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Crop-to-weed gene flow

Hybridization of crops with nearby weeds may enable weeds to acquire traits we wish they didn't have, such as resistance to herbicides. Research (Kaiser, 2001b; http://www.osu.edu/researchnews/archive/radweed.htm) suggests that crop traits may escape from cultivation and persist for many years in wild populations. Genes that provide a competitive edge, such as resistance to viral disease, could benefit weed populations around a crop field (Kaiser, 2001b).

Gene flow from crops to weeds requires:

  • the presence of sexually compatible wild or weedy relatives close to the crop,
  • an overlap of flowering times between the crop and the wild relatives,
  • the presence of a pollinating agent such as a bird or an insect unless pollination is accomplished by the wind.

Many cultivated crops have sexually compatible wild relatives with which they hybridize under favorable circumstances. The table below provides some of the documented instances of hybridization between crops and weeds.

crop species pollination mode area where studied compatible relatives
Medicago sativa
mostly cross-pollinated USA wild alfalfa
Medicago sativa
Asparagus officinalis
mostly cross-pollinated USA wild asparagus
Asparagus officinalis

Wild asparagus.
Photo: Kathleen Durbin,

Vaccinium angustifolium

Photo: www.noursefarms.com

mostly cross-pollinated USA wild blueberry
Vaccinium angustofolium
burmuda grass
Cynodon dactylon
mostly cross-pollinated USA wild burmuda grass
Cynodon dactylon
Daucus carota
mostly cross-pollinated USA wild carrot
Daucus carota
Apium graveolens
self- and cross-pollinated USA wild celery
Apium graveolens
Chicorium intybus
mostly cross-pollinated USA wild chicory
Chicorium intybus
Trifolium spp.
some cross-, some self-pollinated USA wild clover
Trifolium spp.
Zea mays subsp. mays
mostly cross-pollinated Mexico and Central America wild relatives of corn
Zea mays subsp. mexicana
Zea mays subsp. parviglumis
Zea mays subsp. huehuetenangensis
Zea diploperennis
Zea perennis
Zea luxurians
Vaccinium macrocarpon

Source: USDA

mostly cross-pollinated USA wild cranberry
Vaccinium macrocarpon
foxtail millet
Setaria italica
mostly self-pollinated France green foxtail
Setaria viridis
Lactuca sativa
mostly self-pollinated USA wild lettuce
Lactuca serriola

Avena sativa

Source: Jim Manhart,
Texas A&M Herbarium,

mostly self-pollinated USA wild oats
Avena fatua
oilseed rape, canola
Brassica napus
mostly self-pollinated France, USA wild radish
Raphanus raphanistrum
wild brassicas
Brassica napus
Brassica campestris
Brassica juncea
Chenopodium quinoa
mostly self-pollinated USA wild quinoa
Chenopodium berlandieri
Raphanus sativus
mostly cross-pollinated USA wild radish
Raphanus raphanistrum
Oryza sativa

Source: USDA

mostly self-pollinated USA red rice
Oryza sativa
Nicotiana tabacum
most self-pollinated USA tobacco escaped from cultivation
Nicotiana tabacum
Sorghum bicolor
mostly self-pollinated USA Johnsongrass
Sorghum halapense
Cucurbita pepo
mostly cross-pollinated USA wild squash
Cucurbita texana
Fragaria X ananassa

Source: USDA

self- and cross-pollinated USA wild strawberry
Fragaria virginiana
sugar beets
Beta vulgaris
mostly cross-pollinated France wild beets
Beta vulgaris
Helianthus annuus

Source: USDA

mostly cross-pollinated USA wild sunflower
Helianthus annuus
Juglans regia
self- and cross-pollinated USA California walnut
Juglans hindsii
Triticum aestivum

Source: USDA

mostly self-pollinated USA jointed goatgrass
Aegilops cylindrica

Source: California Department of Food and Agriculture

Sources for table:
Arriola and Elstrand, 1996; Baranger et al., 1995; Desplanque et al., 1999; Doebley, 1990; Klinger et al., 1992; Saeglitz et al., 2000; Snow and Palma, 1997; Till-Bottraud et al, 1992; Zemetra et al., 1998.

The likelihood that transgenes will spread can be different for each crop-and-weed combination in each area of the world.

For example, there are no wild relatives of corn or soybean in the United States. If pollen grains from transgenic corn and soybean plants are released in these areas, they do not encounter any compatible weeds to pollinate, so there is no risk of gene flow. But wild relatives of corn do live in Mexico, so there is a risk that these plants could acquire foreign genes if transgenic corn is grown in Mexico.

Alfalfa pollen is carried by bees from one plant to another. U.S. government researchers report that bees can carry alfalfa pollen at least two-thirds of a mile (http://www.ars.usda.gov/is/AR/archive/oct01/pollen1001.htm). Thus, if transgenic alfalfa is grown in the future, bees might carry the pollen to wild alfalfa populations, resulting in wild plants that have whatever trait was transferred into the cultivated plants.

The risk of gene transfer via pollen is much smaller when crops and weeds are self-pollinating. Self-pollinating plants such as wheat and rice employ their own pollen to produce seeds, They make little use of pollen that might be available from nearby plants. Nevertheless, some exchange among plants does occur at a low rate.

The small risk of gene flow in certain situations must be balanced against the magnitude of the consequences if some gene flow does occur. Jointed goatgrass, a wild relative of wheat, is a serious weed problem in the United States. The consequences of jointed goatgrass acquiring herbicide resistance must be considered when decisions are made about whether to grow herbicide-resistant wheat.

Rice growers struggle to suppress a weed called red rice. A University of Arkansas researcher is tracking the escape of transgenes from cultivated rice to red rice. (http://pigtrail.uark.edu/pubs/Research_Frontiers/fall_2000/05_Feature1.4.html)

A report by the European Environment Agency assessing the potential for gene flow to the natural environment from six major crops--oilseed rape, sugar beet, potatoes, maize, wheat, and barley--is available at http://reports.eea.eu.int/environmental_issue_report_2002_28/en.

The proceedings from a conference on the risks of gene flow from crops to wild relatives are available at http://www.biosci.ohio-state.edu/~lspencer/gene_flow.htm.

The phenomenon of crop-to-weed gene flow is complex. Each crop must be evaluated individually for the risk of gene flow in the area where it will be grown.

Page last updated : March 11, 2004

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