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TITLE:  A new inheritance pattern for Bt resistance
SOURCE: ISB News Report, by John T. Lohr
DATE:   July 6, 1999

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A NEW INHERITANCE PATTERN FOR Bt RESISTANCE

The European corn borer (Ostrinia nubilalis) is a major corn pest
in the US. Estimated corn crop losses in North America caused by
this pest exceed $1 billion annually. The European corn borer
(ECB) is susceptible to Bt toxin, a natural insecticide produced
by the soil bacterium Bacillus thuringiensis, and corn
genetically engineered to express the Bt toxin is resistant to
ECB devastation. However, there is much concern that due to
selective pressure, the ECB might develop resistance to the
insecticide expressed by Bt corn.

Currently, it is suggested that farmers grow transgenic Bt crops
adjacent to refuge areas containing a non-Bt variety of the same
crop. The refuge area is intended to supply a source of
susceptible mates for any resistant insects that survive exposure
to high doses of the Bt toxin. This approach, called the high
dose/refuge strategy, is designed to limit the development of
insect resistance to Bt and is based on the assumption that
insect resistance is a recessive trait.

However, research by Randall Higgins and collaborators (1) shows
that resistance of ECB to Bt toxin is not recessive as previously
thought, but appears to display the inheritance pattern of an
incompletely dominant autosomal gene. Consequently, the high
dose/refuge strategy may be insufficient to limit the development
of resistance to Bt by the corn borer.

Higgins grew two laboratory colonies of ECB under controlled
conditions--a control colony of Bt susceptible insects and a
second strain that displayed resistance to Dipel ES, a commercial
formulation of Bt. The resistant strain is 70 times more
resistant to Dipel ES than the susceptible colony. The eighth and
ninth generations from each colony were tested for the study.

Four types of crosses were used to test the transmission of the
resistance gene:
1) reciprocal parental crosses between resistant and susceptible
   colonies;
2) F1 X F1 crosses;
3) back crosses of F1 with susceptible borers; and
4) successive back crosses between heterozygous susceptible
   borers.

The data indicated that resistance to Dipel ES in ECB is limited
to a single gene, which agrees with similar studies in other
insects. However, in this study, resistance appeared to be
inherited as an incompletely dominant autosomal gene, whereas
other studies report that the resistance trait is recessive.

When comparing the various studies of insect resistance patterns
to Bt toxin, it is helpful to note the species tested, the form
of Bt used, and method of Bt delivery. All of these factors can
affect the results. Often, resistance to one form of Bt does not
imply resistance to other forms. Hence, switching from one type
of Bt-corn to another may be an effective way to limit resistance
development in the European corn borer. Counter to this argument
is the possibility that some insects may develop resistance to
multiple forms of Bt, a pattern seen in bacterial resistance to
antibiotics.

These studies suggest that existing refugia strategies designed
to limit the development of insect resistance to Bt may not be
effective. If insects from the field are shown to develop similar
dominant gene resistance, alternative strategies for the use of
Bt-crops would need to be developed.

Source

Huang F, Buschman LL, Higgns RA, and McGaughey WH. 1999.
Inheritance of resistance to Bacillus thuringiensis toxin (Dipel
ES) in the European corn borer. Science 284: 965-967.

John T. Lohr
Assistant Director, Education & Outreach
Utah State University
johnlohr@cscfs1.usu.edu 



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