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Unintended Effects of Genetic Manipulation
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Transgenic tomatoes altered with a marker gene construct showed significant changes in morphological and physiological characteristics.

Manipulated Organism: Tomato (Lycopersicon esculentum mill.).

Inserted Transgene: GUS (uidA) gene for the synthesis of the enzyme beta-glucuronidase (GUS); GUS is a marker gene and was fused to the cauliflower mosaic virus (CaMV-35S) promoter so that it would be expressed in all tissues of the plant. The construct was inserted into tomato tissue via Agrobacterium.

Goal of This Study: Investigate whether transgenic tomatoes carrying gene constructs with a marker gene, which would not be expected to change a plant's morphology and physiology, show any differences from the parent variety out of which the transgenic tomatoes were developed. Three transgenic tomato lines were compared with the parent variety: one line that showed high expression of GUS; a second line where GUS expression was silenced; and a third line that had lost the GUS gene through outcrossing so that it was genetically equivalent to the unmanipulated controls but had been subjected to the genetic manipulation and to tissue culture regeneration.

Results of This Study: In 11 of 25 morphological and physiological characteristics that were subjected to statistical analysis, the unmanipulated parent variety differed significantly from all three genetically engineered lines. With respect to an additional seven characteristics, the parent variety differed significantly from one or two of the genetically engineered lines. The largest number of differences (15) were found between the parent variety and the line with the silenced GUS gene. Surprisingly, the line that had lost the gene construct through outcrossing differed from the parent variety just as much as the line that had high expression of GUS. The transgenic lines were more similar to each other morphologically and physiologically than they were to the parent variety. The characteristics in which the parent variety differed from one or more of the transgenic lines included plant height, thickness of the main shoot, leaf shape and size, leaf color, number and organization of fruit branches (inflorescence branching pattern), and fruit number and quality.

For example, in comparison to the parent variety, all three transgenic lines (except where noted) had:
  • On average two fewer fruit-bearing branches per plant, but in proportion more subdivided fruit-bearing branches (changed inflorescence architecture) and two fruits more per branch.

  • On each inflorescence, the first fruit that formed originated at a further distance from the main stem, which might increase the likelihood of the fruit-bearing branches tearing and breaking off the main stem.

  • A significantly larger number of tomatoes with a green collar around the upper part of the tomato - an area of thickened tissue that stays green (a commercially undesirable trait).

  • Significantly lower total acid content in fruits of two of the three transgenic lines.

  • Larger and darker green leaves that were more rolled in at the margins than those of the parent variety.

Exactly how the genetic manipulation was related to these changes in morphology and physiology is unclear. Since both the line with the silenced marker gene and the line that had lost the construct differed in many features from the parent variety and were more similar in those features to the line that expressed the marker gene, the changes cannot be due solely to the expression of the marker gene but rather to the whole process of genetic transformation.

Additional Comments: The fact that three different transgenic lines could be chosen for investigation in this study already indicates that the production of the transgenic lines involved unintended effects. The goal had been to create transgenic tomatoes with high expression of the GUS gene. The lines showed variable expression of the GUS gene, and one high-expressing line was used in this study. The line with the silenced GUS gene represents, of course, an unintended effect.

Source: Schätzl, A. (2007). "Nicht Beabsichtigte Effekte von Gentechnischen Veränderungen bei Tomaten (Lycopersicon esculentum Mill.)." [Unintended Effects of Genetic Modifications of Tomatoes], Diploma Thesis, Fachhochschule Weihenstephan, Germany.

Author Affiliations: This study was a thesis research project.

Funding: Not mentioned

Product Status: Not on the market as of 2008.

Copyright 2008 The Nature Institute.

This document: http://natureinstitute.org/nontarget/reports/tomato_004.php

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