Contents - Faperta

Transgenic Resistance to Insects: Gene Flow 409
Role of Viruses in Gene Flow
Plants expressing viral sequences have potential risks such as heterologous encapsidation,
which can temporarily confer the ability to a virus to be transmitted by a novel vector.
However, this is not different from the nontransgenic plants infected by viruses, except
when a coat protein gene is introduced into a nonhost plant (Tepfer, 2002). To overcome
this problem, genes that code for a coat protein that no longer interacts with the vector may
be used. Other options are to delete the motif recognized by the vector or delete the motifs
allowing particle assembly. Plant to plant gene fl ow by sexual outcrossing may lead to
creation of virus-resistant weeds, and can be overcome by using terminator technology,
and carrying out detailed studies of the potential ecological impact. Plant to virus gene
fl ow by recombination results from covalent joining of DNA or RNA sequences that are
not normally adjacent (Hammond, Lecoq, and Raccah, 1999). It can occur by cleavage and
ligation (cut and paste) or in the case of RNA recombination, by the RNA polymerase
changing matrix during strand synthesis. Plant to virus gene fl ow by recombination could
create a virus with novel properties, worsening of symptoms, and changes in other properties
such as host range. Recombination can also occur in nontransgenic plants infected
by more than one virus. Under conditions of minimum selection pressure, there is a need
to examine the nature of recombinants produced in single-infected transgenic plants, and
doubly infected nontransgenic plants. If the nature of recombinants produced is the same,
it is reasonable to predict that, selection pressures being equivalent, the fi tness of recombinants
occurring in transgenic plants will be the same as in doubly infected plants. In nontransgenic
plants infected with caulifl ower mosaic virus and tomato aspermy virus,
recombination has been observed frequently. All crossovers occurred in a region of high
sequence identity (Gibs, 1994; Aaziz and Tepfer, 1999; Hammond, Lecoq, and Raccah, 1999).
In all plants where recombination occurred, several types of recombinants have been
observed.
Gene Flow into the Wild Relatives of Crops: Vertical Gene Flow
To assess the potential ecological impact of commercial release of transgenic crops in a given
region, the likelihood and impact of vertical gene fl ow for that crop in that region should
be taken into consideration. To guide this assessment, the concept of gene fl ow indices or
botanical fi les (Frietema de Vries, van der Meijden, and Brandenburg, 1992, 1994; Frietema
de Vries, 1996; Amman, 2001) has been developed. Gene fl ow indices give an indication
of the likelihood of a given species to hybridize with the wild relatives and its impact on
the ecosystem. Botanical profi les should be established for each region, consisting of data
on plant species, and should provide an index of the likelihood for dispersal of pollen,
dispersal of reproductive plant parts such as seeds or fruit, and distribution frequency of
wild relatives. Each of these factors can be subdivided into different levels of potential
(or unknown) risk.
Botanical profi les indicate the likelihood of gene fl ow from a particular transgenic crop
plant to its wild relatives, but ignore the potential impact of the transgene on crops and
recipient wild relatives. Therefore, botanical profi les should be combined with information
on the transgene used for transformation, and the transformation event (Amman, 2001).