollination in mixed mating systems can take place within a single flower (autogamy) or between different flowers on the same plant (geitonogamy). Additionally, selfing within a flower may happen naturally without assistance (autonomous) or with the help of external pollinators (facilitated) (Goodwillie et al. Reference Goodwillie, Kalisz and Eckert2005; Lloyd and Schoen Reference Lloyd and Schoen1992). This extent of outcrossing can facilitate gene flow, a critical concern in the context of resistant gene escape in weeds. Therefore, pollinator availability can also influence the proportion of cleistogamous and chasmogamous flowers, thereby can contribute to gene flow. For example, rice is primarily a selfing species; however, wind and insect abundance can increase outcrossing rates up to 3% in hybrid systems (Song et al., Reference Song, Lu, Zhu and Chen2003). Similarly, in soybean, bees have been shown to enhance outcrossing rates from <1% to 5% under certain conditions (Abrol Reference Abrol2012). Thus, cleistogamy and chasmogamy together create an evolutionary balance that maintains outcrossing potential in primarily autogamous species, including weeds, contributing to spread of aggressive biotypes and herbicide resistance (Oakley et al. Reference Oakley, Moriuchi and Winn2007).

Practical Implications of PMGF in Self-Pollinated Weeds

While natural cross-pollination rates are typically lower in self-pollinated weeds compared to cross-pollinated species, even this small amount of gene flow could have significant implications for the spread of herbicide resistance alleles (Jhala et al. Reference Jhala, Beckie, Mallory-Smith, Jasieniuk, Busi, Norsworthy and Geddes2021a, Reference Jhala, Norsworthy, Ganie, Sosnoskie, Beckie, Mallory-Smith and Stoltenberg2021b). Numerous studies have examined the life-history traits that can influence the invasive behavior of specific weed species (Noble Reference Noble, Drake, Mooney, di Castri, Groves, Kruger, Rejmanek and Williamson1989). One key attribute believed to affect the invasiveness of plants is self-compatibility, which enables reproduction with just a few individuals or even a single individual in a newly formed population (Baker Reference Baker, Baker and Stebbins1965; Caño et al. Reference Caño, Escarré, Blanco-Moreno and Sans2008). Given the increasing cases of the evolution of herbicide resistance and spread across the weed species, aggravated outcrossing in self-pollinated weeds may intensify the threat of herbicide resistance management. Understanding the extent and distance of gene flow in self-pollinated weed species is crucial for understanding how genetic material spreads within and among populations. This information helps assess factors such as genetic diversity, adaptation to changing environments, and the potential for herbicide resistance evolution and spread. It also helps inform decisions on weed management and conservation strategies. Moreover, they can help forecast the adaptive evolution of weeds and are a critical consideration for improving agricultural practices.

Environmental conditions play a major role in pollen movement, even in self-pollinated weeds. Factors such as wind, temperature, humidity, rainfall, and field structure influence pollen viability and its travel distance (Ganie and Jhala Reference Ganie and Jhala2017; Hulme Reference Hulme2023). Turbulent winds can carry pollen far away beyond its self-pollination ranges, while high humidity or rainfall may shorten pollen viability. Dry weather periods can also extend pollen flight distance from pollen source. Therefore, climate change is likely to amplify these effects via rising temperatures, changing wind patterns, and unpredictable weather, which can disrupt the flowering cycle and increase outcrossing chance even in self-pollinated plants (Aboulaich et al. Reference Aboulaich, Achmakh, Bouziane, Trigo, Recio, Kadiri and Kazzaz2013; Bagavathiannan and Norsworthy Reference Bagavathiannan and Norsworthy2014; Huang et al. Reference Huang, Ye, Qi, Li, Miller, Stewart and Wang2015).

In summary, gene flow in self-pollinated species can affect genetic diversity, adaptation, inbreeding, herbicide resistance, conservation efforts, population structure, evolutionary dynamics, and biodiversity management. While long-distance PMGF in most of the self-pollinated weeds is unlikely, herbicide resistance can still spread within and between neighboring fields, leading to the same scenarios as in outcrossing species over time. Even at low frequencies, gene flow can introduce resistance alleles into unselected populations, increasing the likelihood of resistance evolution over time. The movement of resistance traits through gene flow can accelerate the establishment of resistant populations, underscoring the need for proactive management.

Based on our findings, growers can reduce gene flow in weeds –selfing or outcrossing – by combining collaborative, cultural, and timely management strategies. Collaborative and awareness strategies among neighboring producers can help reduce the movement of resistance alleles across farm boundaries (Bagavathiannan and Norsworthy Reference Bagavathiannan and Norsworthy2014; Kuroda et al. Reference Kuroda, Sato, Bounphanousay, Kono and Tanaka2005). At the field level, eliminating weeds before they flower, maintaining isolation zones or tall border rows, and cleaning farm machinery can reduce both pollen and seed dispersal (Hulme Reference Hulme2023; Maity et al. Reference Maity, Young, Subramanian and Bagavathiannan2022; Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster and Barrett2012). Providing spatial and temporal isolation between resistant and susceptible populations can also help minimize pollen movement (Ganie and Jhala Reference Ganie and Jhala2017). Integrating different practices such as rotational cropping, reduced tillage, rotating herbicide modes of action, and implementing long-term seedbank depletion practices can disrupt weed life cycles and reduce the spread of resistance genes (Maity et al. Reference Maity, Young, Subramanian and Bagavathiannan2022; Vencill et al. Reference Vencill, Nichols, Webster, Soteres, Mallory-Smith, Burgos and McClelland2012). In high-risk areas, including field edges and irrigation ditches, targeted control of escapes before flowering is especially important, because irrigation may enhance gene movement by dispersing seeds and creating refugees for resistant weeds (Hulme Reference Hulme2023). Generally, these findings highlight that reducing gene flow requires an integrated weed management approach with diverse strategies and strong grower cooperation (Gani et al. Reference Ganie, Sandell, Jugulam, Kruger, Marx and Jhala2016; Ganie and Jhala Reference Ganie and Jhala2017). In this regard, gene flow management in self-pollinating weeds needs consolidated efforts similar to that in cross-pollinating weeds.