Phenotypic diversity among maize landraces is obvious in kernel
color, ear and kernel shape and size, and agronomic traits such as
plant height and length of the growing cycle. Traditionally, this
phenotypic diversity has been used to classify populations into
"races." Currently, 59 races of maize have been described in
Mexico. According to DNA analyses, these races are organized as a
continuum and their differentiation is mainly due to isolation by
distance. Recent genetic studies of landraces in Oaxaca (Mexico)
have shown that gene flow between maize populations is
quantitatively important but that management of seed by farmers
maintains strong agromorphological differentiation between maize
populations.
In traditional agricultural systems, farmers play a wide range of
roles with regard to seed production and maintenance. They conserve
the genetic resources; they select and plant seed from their
varieties; and, after harvest, they are the main consumers of their
products. In Mexico, the traditional system coexists with the
modern agricultural sector but the subsistence-oriented system is
much more widely distributed. In this system, landraces are the
result of continued evolution influenced by various factors: 1)
Seed recycling: Saving seed from one season to the next is an
almost universal practice among small-scale Mexican farmers. At
least from the farmer's perspective, seed selection may also be
fundamental to maintaining the integrity of a landrace, which can
be lost easily through hybridization; and 2) Seed flows: Mexican
farmers commonly acquire seed from other farmers or sources within
or outside the community for several reasons, including
experimentation, starting to farm, and lack of sufficient seeds.
Thus, traditional farmers actively maintain landraces as dynamic
entities.
Farmers are willing to modify introduced maize cultivars through
recurrent hybridization with the local genetic material (process of
creolization) to improve their local performance and consumer
acceptability. They do not consider this process as
"contamination." However, if the introduction of modern varieties
becomes a permanent and pervasive process, a threshold could be
reached above which gene swamping from those cultivars would reduce
or eliminate the genetic diversity of local landraces. Furthermore,
the evolution of landraces, which is based on hybridization,
recombination, and selection, may follow paths unsuspected for
modern varieties. Through recombination, genes belonging to a
specific variety can migrate into new genetic backgrounds where new
linkages and gene interactions may modify the expression of
transgenes in an unpredictable fashion.
Teosinte, the closest wild relative of maize, is considered a weed
in Mexico and farmers attempt to control it in their fields.
Co-occurrence of maize and teosinte exists in several places in
Mexico (Balsas, Chalco, the Central Plateau). Teosinte usually
flowers two to three weeks later than maize but overlap in
flowering times may occur. Furthermore, there are genetic systems
that limit but do not completely exclude crossing between maize and
teosinte. Evidence of the extent of introgression of maize alleles
into teosinte is mixed. The most convincing example of
introgression is provided by Z. diploperennis. In contrast, Kato
(1984) was not able to find genetic evidence for introgression. The
presence in the fields of plants that appear to be F1 hybrids
between maize and teosinte is well documented, but information is
lacking about the behavior of the advanced hybrid generations. It
also remains to be determined whether
hybridization between different genomes results in genome
instability and gene movement within the maize genome.
Different agroecosystems are likely to present very different
selection pressures. Thus, any beneficial effects of a transgene,
for example, in an industrial agricultural setting cannot be
assumed to exist in a traditional agriculture setting or among
wild-growing relatives such as teosinte. Major pests of maize in
Mexico are Lepidoptera, which may be susceptible to most of the Bt
varieties that have been commercialized in the United States.
However, further information is needed on how limiting these pests
are in traditional maize agriculture and in teosinte. If herbicide
tolerance genes are introduced into other genotypes, these in turn
may become herbicide tolerant. This could be of benefit for local
farmers, if they can or are willing to use the appropriate
herbicides, and if the patent owner tolerates this inadvertent
escape without suing the farmers. If herbicide tolerance is
introduced through gene flow into teosinte, a potential tool for
control of teosinte in maize fields may be lost.
Whether or not a transgene will spread into landraces or wild
populations depends on a number of factors, including the level of
gene flow in any given growing season and in successive seasons,
and the selective effect of the transgene. If transgenes are
selectively favored, selection may lead to a reduction of genetic
diversity in the genomic vicinity of the gene in question. The size
of the affected region subject to reduction in genetic diversity
("genomic window") is proportionate to the selective advantage of
the gene under selection and inversely proportionate to the level
of recombination. In maize, which has high rates of recombination,
the region showing reduced diversity as a result of selection on a
single gene would be small (one hundredth of one per cent). Thus,
any effects of a single transgene on the genetic diversity of a
landrace or of teosinte are likely to be insignificant from a
biological point of view, unless there are high levels of gene flow
from transgenic cultivars to landraces and teosinte populations, in
which case the native genetic diversity may be displaced by the
limited diversity of the transgenic cultivars.
The consensus at this stage is that transgenic sequences are
present in Mexican maize landraces in the field although not in the
CIMMYT gene bank. However, this leaves many open questions,
including the geographic magnitude of the transgenes (how
widespread are they?), their local intensity (what is their local
frequency?), the identity of the transgenes (are they only from
commercialized varieties, such as those conferring Bt and herbicide
resistance, or are they from as yet uncommercialized genotypes,
such as maize transformed for pharmaceutical production?), their
possible presence in teosinte, their source(s) (local government
stores, emigrants to the United States, seed companies, or other
origins), the fate of transgenes in landraces and teosinte, and the
role of farmers and others in gene flow by pollen and seed,
involving transgenic sequences.
The processes that generate and maintain the genetic diversity of
maize are very dynamic and preservation of diversity should be
based on the preservation of these processes as well as the
conservation of landraces, per se. We contend that establishing
more effective selection schemes on the part of farmers may
constitute an effective incentive for farmers to maintain their
landraces. Farmers can actually be trained to conduct some degree
of plant breeding, if they do not already do so. This type of
outreach would require an active extension service, a network of
NGOs or local cooperatives, or a network of technical schools.
Preservation of genetic diversity can be achieved through more
active use of the diverse landraces and their diverse products,
especially in urban areas where many of the maize products are
currently very standardized and uniform. Education of urban
consumers about diversity of maize and its products would help
promote the use of products from these landraces, permitting the
conservation of the diversity in the fields. Additional research is
needed in the areas of flowering biology, gene flow, and
reproductive isolation of maize. Furthermore, studies that document
the extent of the distribution of transgenes among maize landraces
and teosinte populations need to be conducted and published in
peer-reviewed journals, following protocols that rely on
replications and blind tests under the supervision of outside
advisors.
|
