C Kameswara Rao
Foundation for Biotechnology Awareness and Education, Bangalore, India
There are two kinds of ‘gene flow’, the natural processes of transfer of genes from one species/variety to another. Gene flow through natural interspecific or intervarietal hybridization mediated by pollen is ‘vertical gene flow’, which results in the transfer of entire genomes. The genes are transferred from the parent to the offspring and express only in the next generation, the offspring.
The other kind is ‘horizontal or lateral gene flow’, which is a rare natural biological event that transfers a few genes at a time from one species to an unrelated species, without involving sexual reproduction.
Crossability Barriers and Reproductive Isolation
Species and even their varieties do not cross with each other freely. They are reproductively isolated, with their identity being maintained through several genetically controlled physical and/or physiological crossability (reproductive) barriers. Species and varieties differ in chromosome number and structure, and genetic constitution, which are often serious impediments to the fertility of the offspring of hybrid crosses. Reproductive barriers may also operate at one or more stages of pollen germination, viability, fertilization, embryo development and seed germination. When reproductive isolation from reproductive barriers is strong, even persistent attempts at artificial hybridization have failed. When such restrictions are weak, natural intervarietal or interspecific hybridization and gene transfer may occur. Even intergeneric gene transfer such as known from wheat (Triticum aestivum) to a distant wild relative (Aegilops peregrina) is a possibility, though extremely rare. Repeated (back) crossing with the donor parent results in introgression, the stable incorporation of genes from one differentiated gene pool into another.
Biosecurity of Genetically Engineered Products
Biosecurity, the safety of genetically engineered (GE) products as food and feed and the safety of the environment, is a matter of concern both for the biotechnologists and the critics. It was the scientists, not the anti-GE activists, who first thought of biosecurity issues, investigated the extent of risk and devised means to mitigate any possible risk. Before commercialization, each GE product is subjected to a very lengthy and stringent biosecurity regime and only when the product is found to be safe and has a favorable risk-benefit ratio, it is released for public use.
The opponents of GE (transgenic) crops and products made gene flow a contentious issue and raised several biosecurity concerns in their anti-GE arguments.
Vertical gene flow
The apprehension is that pollen from the transgenic crops pollinate non-GE varieties (gene escape) causing genetic contamination or pollution. According to the anti-tech activists such an event would cause untold horrors to the crops and the environment. The activists argue that the non-target organisms would be affected and the transgenics would become super weeds.
Issues of gene flow should take into consideration several factors related to the reproductive biology of the species involved. Vertical gene flow requires a) populations of two different varieties/species that grow together in the same geographical area (sympatric), b) they should be synchronous in flowering time, pollen viability and stigma receptivity, and c) be genetically compatible allowing random mating (panmictic) between the two populations, which means that there are no reproductive barriers preventing hybridization or at least that they are weak.
The means of pollination, extent of natural cross-pollination, duration of pollen viability and stigma receptivity, extent of cross-fertilization, embryo survival and seed and fruit set are the critical events in gene flow. The degree of fertility of the hybrids depends upon the degree of genetic compatibility. Analysis of the hybrids would indicate the extent of gene transfer. If the transferred genes add to the competence of the species/variety (adaptive value), and if there is repeated back crossing with the donor parent, the genes will be fixed in the population (introgressed) which in course of time differentiates as a new variety or even a new species.
Gene flow can occur between transgenics and the isogenic varieties of a crop, to the same extent as was happening earlier between non-transgenic populations of that crop. Transgenes do not promote promiscuity. Unintended transgenic hybrids resulting from gene flow from GE crops to wild or weedy relatives is possible, but cannot easily or routinely occur.
Hybridization experiments have shown that intercrossing among the indica varieties of rice was 3.9 per cent and among japonica varieties it was 3.6 per cent, while practically there was no intercrossing between indica and japonica varieties. Gene flow of an herbicide resistant gene from a cultivated rice variety to a weedy rice variety was 0.011 to 0.046 per cent and from a cultivated variety to a wild variety was 1.21 to 2.19 per cent.
A comparative analytical study of GE and non-GE corn has shown that the transgenic content of a non-GE population was 0.9 per cent at 10 m from the GE population. The claim that corn transgenes have introgressed into native corn varieties in Mexico was disproved.
In Canola, the presence of an herbicide resistant gene in a non-GE population was 0.1 per cent at 5 m, 0.02 per cent at 10 m and zero at more than 15 m distance, from the GE variety.
Since the conditions under which these experiments were conducted do not exist in nature, the possibility of gene flow and of the impact of such an event would be even less.
The mere presence of a transgene in a plant is of no consequence. The question is what is the impact of few miniscule weedy populations with rapidly deteriorating vigor in agronomic terms? A Bt gene has no impact if the intended pest is not present and an herbicide tolerant gene is of no consequence if an appropriate herbicide is not sprayed. The gene should introgress and should impart an advantage to the recipient population over the others which do not have the transgene.
Horizontal gene flow
The other set of anti-GE arguments relate to the horizontal or lateral gene flow, raised in the context of antibiotic resistance genes used as selectable markers in GE technology. It is argued that these genes would be taken up by soil pathogens which would become resistant to the antibiotics, and people infected by them cannot be cured by that antibiotic, leading to incurable diseases and death.
In horizontal gene flow which does not involve sexual reproduction and bypasses reproductive isolation, the few genes transferred can express in the same cell generation. In nature, horizontal gene flow is a fundamentally prokaryotic (organisms with no nucleus or cell organelles such as bacteria) phenomenon. Other natural examples of horizontal gene flow relate to endoparasites and their hosts, which share several genes as a result of millions of years of co-habitation and co-evolution.
Horizontal gene flow occurs through one of five different biological processes known as transformation, conjugation, transfection, lysogenization and transduction. Recombinant DNA (rDNA) technology that involves transformation is a good example of artificial horizontal gene transfer that makes transgenics possible irrespective of genetic relationship.
The issue of horizontal transfer of antibiotic resistance marker genes from transgenic plants to bacteria was discussed in depth at an international symposium on the Biosafety of GE organisms in 2000. The consensus was that such an event is very unlikely and even if it happens it would not contribute significantly to the horizontal spread of the genes in question, for three reasons: a) the antibiotic resistance genes are already widespread in bacterial populations, b) there was no experimental evidence for horizontal gene transfer from plants to bacteria, and c) horizontal gene transfer events from transgenic plants to bacteria have not been detected. There is no change in this position till to date.
A recent paper in Transgenic Research (June, 2007) concluded, supported by numerous studies, most of which are commissioned by some of the very parties that have taken a position against the use of antibiotic selectable marker gene systems, is that there is no scientific basis to argue against the use and presence of selectable marker genes in transgenic plants.
February 22, 2008