This study compares the survival and seed production of wild and crop-wild hybrid sunflower with and without wheat competition. Nine wild sunflower populations were crossed with conventional and herbicide-resistant sunflower crop lines to create a diversity of crop-wild hybrids. Wild sunflowers consistently produced more seed than hybrids, but had lower probability of survival; the wheat treatment decreased the survival and seed production of all of the sunflowers. The crop-wild hybrids produced by three different crop parents differed in seed production.
In recent years, concern over genetic contamination due to pollen flow from genetically engineered crops has grown. Whether it is canola in Canada, or maize in Mexico, outcrossing crops have the potential to hybridize with neighboring crops. Additionally, concern that the movement of crop genes into related native plants and weed species might affect their invasive or weedy characteristics has also increased.
Our research focused on two factors that could influence the ability of a crop-wild hybrid to be a good weed: the environmental conditions and its genetic background. Much of the research performed on crop-wild hybrids has focused on assessments of seed production in a non-competitive environment. Since farmers may be concerned about how the wild and crop-wild hybrids compare within environments that are relevant to agricultural conditions, we compared them with and without competition from a dense wheat stand. In addition, the genetic variability we know exists across a wild species’ range had not been assessed for its influence on the characteristics of the hybrids. We chose to compare survival to reproduction, seed production and relative fitness of hybrids and wilds across nine wild populations and three crop lines.
Our objectives for this study were (1) to quantify the competitive abilities of sunflower weeds and crop-wild hybrids under competition with wheat and (2) to investigate possible fitness differences between herbicide resistant and susceptible crop-wild hybrids. This research provides empirical data that helps us to better understand both the relative competitive ability of these particular hybrids, and how quickly crop genes can spread through wild populations.
Worldwide, cross-pollinating crops hybridize with related species (reviewed in Snow and Palma 1997; Ellstrand et al. 1999) and researchers have suggested that many of the traits in crop plants are likely to ‘introgress’, or move into natural plant populations increasing competitive ability and weediness, potentially endangering both agricultural and natural ecosystems (Kareiva et al. 1996; Snow and Palma 1997; Hails 2000). For example, crop-weed gene flow from sorghum to weedy johnson grass has been implicated in the increased competitive ability of this common weed (Arriola and Ellstrand 1996). Similarly, recent studies in rice fields show evidence of movement of crop genes from cultivated herbicide resistant rice to weedy red rice (Gealy et al. 2002). Ecologically important traits bred into crops, such as resistance to herbicide or insects have the potential to influence the weediness of a species.
When considering the effects of crop gene introgression, the novel trait itself, the genetics of the wild population, and the environment, including all relevant biotic and abiotic selection pressures, must be considered. The risks associated with crop gene introgression have been investigated in the context of resistance to viruses, insects and herbicides. Fuchs and Gonsalves (1997) studied the effect of gene flow in virus-resistant transgenic squash, where virus resistant crop-wild hybrids had higher fitness than susceptible plants under high disease conditions. Snow et al. studied the effects of the Bt gene (cry1A) for Lepidopteran insect-resistance in sunflower hybrids. Results show increased lifetime fitness of the hybrids under field conditions due to a decrease in damage by both head-feeding and stem-boring Lepidopteran (Snow et al. 2003). Finally, studies of the potential fitness cost of genes conferring herbicide resistance show mixed results. No fitness cost of the transgene for glufosinate resistance was detected in B. napus x B. rapa hybrids (Snow et al. 1999), while Purrington and Bergelson (1997) show a 26% fitness cost to ALS-resistance in Arabidopsis. Clearly, herbicide-resistance would be expected to increase fitness of hybrids when the herbicide is applied; however, when no herbicide is used, resistance genes may or may not reduce fitness.
Common sunflower (Helianthus annuus) can be found along roadsides and in agricultural fields, including as a weed in cultivated sunflower (also H. annuus). Helianthus species are outcrossing and insect pollinated, and gene flow between cultivated and wild H. annuus is known to occur readily. Arias and Rieseberg (1994) showed that the amount of gene flow in one year from cultivated fields into neighboring weedy populations resulted in hybrids in 27% of weedy offspring at 3-m from the field. In a similar study by Whitton et al. (1997), molecular techniques were used to determine the persistence of crop genes in wild populations for five years after hybridization. They found 42% initial hybridization at close range, and that over the course of five generations without any more crop-weed gene flow, two crop genes were shown to be maintained at a moderate frequency.
Although it is now clear that hybridization does occur between wild and cultivated sunflower, our understanding of the fitness consequences of hybridization are not yet well understood. Snow et al. (1998) investigated the fecundity, phenology and seed dormancy of F1 hybrids using wild germplasm collections from Texas, Kansas and North Dakota, and two crop cultivars. Interestingly, they noted regional differences in relative hybrid performance in seed number and germinability. Hybrid seeds generally showed less dormancy, germinated more readily, flowered earlier, and produced fewer flower heads than weedy types. Sunflower hybrids may have fitness reduction, but the degree of reduction is regional. More work in this area is needed to clarify the effects that sunflower hybridization can have on weed competitive ability and population dynamics under various conditions.
We studied how gene flow from crop fields into wild populations influences survival, seed production and relative fitness across nine wild populations with and without competition to better understand the evolutionary consequences of crop-wild gene flow.
1. To quantify the competitive abilities of wild sunflower and crop-wild hybrids under competition with wheat.
2. To investigate possible fitness differences between herbicide resistant and herbicide susceptible crop-wild hybrids.
Nine wild populations of sunflower from 43-47 degrees North latitude were obtained from the UDSA collections. Three inbred crop lines, also from the USDA, were acquired: a conventional elite line, an imidazolinone resistant line, and a sulfonylurea resistant line.
A total of 36 types of crosses were performed at the University of Minnesota during the spring and summer of 2002 using the nine wild populations as the maternal parents. Crosses were made within the populations to produce wild seed, and between each population and the three crop lines to produce crop-wild hybrids. These crosses supplied the seed for the 2003 field experiment.
Split-plot and field layout
The field experiment was designed as a split-plot with seven replications. The main plots were comprised of four treatments – sulfonylurea application (field rate), sulfonylurea application (three times the field rate), wheat competition, and control. Only data from the wheat competition and control plots are presented here. Each treatment was assigned in a random order within each replication. Each treatment plot was then split into cross type subplots (split-plots). In the control and wheat treatments, we randomized subplots of nine wild types, 27 crop-wild types, and three crop lines, for a total of 39 cross type sub-plots. Each subplot was composed of four individuals representing that cross type. In total, there were 4,544 individual plants in the experiment.
Field preparation, seed preparation and planting
Using a mold board plow and a disk, five acres were prepared on the St. Paul Experiment Station of the University of Minnesota, May 2003. Treflan (trifluralin) was incorporated at a field rate for pre-emergence weed control. Fifty lbs/acre of nitrogen fertilizer was applied in the form of ammonium nitrate. On May 24, we planted the wheat treatment plots with the variety, Alsen, using an Almaco planter with ten six-inch rows. The wheat was planted to a 1.5” depth and at a rate of approximately 180 lbs/acre; we doubled the normal planting rate because wheat is sensitive to Treflan.
Seed from fifteen to twenty maternal families were bulked together from each cross type made up of a particular maternal and paternal parent combination. These seed were put through a cold treatment and germinated in a growth chamber. May 30 to June 3 we planted pre-germinated sunflower seeds with at least a 5-mm radicle into the subplots. A small percentage of the seeds had developed cotyledons. Seedlings were hand-watered until June 12 due to a hard soil crust.
Due to uneven development of the seedlings, we classified all the individuals in the experiment as coming from one of six ‘cohorts’. These cohorts and the damage done to cotyledons by flea beetles were used as factors in the analysis.
In this experiment, competitive ability was defined as the ability to survive to reproduction and produce more seeds under competitive conditions. The seed production of each plant was estimated by counting the total number of heads per plant and estimating the number of seeds per head from the diameter of three heads per plant. Plants that survived to reproduce were those that produced seeds; those that did not survive to reproduce died before setting seed, either mid-season or at frost. Fitnesses of hybrids and wilds from each population, with and without competition, were calculated as the product of seed production and survival to reproduction. Relative fitness was calculated as hybrid fitness divided by wild fitness for each population within each level of competition.
ANOVAs were performed on the survival to reproduction and seed production. The first model (for objective 1) assessed the effects of competition, wild population, and hybridization on fitness components. The second (for objective 2) assessed the effects of competition, wild population and specific crop parent on fitness components. Seed production data were analyzed transformed to their fourth root. Survival to reproduction was analyzed as a probability based on subplot characteristics and arcsine transformed. Back-transformed least squared means are presented here.
Both with and without wheat competition, the wild sunflowers consistently produced more seed than their crop-wild hybrid counterparts. Without competition, the wilds produced over five times the number seeds produced by the hybrids (wild 4,275; hybrid 801). Without competition, the wilds produced over six times the seeds of hybrids (wild 44,778 seeds; hybrid 7,208 seeds). Thus the wilds produce more seeds regardless of the environment. However, under competition, the hybrids are at less of a disadvantage than they are without competition. In addition, hybrid survival to reproduction was greater than wild survival to reproduction. This was especially true under competitive conditions where the survival to reproduction was 0.80 for the wilds and 0.87 for the hybrids.
We calculated relative fitness (hybrids/wilds) for all nine wild populations under both competitive and non-competitive environments. Overall, the relative fitness values range from 0.13 (Iowa population without competition) to 0.55 (Idaho population with competition). For seven of the nine populations, the competitive environment increased the relative fitness of hybrids. Hybrids from Idaho and Wyoming I had a large increase in relative fitness under competition. Wilds from Iowa and Washington were unexpectedly fit under competition, which lowered the relative fitness of their hybrids. Hybrids from Washington showed a substantial decrease in relative fitness under competition.
Therefore, wilds are consistently more fit than hybrids, regardless of the wild population or the environment. Thus, we would expect the wild seeds to dominate the weed seed banks. However, the seed production of hybrids is significant, indicating that crop genes will not be purged from the population immediately and should make a contribution. Due to the variability in relative fitness across populations, the contribution of the hybrids to each individual population will vary. And due to the higher relative fitness of hybrids under competition, the contribution made by the hybrids should be greater in competitive environments.
We compared one conventional and two herbicide resistant crop-wild hybrids to one another in the absence of herbicide. Seed production was highest for the imidazolinone resistant hybrid (3,540 seeds), second for the conventional hybrid (2,793 seeds) and lowest for the sulfonylurea resistant hybrid (2,554 seeds). Most of this variability came from differences among the hybrids from the Iowa and Idaho populations. Hybrids were more similar to one another in the other populations. In the absence of herbicide, herbicide resistant hybrids did not consistently produce more or fewer seeds than the conventional hybrid. They shifted rankings in every population. Thus, in most populations, crop parent did not significantly influence seed production. However, in some populations, the specific crop line used to produce the crop-wild hybrid did influence seed production.
This research has important implications for both farmers and for researchers. For farmers, information on the relative competitive ability of crop-weed hybrids increases their understanding of how gene flow may or may not affect their weed populations. For researchers, the data we obtained from our study can improve generalizable models concerning gene flow from both genetically engineered and conventionally bred crops by adding to our understanding of how a range of environments and a range of genetic material may influence estimates of crop gene introgression.
Arias, D. M. and L. H. Rieseberg (1994). Gene flow between cultivated and wild sunflower. Theor. Appl. Genet. 89: 655-660.
Arriola, P.E. and N.C. Ellstrand (1996). Crop-to-weed gene flow in the genus Sorghum (Poaceae): spontaneous interspecific hybridization between johnson grass, Sorghum halapense and crop sorghum, S. bicolor. Am. J. Bot. 83: 1153-1160.
Ellstrand, N. C., H. C. Prentice, et al. (1999). Gene flow and introgression from domesticated plants into their wild relatives. Annu. Rev. Ecol. Syst. 30: 539-563.
Fuchs, M. and D. Gonsalves (1997). Risk assessment of gene flow associated with the release of virus-resistant transgenic crop plants. Virus-Resistant Transgenic Plants: potential ecological impact. M. Tepfer and E. Balázs. Berlin, Springer-Verlag: 114-120.
Gealy, D. R., L. E. Estorninos, et al. (2002). Using microsatellite markers to confirm hybridization between rice and red rice. Weed Science Society of America Annual Meeting Abstracts, Reno, NV.
Hails, R. S. (2000). Genetically modified plants — the debate continues. TREE. 15: 14-18.
Kareiva, P., I. M. Parker, et al. (1996). Can we use experiments and models in predicting the invasiveness of genetically engineered organisms? Ecol. 77: 1670-1675.
Purrington, C. B. and J. Bergelson (1997). Fitness consequences of genetically engineered herbicide and antibiotic resistance in Arabidopsis thaliana. Genetics. 145: 807-814.
Snow, A. A., B. Andersen, et al. (1999). Costs of transgenic herbicide resistance introgressed from Brassica napus into weedy B. rapa. Mol. Ecol. 8: 605-615.
Snow, A. A. and P. Moran-Palma (1997). Commercialization of transgenic plants: potential ecological risks. Bioscience. 47: 86-96.
Snow, A. A., P. Moran-Palma, et al. (1998). Fecundity, phenology, and seed dormancy of F1 wild-crop hybrids in sunflower (Helianthus annuus, Asteraceae). Am. J. Bot. 85: 794-801.
Snow, A. A., D. Pilson, et al. (2003). A Bt transgene reduces herbivory and enhances fecundity in wild sunflower. Ecological Applications. 13(2): 279-286.
Whitton, J., D. E. Wolf, et al. (1997). The persistence of cultivar alleles in wild populations of sunflowers. Theor. Appl. Genet. 95: 33-40.
Educational & Outreach Activities
Mercer, K.L. 2005. Effects of crop-wild gene flow on evolution in annual sunflower. PhD Dissertation from the University of Minnesota, St. Paul, MN.
Mercer, K.L, R.G. Shaw, and D.L. Wyse. (In Preparation) Evolutionary implications of crop-wild gene flow in sunflower. Evolution.
Mercer, K.L, R.G. Shaw, and D.L. Wyse. (In Preparation) Fitness and related characteristics of wild and herbicide resistant and susceptible crop-wild sunflower hybrids under herbicide pressure. Weed Science.
Mercer, K.L, R.G. Shaw, and D.L. Wyse. (In Preparation) Variability in seed germination and dormancy in wild and crop-wild sunflower hybrids from nine populations. Journal of Applied Ecology.
Mercer, K.L., R.G. Shaw, and D.L. Wyse. 2004. Fitness of crop-wild sunflower hybrids from diverse genetic backgrounds. Oral presentation, Agronomy Society of America Meeting, Seattle, WA.
Mercer, K.L., R.G. Shaw, and D.L. Wyse. 2004. Genetic variation for seed germination and dormancy among crop-wild hybrids in sunflower. Oral presentation, Ecological Society of America Meeting, Portland, OR.
Mercer, K.L., R.G. Shaw, and D.L. Wyse. 2004. Gene flow from crop to wild sunflowers: Genetic variation for seed germination and herbicide resistance. Oral presentation, National Sunflower Association Research Forum, Fargo, ND.
Mercer, K.L., R.G. Shaw, and D.L. Wyse. 2003. Genetic Variation Among Crop-wild Hybrids in Sunflower (Helianthus annuus). Poster presentation, Invasive Plants in Natural and Managed Systems Conference, Ft. Lauderdale, FL.
Areas needing additional study
Research done by our lab (Mercer 2005) indicates that hybrids and wilds differ considerably in germination and dormancy (also see Snow et al. 1998). These results can be combined with our understanding of fitness differences gained through these experiments to better estimate crop gene introgression. In the future, I hope to develop a model using these relative fitness values and other parameters to predict crop gene introgression.
This research has served the useful purpose of producing considerable empirical data about the ways that competition and genetics influence differences among wild and crop-wild hybrid sunflower. This has helped us to generate hypotheses about the rate of spread of crop genes in wild populations. However, our sunflowers were grown in equal numbers in spaced plantings from germinated seeds. We will be able to make better estimates of introgression dynamics in actual populations if we better understand the differences in life history, phenology and contributions to the seed bank in a more natural setting. This research would help to clarify how these differences might make either the wild or the hybrid a “better weed” or more dominant. More research into the seed bank dynamics of mixed wild and hybrid seed lots that explores mortality and emergence would improve our understanding of the different life histories of wilds and hybrids. Similarly, mixed stands of wilds and hybrids should be grown together to understand how they compete with one another, and which ends up exerting the most pressure on a crop stand.