Genetics of Resistance to Transgenic Bacillus thuringiensis Poplars in Chrysomela tremulae (Coleoptera: Chrysomelidae)

Abstract

The area under genetically engineered plants producing Bacillus thuringiensis (Bt) toxins is steadily increasing. This increase has magnified the risk of alleles conferring resistance to these toxins being selected in natural populations of target insect pests. The speed at which this selection is likely to occur depends on the genetic characteristics of Bt resistance. We selected a strain of the beetle Chrysomela tremulae Fabricius on a transgenic Bt poplar clone Populus tremula L. x Populus tremuloides Michx producing high levels of B. thuringiensis Cry3Aa toxin. This strain was derived from an isofemale line that generated some F2 offspring that actively fed on this Bt poplar clone. The resistance ratio of the strain was >6400. Susceptibility had decreased to such an extent that the mortality of beetles of the strain fed Bt poplar leaves was similar to that of beetles fed nontransgenic poplar leaves. Genetic crosses between susceptible, resistant, and F1 hybrids showed that resistance to the Cry3Aa toxin was almost completely recessive (D(LC) = 0.07) and conferred by a single autosomal gene. The concentration of Cry3Aa produced in the transgenic Bt poplar used in this study was 6.34 times higher than the LC99 of the F1 hybrids, accounting for the complete recessivity (D(ML) = 0) of survival on Bt poplar leaves. Overall, the genetic characteristics of the resistance of C. tremulae to the Cry3Aa toxin are consistent with the assumptions underlying the high-dose refuge strategy, which aims to decrease the selection of Bt resistance alleles in natural target pest populations.

Frequency and fitness cost of resistance to Bacillus thuringiensis in Chrysomela tremulae (Coleoptera: Chrysomelidae)

“…In a previous investigation, we showed that an allele conferring resistance to Bt poplar in a field population of C. tremulae of the Centre region of France was segregating at a frequency of 0.0037 (Génissel et al, 2003a). We subsequently demonstrated that this resistance was completely recessive and determined by a single allele at one autosomal locus (Augustin et al, 2004).…”

“…The two resistant strains, R#60 and R#116, were established from two different resistant isofemale lines selected on the foliage of a transgenic Bt poplar line producing the Cry3Aa Bt toxin (Génissel et al, 2003a). Both the R#60 and R#116 strains are fixed for a recessive allele conferring resistance to the Cry3Aa toxin (Augustin et al, 2004; Augustin, unpublished data).…”

“…353-38) expressing a synthetic cry3Aa gene from Bacillus thuringiensis spp tenebrionis (Génissel et al, 2003b). Its leaves contained enough Cry3Aa toxin to kill susceptible homozygous and heterozygous C. tremulae in less than 3 days (Augustin et al, 2004). Thus, all larvae that had fed actively on Bt poplar and survived for 4 days or more were classified as resistant.…”

“…As this resistance is recessive and determined by a single allele at one autosomal locus (Augustin et al, 2004), q and its 95% credible interval were estimated as follows. Assuming that the sample was from a panmictic population, the frequency of RR should be q 2 and the frequency of RS should be 2q(1Àq).…”

Wood production, wood technology, and biotechnological impacts

“…For example, resistant Chrysomela tremulae beetles were found feeding on transgenic Bt poplar with a mortality similar to that of beetles fed with nontransgenic poplar leaves. Such findings call for implementation of suitable resistance-management strategies together with any release of transgenic resistant trees into nature (Génissel et al 2003, Augustin et al 2004. Generally, effects of transgenic trees on ecosystems have carefully to be evaluated (van Frankenhuizen & Breadmore 2004; see Chapter 7 of this book).…”