CytA enables CryIV endotoxins of Bacillus thuringiensis to overcome high levels of CryIV resistance in the mosquito, Culex quinquefasciatus

Abstract

Cry proteins produced by Bacillus thuringiensis are selective biodegradable insecticides used increasingly in bacterial insecticides and transgenic plants as alternatives to synthetic chemical insecticides. However, the potential for development of resistance and cross-resistance in target insect populations to Cry proteins used alone or in combination threatens the more widespread use of this novel pest control technology. Here we show that high levels of resistance to CryIV proteins in larvae of the mosquito, Culex quinquefasciatus, can be suppressed or reduced markedly by combining these proteins with sublethal quantities of CytA, a cytolytic endotoxin of B. thuringiensis. Resistance at the LC 95 level of 127-fold for a combination of three CryIV toxins (CryIVA, B, and D), resulting from 60 generations of continuous selection, was completely suppressed by combining sporulated powders of CytA in a 1:3 ratio with sporulated powders of a CryIVA, CryIVB, and CryIVD strain. Combining the CytA strain with a CryIVA and CryIVB strain also completely suppressed mosquito resistance of 217-fold to the latter toxins at the LC 95 level, whereas combination of CytA with CryIVD reduced resistance in a CryIVD-selected mosquito strain from greater than 1,000-fold to less than 8-fold. The CytA͞CryIV model provides a potential molecular genetic strategy for engineering resistance management for Cry proteins directly into bacterial insecticides and transgenic plants.

Recombinant bacteria for mosquito control

“…Laboratory studies suggest that this lack of resistance is due primarily to the presence of Cyt1A in the parasporal body Wirth et al, 1997). Cyt1A's capacity to synergize endotoxin proteins, including the B. sphaericus Bin toxin against resistant and non-sensitive mosquitoes (Wirth et al, 2000a), and to delay resistance are important properties for the improvement of mosquito larvicides.…”

Cyt1Ca from Bacillus thuringiensis subsp. israelensis: production in Escherichia coli and comparison of its biological activities with those of other Cyt-like proteins

“…aegypti (Crickmore et al, 1995;Khasdan et al, 2001). Moreover,higher synergy values (between 16?6 and 70?5) were obtained when resistant strains of Culex quinquefasciatus were tested, thus demonstrating the significance of Cyt1Aa in suppressing resistance (Wirth et al, 1997). Cyt2Ba has been demonstrated to synergize with B. sphaericus (Wirth et al, 2001), but synergy with Cry4Aa has never been demonstrated.…”

“…The most studied Cyt family toxin, Cyt1Aa, is synergistic with Cry toxins (Crickmore et al, 1995;Khasdan et al, 2001), and delays or prevents the selection for resistance of target insects (Wirth et al, 1997(Wirth et al, , 2005. Discovering a potentially cytolytic protein from the Cyt family with a binding domain may indicate that it is targeted to the cell via a receptor, as for Cry family toxins.…”

“…These Cyt/Cry synergies are significantly greater than any synergistic interactions between the Cry toxins themselves (Crickmore et al, 1995;Khasdan et al, 2001). The role of Cyt1Aa in retarding resistance to the Cry proteins is crucial (Wirth et al, 1997(Wirth et al, , 2005. In addition, recombinant Escherichia coli (Douek et al, 1992) loses its colony-forming ability upon expressing Cyt1Aa, a lethal action that is circumvented by the accessory protein P20 (Manasherob et al, 2001).…”

Behaviour of Laboratory-Selected Cry1Ac-Tolerant Strain of <i>Helicoverpa armigera</i> (Hubner) (Lepidoptera:Noctuidae) on Bt-Cotton

“…Bt resistance under laboratory has been reported in 13 insect species, 11 of which, i.e. Ostrinia nubilalis (Hübner); (European corn borer), Heliothis virescens (Fabricius); (tobacco budworm), Pectinophora gossypiella (Saunders); (pink bollworm), Culex quinquefasciatus Say (mosquito), Caudra cautella (Walker); (almond moth), Chrysomela scripta Fabricius (cottonwood leaf beetle), Spodoptera exigua (Hübner) (beet armyworm), Spodoptera littoralis (Boisduval); (Egyptian cotton leafworm), Trichoplusia ni (Hübner); (cabbage looper), Aedes aegypti (Linnaeus); (yellow fever mosquito) and Leptinotarsa decemlineata (Say); (Colorado potato beetle) have developed resistance to various strains of Bt in the laboratory but not in the field [2][3][4][5][6][7][8] . High survivorship of pink bollworm, P. gossypiella was found from Bollgard cotton fields in the adjoining states of Maharashtra and Madhya Pradesh in Central India 9 .…”