Contents - Faperta

Development of Resistance to Transgenic Plants 375
15 generations, and reached 250-fold after 36 generations of selection with Dipel ®
(McGaughey and Beeman, 1988). However, the resistant colony was still susceptible to a
strain of Bt expressing toxin proteins different than Cry1A. The resistant colony was
800-fold resistant to Cry1Ab, but fourfold more susceptible to Cry1Ca (McGaughey and
Johnson, 1987). However, selection for resistance to Dipel in another colony increased
resistance to Cry1Ab, without increasing susceptibility to Cry1Ca (McGaughey and
Johnson, 1992, 1994; Herrero, Oppert, and Ferre, 2001). Colonies of P. interpunctella selected
for resistance to other strains of Bt showed low levels of resístance to Cry1Ca probably
because of low frequencey of resistance alleles to this toxin (Ferre and Van Rie, 2002).
Almond moth, Cadra cautella (Walker) (McGaughey and Beeman, 1988), cotton leaf beetle,
Chrysomela scripta (F.) (Bauer et al., 1994), tobacco budworm, H. virescens (Stone, Sims, and
Marrone, 1989; MacIntosh et al., 1991; Gould et al., 1992), European corn borer, O. nubilalis
(Bolin, Hutchinson, and Andow, 1995; Bolin, Hutchinson, and Davis, 1996), cotton leafworm,
Spodoptera littoralis (Boisd.) (Moar et al., 1995), and cabbage looper, Trichoplusia ni
(Hubner) (Estada and Ferré, 1994) have also been selected for resistance Bt toxins under
laboratory conditions.
Diamondback moth, P. xylostella, is the only insect known to have developed resistance
to Bt under fi eld conditions in regions heavily sprayed with Bt formulations. The highest
levels of resistance recorded have been 30-fold (Tabashnik et al., 1990). Selection under
laboratory conditions has resulted in up to 1,000-fold resistance (Tabashnik et al., 1993),
and the resistant colony displayed high levels of resistance to Cry1 toxins, while the levels
of resistance to Cry2a were not signifi cant (Tabashnik et al., 1993, 1994a, 1996). Over 90% of
the larvae from the resistant colony survived on transgenic canola expressing cry1Ac,
while the unselected ones died on the transgenic plants (Ramachandran et al., 1998b).
A resistant strain from the Philippines expressed 200-fold resistance to Cry1Ac (Ferre
et al., 1991), but was susceptible to Cry1Ba and Cry1Ca. Following selection with Cry1Ab
and later with hybrid Cry1A protoxin, partial resistance was detected against Cry1A toxins,
but not against Cry1Ca, Cry1Fa, and Cry1Ja (Tabashnik et al., 1997b). In a colony from
Florida, 1500-fold resistance was observed in the second generation, which fell to 300-fold
in subsequent generations in the absence of selection pressure, and remained stable thereafter
(Tang et al., 1996). These insects were resistant to Cry1A toxins, but were susceptible
to toxin proteins such as Cry1Ba, Cry1Ca, Cry1Da, and Cry9Ca (Lambert, Bradley, and van
Duyn, 1996; Tang et al., 1996). Selection under laboratory conditions with Cry1Ca, and later
on transgenic broccoli expressing cry1Ca, increased resistance levels to 12,400-fold (Zhao
et al., 2000a), suggesting that P. xylostella has the capability to develop resistance against
several Bt proteins in a short span of time. Several colonies of P. xylostella have been selected
for resistance to Bt proteins, and resistance is retained in the absence of selection pressure
for several generations, but the expression of resistance to different toxins is quite variable
(Sayyed et al., 2000).
High levels of resistance (10,000-fold) have been obtained in a colony of H. virscens to
Cry1Ac (Gould et al., 1995), which also exhibited resistance to Cry1Ab and Cry1Fa, but no
resistance to Cry1Ba and Cry1Ca. Following continued selection for resistance to Cry1Ac
or Cry1Ab, moderate levels of resistance were recorded against Cry2a (Stone, Sims, and
Marrone, 1989; Sims and Stone, 1991; Kota et al., 1999). Further selection for resistance to
Cry2Aa resulted in development of resistance to Cry1Ac and Cry2a. However, the larvae
from the resistant colony suffered 100% mortality on tobacco plants expressing cry2Aa
(Kota et al., 1999). Larvae of H. armigera have shown potential to develop resistance to
Cry1Ac under selection pressure; 76-fold resistance was observed after nine generations
(Kranthi et al., 2000) and 31.4-fold after six generations (Chandrashekar and Gujar, 2004).