Biology Reference
In-Depth Information
by the degree of self-incompatibility of the plants involved, the mode of
pollen transport, and many conditions of the physical habitat. In the case
of the common sunflower (
Helianthus annuus
), for example, crop varieties
are highly self-compatible, whereas wild plants are self-incompatible. Both
are insect-pollinated, primarily by bees. This relationship creates a strong
potential for crop pollen to be transferred to wild plants, with the insect
pollinators being capable of long-distance transport (Arias and Riesberg
1994). Crop and wild sunflowers occur in close proximity throughout
most of the area of sunflower cultivation in the Unites States, and
hybridization of crop and wild varieties is very frequent (Burke et al.
2002). For other species, including sorghum, radish, and squashes,
hybridization resulting from insect pollination has been detected up to 1
km from crop plantings. Most grain crops, on the other hand, are wind
pollinated and strongly self-compatible. Crop sorghum (
Sorghum bicolor
),
for example, has an outcrossing rate of only about 10-15% and John-
songrass, with which it is capable of hybridizing, is also primarily, but not
completely, self-pollinating (Arriola and Ellstrand 1996), as we noted at
the chapter outset. Field experiments have nevertheless detected
hybridization between sorghum and Johnsongrass at distances of 100 m
from sorghum plantings.
Experimental studies have revealed that genetic engineering itself can
influence the frequency of outcrossing. The annual crucifer
Arabidopsis
thaliana
, used widely in laboratory studies of plant genetics, is normally
self-pollinating. Various transgenic lines carrying a gene for herbicide
resistance differed substantially in outcrossing frequency, with several
showing increased outcrossing tendency. One line was 17 times more
likely to outcross than untransformed plants (Bergelson and Purrington
2002).
Genetic Engineering and Transgenes
Genetic engineers now have perfected techniques for transferring genes
between microorganisms, from microorganisms to higher plants and ani-
mals, or from one higher plant to another.The genes themselves may vary
in structure depending on their source and may be modified in structure
before being inserted into a host organism. The performance of such
transgenes depends to some extent on the location at which they are
inserted in the host genome.Thus, many different transgenic lines of host
organisms may be created, even with a single gene. Several types of genes
