Transgenic Bt Technology: 4. Variation in Gene Expression
Prof. C. Kameswara Rao
Foundation for Biotechnology Awareness and Education,
Bangalore, India krao@vsnl.com, www.fbae.org, www.fbaeblog.org

Natural variation in gene expression:
The tendency to vary is the only consistent feature of Nature.   All species organisms, whether wild or cultivated, show naturally inherent variation in physical, chemical and physiological features, which is also the basis for distinguishing different species, varieties.   Each species or variety shows some variation in several features both between and within its populations.   Nevertheless, species and varieties have a set of discernible and invariable features characterizing their identity.   For example all transgenic Bt cotton varieties contain some quantity of Bt toxin, though the actual quantities of the toxin may vary from one variety to the other, as well as within each variety.  

By centuries of experience, biologists in general and agricultural scientists in particular, fully understand that the expression of the same gene or set of genes is influenced by several factors, some inherent in the organism (see Expression of transgenes, http://www.fbae.org/Channels/Views/transgenic_bt_technology3.htm) and some in the environment. Some of this variation, called genotypic variation, is based in the differences in the genetic constitution (genotype) between the varieties.   Another kind, called phenotypic variation, is the result of an interaction between the genotype and the environment, so much so the same genotype behaves differently in different areas and seasons.     Cultivation and management practices also influence a crop’s performance.  Consequently, no crop variety, either conventional or genetically engineered, can be expected to perform uniformly throughout the entire area, or history, of its cultivation.  

The full expression of the transgenes in a transgenic crop variety is crucial, but transgenic varieties may behave differently depending upon the genotype of the recipient variety and on where and how it is being cultivated, as has happened also in conventional agriculture all through.   Most of the factors that affect gene expression are beyond the control of the plant breeders and biotechnologists, once an optimal variety is chosen.  

Agro-climatic zones and crop varieties:

The physical and chemical characteristics of a) the soil, b) the quantity, periodicity and distribution of rainfall, and c) the range of temperature, are factors important for a healthy crop life.   These factors, which vary from country to country and even within a country from region to region, are very critical to successful agriculture.  Taking all such relevant factors together, several agro-climatic zones, each characterized by a set of soil, rainfall (or irrigation facilities), and temperature parameters, are identified in countries with diverse geographical features.   The Planning Commission of India has recognized 15 agro-climatic zones in India (http://www.manage.gov.in/managelib/faculty/bhaskar.htm) and these are further divided into about 120 sub-zones.   Each agro-climatic zone or sub-zone requires varieties of crops particularly suitable to be grown there.   Consequently, a very large number of varieties of different crops was developed by farmers and agricultural scientists in different parts of the world, over centuries, either to suit a particular agro-climatic zone and for certain beneficial traits in them.   As a result, there are over 1,00,000 varieties of cultivated rice, some 80,000 varieties of wheat, and about 15,000 varieties each of potato and the bean.  

Transgsenic Bt crop varieties:

Transgenic Bt cotton containing Cry1Ac was originally developed using the American cotton variety Cocker 312, and this variety is not suitable for cultivation outside America.   Different local varieties of cotton are chosen for developing transgenic Bt cotton varieties, incorporating the same Cry1Ac event, for cultivation in different agro-climatic zones in different countries.    There are now 20 approved varieties of transgenic Bt cotton in India, containing the same Cry1Ac transgenic event, and marketed under the Monsanto’s trade name Bollgard.    The situation is similar with all transgenic events.   The Golden Rice event was first inserted into the genome of a temperate japonica variety and the event has to be transferred to the indica varieties for cultivation in different rice growing agro-climatic zones in tropical countries.   The costs of developing so many varieties with the same transgenic event and the costs of the associated regulatory processing of all these varieties escalate steeply by the time the transgenic products reach the consumer.

Variation in the expression of Bt genes:
Even when Bt crop varieties are cultivated in the recommended agro-climatic sub-zone, there would be significant differences in the expression of Cry1Ac gene in them (see Expression of transgenes, http://www.fbae.org/Channels/Views/transgenic_bt_technology3.htm).  

The general health of the crop is an important factor in realizing the full genetic potential of a crop variety.   The expression levels of a gene may decrease as the age of the crop advances.   There may be differences in expression levels between young and older parts such as the leaves or between comparable parts in vegetative and reproductive phases.   Such variation in the expression of Bt event in cotton was observed in Australia and India (http://www.ias.ac.in/currsci/jul252005/contents.htm).

Soil characteristics, rain fall, the severity of pests and diseases, adequate, appropriate and timely farming inputs such as irrigation, weeding, fertilizer, supportive pesticide application, all have a direct or indirect influence on the performance of the crop and may affect the expression of the transgenes and so the benefits to be derived from transgenic technology.   All these factors, inherent in the varieties and the environment vary from crop season to season, make the difference between optimal or below optimal performance of a crop or its failure.

Transgenic Bt technology produces crop varieties that are only tolerant of the targeted pests and not fully resistant to them.   The farmer has to be advised on the varieties suitable for cultivation in an area and the appropriate practices and precautions needed in every crop season, in order to derive the maximum possible benefit during each season.  The objective of transgenic technology is to derive cost effective benefits of the technology over a considerable period of time and not in a particular season or in a particular region in a season.   No crop variety has ever performed uniformly season after season in all regions of its cultivation.  

Ignoring the factors that control crop performance is poor crop husbandry.   Technology should not be blamed for ills befalling for reasons that lie beyond the realm of particular technology.

August 28, 2005