GENET archive


2-Plants: Some remarks on insect Bt-resistance

-------------------------- GENET-news ---------------------------

TITLE:  A) Bt toxin: Assessing GM strategies
        B) Resistance to Bt toxins
        C) [Without title]
SOURCE: all three letters from Science, edited by AGNET, Canada
        A) by T. Wallimann
        B) by B. E. Tabashnik, R. T. Roush, E. D. Earle
        C) by F. Huang, L. Buschman, R. Higgins, W. McGaughey
DATE:   January 7, 2000

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Bt toxin: Assessing GM strategies

Theo Wallimann, Institute of Cell Biology, ETH-Honggerberg, CH
8093 Zurich, Switzerland,
writes in this letter that the debate over possible deleterious
effects on the Monarch butterfly of genetically engineered plants
expressing a biological insecticide (Bt toxin) is described in
the News Focus article "Risks and benefits:GM crops in the cross
hairs" by Dan Ferber (26 Nov., p. 1662). In response, says
Wallimann, some critical points should be raised.

The issue is broader than whether Bt toxin (from the bacterium
Bacillus thuringiensis) produced by genetically modified (GM)
crops imperils Monarch butterflies. The real issue is that a
strategy to constitutively express an insecticidal compound in
large-scale crop monocultures (15 million acres of Bt corn was
planted in the United States in 1998, 20% of the total acreage of
corn), and thus expose a homogeneous subecosystem continuously to
the toxin, seems bound to create Bt-toxin-resistant pests because
of heavy selection pressure.

Sooner or later we will likely see Bt-toxin resistance in those
insects that are continuously in contact with these monocultures
and feed on them. If or when this occurs, we will have lost the
use of a valuable bio-insecticide. For about 30 years Bt toxin
has been applied on the spot (by spraying B. thuringiensis
directly onto plants) and only when there are signs of
infestation of the crops by insects. It is the most successful
biological insecticide control system we have and would probably
retain its potency against pests for many more years to come. Bt
toxin has been found to leak through the root system of Bt-toxin
GM maize into the soil, which could possibly affect a myriad of
insects in the soil and give rise to horizontal gene transfer,
for example, through soil bacteria (1). Perhaps we should
consider going back to the drawing board and designing better GM
strategies with less or none of such drawbacks.

1. D. Saxena, S. Flores, G. Stotzky, Nature 402, 480 (1999).


Resistance to Bt toxins

Bruce E. Tabashnik, Department of Entomology, University of
Arizona, Richard T. Roush, Waite Institute, University of
Adelaide, Australia, and Elizabeth D. Earle, Department of Plant
Breeding, Cornell University,
write that although we share the general concerns about pest
resistance to transgenic crops discussed by F. Huang et al.
[Reports, "Inheritance of resistance to Bacillus thuringiensis
toxin (Dipel ES) in the European corn borer," 7 May 1999, p.
965], reconsideration of several aspects of their report is
warranted. They examined resistance to Dipel ES, a commercial
formulation of B. thuringiensis (Bt) toxin, in a laboratory
selected strain of the European corn borer, a major lepidopteran
pest. Bt genes encoding insecticidal proteins have been
introduced into the genome of maize to provide protection from
larvae of the European corn borer. The transgenic hybrids are
called Bt maize.

The results presented by Huang et al., however, are not directly
relevant to potential resistance of the European corn borer to Bt
maize because Dipel ES differs substantially from the toxins
produced by Bt maize. Dipel ES contains Bt spores and at least
three Bt toxins (Cry1Aa, Cry2A, and Cry2B) that are not present
in Bt maize. Thus, the European corn borer strain studied by
Huang et al. could be resistant to these components of Dipel ES,
rather than to the toxins in Bt maize.

Huang et al. mention in note 7 that neonates from their European
corn borer strain with 65-fold resistance to Dipel ES caused more
damage than susceptible insects when placed on certain Bt maize
hybrids. However, damage by neonates is not a reliable indicator
of survival on transgenic plants. Results with the Colorado
potato beetle show that neonates with greater than 400-fold
resistance to Bt toxin Cry3A do not survive on Bt potato plants
that produce Cry3A (1). Therefore, in some cases, pests may need
extremely high levels of resistance to overcome the high
concentrations of toxin in Bt plants.

The critical point about the inheritance of resistance and its
implications for resistance management is whether heterozygotes
die on transgenic plants. Huang et al. provide no evidence that
either larvae from their Dipel ES-resistant strain or
heterozygous larvae can survive to maturity on Bt maize, which
means that no conclusions can be drawn about inheritance of
resistance to Bt maize. In contrast to survival of resistant
diamondback moth on Bt broccoli and Bt canola (2) and resistant
pink bollworm on Bt cotton (3), as far as we know, no one has
reported results showing survival of European corn borer on Bt
maize. The failure to find such resistance in European corn borer
despite extensive efforts (4) bodes well for managing resistance
of this pest to Bt maize.

Several examples of nonrecessive inheritance of resistance to Bt
toxins are known (5), but in the few cases of resistance to Bt
plants analyzed so far, inheritance of resistance to the Bt
plants is recessive (3, 6).

1. J. M. Wierenga, D. L. Norris, M. E. Whalon, J. Econ. Entomol.
   89, 1047 (1996).
2. S. Ramachandran et al., J. Econ. Entomol. 91, 1239 (1998); J.
   Tang et al., J. Econ. Entomol. 92, 47 (1999).
3. Y.-B. Liu, B. E. Tabashnik, T. J. Dennehy, A. L. Patin, A. C.
   Bartlett, Nature 400, 519 (1999).
4. D. A. Andow, D. N. Alstad, Y.-H. Pang, P. C. Bolin, W. D.
   Hutchison, J. Econ. Entomol. 91, 579 (1998); D. A. Andow, D.
   M. Olson, R. L. Hellmich, D. N. Alstad, W. D. Hutchison, J.
   Econ. Entomol., in press.
5. B. E. Tabashnik et al., Philos. Trans. R. Soc. London Ser. B
   353, 1751 (1998); R. Frutos, C. Rang, M. Royer, Crit. Rev.
   Biotechnol. 19, 227 (1999).
6. T. D. Metz, R. T. Roush, J. D. Tang, A. M. Shelton, E. D.
   Earle, Mol. Breed. 1, 309 (1995).


C) [Without title]

Fangneng Huang, Lawrent Buschman, and Randall Higgins, Department
of Entomology, Kansas State University, and William McGaughey
(retired) U.S. Department of Agriculture, Agricultural Research
Service Grain Marketing and Production Research Center,
Manhattan, Kansas
write in repsonse that they described European corn borer
resistance to Dipel ES in an earlier manuscript (1). Our report
in Science was written to describe the inheritance of Dipel ES
resistance in the European corn borer. We did not expect that
inheritance of resistance to Dipel ES was more dominant than
recessive. We were careful to point out that there were
limitations in applying this research to corn borer resistance on
transgenic plants. As Tabashnik et al. indicate, if the insects
do not survive to adulthood, they will not be able to pass along
their genes for resistance. Further research is needed to
determine if these Dipel ES-resistant corn borers can survive and
reproduce on various Bt corn hybrids. However, we believe it is
important to report this research as evidence that European corn
borer resistance to some Bt toxins may be more dominant than

1. F. Huang, R. A. Higgins, L. L. Buschman, J. Econ. Entomol. 90,
   1137 (1997).


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