- Agronomic: sorghum (milo), soybeans, sunflower, wheat
- Additional Plants: native plants
- Pest Management: chemical control, competition, genetic resistance, weed ecology
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.