Potential shortfall of pyramided transgenic cotton for insect resistance management

Abstract

To delay evolution of pest resistance to transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt), the "pyramid" strategy uses plants that produce two or more toxins that kill the same pest. In the United States, this strategy has been adopted widely, with two-toxin Bt cotton replacing one-toxin Bt cotton. Although two-toxin plants are likely to be more durable than one-toxin plants, the extent of this advantage depends on several conditions. One key assumption favoring success of two-toxin plants is that they kill insects selected for resistance to one toxin, which is called "redundant killing." Here we tested this assumption for a major pest, Helicoverpa zea, on transgenic cotton producing Bt toxins Cry1Ac and Cry2Ab. Selection with Cry1Ac increased survival on two-toxin cotton, which contradicts the assumption. The concentration of Cry1Ac and Cry2Ab declined during the growing season, which would tend to exacerbate this problem. Furthermore, analysis of results from 21 selection experiments with eight species of lepidopteran pests indicates that some cross-resistance typically occurs between Cry1A and Cry2A toxins. Incorporation of empirical data into simulation models shows that the observed deviations from ideal conditions could greatly reduce the benefits of the pyramid strategy for pests like H. zea, which have inherently low susceptibility to Bt toxins and have been exposed extensively to one of the toxins in the pyramid before two-toxin plants are adopted. For such pests, the pyramid strategy could be improved by incorporating empirical data on deviations from ideal assumptions about redundant killing and cross-resistance. (Résumé d'auteur

Biological activity of Bt proteins expressed in different structures of transgenic corn against Spodoptera frugiperda

“…For resistance management, the larval survival of S. frugiperda on corn ears can favor the resistance evolution, because it allows the survival of heterozygous individuals in the reproductive structures, favoring an increase in the frequency of these insects. Survival of heterozygous insects on silks and grains of YieldGard VT PRO™ and PowerCore™ may also compromise the future efficacy of these ♀ ♂ (BRÉVAULT et al, 2013).…”

Resistance to Bt Crops; Influence, mechanisms and management strategies

“…While, a few cases of study in laboratory and field populations showed that the insect could evolve resistance to pyramid crops (Bernardi et al, 2015;Brévault et al, 2013;Fabrick et al, 2015;Wei et al, 2015;Welch et al, 2015). However, study by who reported that the resistance to pyramids is due to similarity of amino acid sequence between Bt toxins, which affect the cross-resistance when the resistance is associated with mutations, that reduce binding to midgut receptor.…”

Resistência de Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) a eventos "piramidados" de milho que expressam proteínas inseticidas de Bacillus thuringiensis Berliner

“…A premissa básica da piramidação de genes é que as proteínas inseticidas, isoladamente, ocasionem elevada mortalidade da praga-alvo, ou seja, que o os insetos resistentes a uma proteína inseticida sejam controlados pela outra proteína e vice-versa, possibilitando assim o "controle redundante" (GOULD, 1986;McGAUCHEY;WHALON, 1992;ROUSH, 1998;ONSTAD;MEINKE, 2010;BRÉVAULT et al, 2013). Contudo há casos em que as plantas expressam mais de uma proteína, porém não atendem o conceito de evento piramidado (ROUSH, 1998;BRÉVAULT et al, 2013).…”

“…Contudo há casos em que as plantas expressam mais de uma proteína, porém não atendem o conceito de evento piramidado (ROUSH, 1998;BRÉVAULT et al, 2013). Este fato ocorre quando as proteínas inseticidas presentes na planta Bt piramidada apresentam resistência cruzada com outra proteína de Bt e/ou a expressão de uma das proteínas não é suficiente para o controle de insetos resistentes a uma determinada proteína RENSBURG;CARRIÈRE, 2009;TABASHNIK, 2010).…”

“…Além da desfolha, S. frugiperda também ataca as espigas, inicialmente alimentando-se de estilo-estigmas e, a partir do 2º instar larval, dos grãos de milho (CRUZ, 1995 (BURKNESS et al, 2010;SIEBERT et al, 2012;NIU et al, 2013). Além disso, possibilitam aumentar o espectro de controle, bem como manejar de forma mais efetiva a resistência, especialmente, quando as proteínas inseticidas apresentam modo de ação distinto e, não há resistência cruzada entre as mesmas VÉLEZ et al, 2013;BRÉVAULT et al, 2013;. O uso de milho Bt piramidado pode ser enquadrado na estratégia de ataque múltiplo para o manejo da resistência, a qual se baseia na premissa que as proteínas Bt, isoladas ou em combinação apresentam elevada toxicidade contra a mesma praga-alvo ROUSH, 1998, ZHAO et al, 2003, STORER et al, 2012.…”