Contents - Faperta

Transgenic Resistance to Insects: Gene Flow 413
circular plot when the plants were placed at 1 m distance from the transgenic plants.
Circular-fi eld trial designs could also prove to be useful for studying the gene fl ow to
commercial cultivars of rice and other crops to develop strategies to prevent pollen dispersal
from transgenic fi elds to the neighboring nontransgenic fi elds. Studies involving
allozyme and chloroplast DNA analyses have shown that gene fl ow does take place
between maize and teosinte [Zea diploperennis (Iltis et al.), Z. perennis (Hitch.) Reeves
et Mangelsd., Z. luxurians (Durieuet Ash) Bid., and Z. mays (L.) subspecies parviglumis and
subsp. mexicana (Hitch.)]. Introgression is in both directions, but at a low level, allowing
the species to maintain their distinct genetic constitutions (Doebley, 1990). It has been
suggested that an engineered gene in maize could spread to teosinte, and if it were to confer
some adaptive advantage, might spread throughout the teosinte population. As teosinte is
restricted to certain areas of Mexico, the solution would be not to grow transgenic maize
in those regions.
Seefeldt et al. (1999) discovered two imazamox-resistant hybrids from a cross between
Aegilops cylindrica (Host) and an imazamox-resistant wheat (induced mutant FS-4IR). Six
seedlings from BC 1 survived an application of 72g ai ha 1 of imazamox. Management
strategies to reduce the occurrence of herbicide-resistant A. cylindrica wheat hybrids
can be incorporated before, during, and after the cropping season when the herbicideresistant
wheat is planted. Arriola and Ellstrand (1996) detected crop weed hybrids in
sorghum at distances of 0.5 to 100 m from the crop. Interspecifi c hybridization can and
does occur in this system at a substantial and measurable rate. Transgenes introduced
into sorghum can be expected to have the opportunity to escape through interspecifi c
hybridization with Johnson grass, Sorghum halepense (L.) Pers. (Figure 13.2). Traits that
prove to be benefi cial to weeds can be expected to persist and spread. This is an issue
that needs to be addressed when developing biosafety guidelines for commercial release
of transgenic sorghum. Chromosome counts of plants intermediate in appearance
between Setaria italica (L.) Beauv. and S. verticillata (L.) Beauv., collected from a millet fi eld
at Maine et Loire, France, revealed that outcrossing rates within the genus are very low
FIGURE 13.2 Wild relatives of sorghum. (A) Sorghum plumosum, (B) Sorghum halepense, and (C) Sorghum bicolor.
There is frequent gene fl ow between cultivated and wild sorghums, which is a source of tremendous genetic
diversity observed in this crop.