Biology Reference
In-Depth Information
Still another example shows the complexity of relationships that can
occur among crop and noncrop plants. For beets (
Beta vulgaris
), several
cultivated forms, the wild progenitor subspecies, and several weedy vari-
eties exist. The cultivated forms include table beet, Swiss chard, fodder
beet, and sugar beet. All beet forms are fully interfertile and exist in close
contact in parts of Europe. Cultivated beets are exclusively biennial,
whereas wild forms range from annual to perennial.Weed forms are pri-
marily annuals, carrying a dominant gene for the annual habit.Weed beets
have become a major problem for sugar beet cultivation in Europe and to
a lesser extent in California.Transgenic varieties of sugar beets with resist-
ance to herbicides and certain virus diseases have been developed, so the
possibility of escape of transgenes to weed beets and other close relatives
is real (Desplanque et al. 2002).
In Italy, alleles that are usually rare in the wild form (sea beet,
B. vul-
garis
ssp.
maritima
) are common in this form where it occurs near plant-
ings of table beets and Swiss chard (Bartsch et al. 1999). In California,
sugar beets hybridize freely with wild sea beets that were apparently
introduced as a contaminant of crop beet seed. They also hybridize with
a second wild European beet,
B. macrocarpa
(Bartsch and Ellstrand 1999).
Transgenic herbicide-resistant varieties of sugar beets are now available,
and it is clear that their use will probably lead to gene escape into popu-
lations of wild and weedy forms (Boudry et al. 1993; Bartsch et al. 2001).
Other examples of hybridization between crop and weed races of the
same species involve squash (
Cucurbita pepo
), carrot (
Daucus carota
), celery
(
Apium graveolens
), and asparagus (
Asparagus officinalis
) (Spencer and Snow
2001; Ellstrand and Hoffman 1990;Wilson and Payne 1994).
A particular risk exists for genetic engineering of plants that are both
beneficial and weedy. Bermuda grass (
Cynodon dactylon
), for example, is
both a pasture grass and one of the most troublesome weeds of natural
ecosystems (Ellstrand and Hoffman 1990).Transgenes introduced into this
grass would almost certainly spread to grass populations outside the pas-
ture environment, making this plant even more difficult to manage in
areas devoted to natural vegetation.
Potential for Exchange of Transgenes between
Crops and Other Plant Species
Many other hybridization routes exist for the transfer of transgenes
between crop plants and other related species. Well-studied examples
include the crop sorghum and Johnsongrass case described at the begin-
