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Bt Research 2015, Vol.6 No.1 1-8
ISSN 1925-1939
http://bt.biopublisher.ca
5
be used to isolate, to sequence, and to produce Cry1
and Vip3A proteins. Therefore, these genes may be
used in the construction of pyramidized plants which
could represent an alternative for combating the
resistance of insects to toxins present in the currently
used genetically modified (GM) plants that carry
usually only one toxin. For example,
S. frugiperda
showed resistance to the Cry1F protein expressed in
the event TC1507 of transgenic maize in Puerto Rico
(Storer et al., 2012).
We suggest that
vip3A
possibly acts more
synergistically with
cry1Fa
, as this combination
resulted in higher mortality (80%), similar to the
isolates that contained all three genes. The isolates
containing individual genes caused lower larval
mortality. This difference may be related to a possible
synergy between the genes, in which the presence of
the
vip3A
gene, in different combinations with the
other genes, appears to increase the mortality of
S.
frugiperda
larvae.
It is possible that chitinase, along with the Cry1Fa and
Vip3A proteins, may cause mortality by partially
digesting the insect’s peritrophic membrane, thus
aiding in toxin penetration. However, we could not
verify the action of chitinase in this work. The
cry1Fa
+ vip3A
combination resulted in 80% mortality, which
was not significantly different from that caused by the
isolates containing all three genes. Similar findings
were reported by Costa et al. (2010), who found that
the increase in the insecticidal activity of the isolated
genes was not associated with the presence of
chi
,
which was most likely due to the action of other
toxins involved; i.e., the differences in mortality may
also be related to the gene contents of each isolate.
The results showed that the Vip3Aa protein presented
a synergistic effect with the protein Cry1Fa that
probably increased the mortality of
S. frugiperda
through binding different union sites in this insect’s
gut cells (Senna et al., 2009). Given the frequency of
resistance observed, a possible strategy to manage
resistance in field populations may be the use of new
isolates of
B. thuringiensis
with different genetic
profiles, such as those found in the isolates examined
in this study. Thus, these proteins can be applied in the
development of pyramidized maize plants, which
could drastically reduce the likelihood of the
development of resistance. Overall, the isolates were
more efficient than the positive controls in causing
mortality in
S. frugiperda
larvae.
The search for new isolates that contain genes with
different modes of action in the insect’s gut can also
aid pest management programs in avoiding or
reducing the probability of the development of
resistance to Bt toxins. This possibility provides the
rationale for the utilization of
B. thuringiensis
-based
products (Arantes et al., 2002).
The results of the polymorphism analysis also indicate
the absence of random alterations in the studied genes
that could be detected using the selected restriction
enzymes and similarities between the isolates in
relation to the amplified gene regions for all three
proteins. However, it is important to note that the
presence of a gene does not mean that it is expressed
at equal levels or that it does not contain an
unidentified polymorphic region.
The long-term success of crops expressing insecticidal
B. thuringiensis
proteins depends largely on reducing
the capacity of insects to develop resistance to these
proteins and, thus, to circumvent mortality. Therefore,
it is important to seek new isolates with different
profiles to diversify the gene expression of proteins in
the available cultivars. So this work is essential to
select isolates through selective bioassay with
different genes. Therefore, the strategy of combining
three distinct genes is promising. The relationship
between toxicity and content of genes of isolates of
B.
thuringiensis
studied suggests joint action. The
synergy between genes results in an increase in the
toxicity of the isolates to
S. frugiperda
larvae.
Hence, these genes can be used in the construction of
pyramidized plants containing genes expressing
proteins with different modes of action because Cry
and Vip do not compete for the same binding sites.
The assays performed in this study showed that
isolates I_10 and I_17, which contained all three of
the studied genes, can be used as sources of donor
genes to be expressed in maize plants and can be used