CROP REPRODUCTIVE BIOLOGY, GENETICALLY ENGINEERED CROPS AND ENVIRONEMNTAL SAFETY: INTRODUCTION TO FIVE CRUCIAL ISSUES
C Kameswara Rao
Foundation for Biotechnology Awareness and Education, Bangalore, India
Opponents of genetic engineering (GE) have vehemently objected to GE crops on the premise that they hybridize with other plants and transfer the new genes, destroying natural biodiversity and the environment. The media reproduce the fallacious arguments, which become factoids, fed to the public. A large segment of the general public will certainly understand the improbability of the scary scenarios painted by the activists, if the basic facts of plant reproductive biology are explained. In this effort, explanatory essays on the following interconnected crucial issues are presented and linked as indicated:
Pollination, the transference of pollen from the anthers of a flower to the stigma of the same or another flower, is a mere physical process carried out mostly by wind or a several biotic vectors such as bees. Biotic pollen vectors may merely consume nectar and pollen in the flower they visit, without pollinating. In general, whether by abiotic or biotic vectors, pollination is non-species specific, incidental or even accidental.
Fertilization, the fusion of the male and female elements, leads to embryo development and seed and fruit set. Critical events such as pollen germination, pollen viability, stigma receptivity and fertilization are controlled by a number of physical, chemical and biological factors inherent in the flowers, and which differ from species to species.
No species hybridizes with all other species/varieties, because it is reproductively isolated from other species. Without such reproductive barriers, species would have freely interbred with every other species, when there would have been only one, or at the best a few species, and not millions as now.
Among the biotic pollen vectors, the role of honeybees in crop plant pollination has been exaggerated, more by popular belief than by rational science. Honeybees actually visit the flowers for nectar and pollen and not with the intent to pollinate. A scientific analysis of the pollen on the body parts and in the ‘pollen baskets’ of the bees, in the nectar in their ‘honey stomachs’, in their intestines, and in the honey in the hive, provides very important clues to answering such questions as a) why do bees visit flowers, b) which species they had visited, c) what do they do in the flowers, d) what do they do with the nectar and pollen they collect from the flowers, and e) how much pollination they actually effect. Appropriate scientific answers to these questions help in assessing the role honeybees play in crop plant pollination and demystify the romantic notions.
Pollination and reproductive behaviour vary from species to species. The cereal crops such as rice, wheat, corn, crops such as cotton, potato, tomato, aubergine and the legumes differ widely in floral structure, nature of the pollen and the mode of pollination, all of which determine the reproductive behaviour of the species.
Scientifically determined antiquity of the bees and the crop plant species shows that they are not overly dependent upon each other, though biotic vectors are important pollinators of a considerable number of fruit and vegetable crop species.
The cereal and millet crop plants are highly self-pollinated or wind pollinated. They offer no incentives to biotic pollen vectors such as nectar or nutritionally rich pollen or have attractants like fragrances or bright colours.
Cotton, potato, tomato, aubergine and the legume crops are almost entirely self-pollinated, though insects visit their flowers for nectar and pollen.
Wild species of Brassica and many other members of the family Brassicaceae are visited by biotic pollen vectors. These species have genetically determined self-incompatibility factors that prevent true self-fertilization in the wild populations. However, centuries of domestication and cultivation has changed this to the extent that cross-fertilization in these crops is usually less than 30 per cent.
4. GENE FLOW
‘Gene flow’ is a natural process of transfer of genes from one species/variety to another. Vertical gene flow is mediated by pollen and sexual reproduction, resulting in the transfer of entire genomes from the parent to the offspring. The extent of gene flow between GE and non-GE crop varieties would be same as among non-GE varieties since GE does not change reproductive behaviour or enhance promiscuity.
Another kind of gene flow is the ‘horizontal or lateral gene flow’, which is a rare natural biological event in bacteria and endoparasites, but not known among plants or higher animal groups. Horizontal gene flow transfers a few genes from one species to an unrelated species, without sexual reproduction.
It is claimed that vertical gene flow transfers the new genes in the GE crops contaminating other varieties and that horizontal gene flow will result in dangerous antibiotic resistant pathogens. Centuries of experience in cultivating crops and decades of experimental and field data on GE crops have shown that the chances for any such calamity are almost nonexistent.
Crop varieties and cultivation practices may influence the biodiversity of a crop field but not the biodiversity outside, which is entirely different from crop field biodiversity.
Activists have claimed that the new genes in the GE crops will be transferred to other species/varieties, severely impacting biodiversity. The genes used in GE crops have all been demonstrated to be safe to non-target organisms. No aspect of credible science based on ten years of field research and commercial cultivation has indicated that GE crops have harmed biodiversity or the environment. A number of studies have shown that GE crops actually increase crop diversity. No superweeds have emerged from GE crops in a decade of their commercial cultivation.
February 22, 2008