Francis R. Groeters

Fitness costs of resistance to Bacillus thuringiensis in the diamondback moth (Plutella xylostella)

Does adaptation to stress entail fitness costs in optimal environments? Antagonistic pleiotropy sometimes causes trade-offs in fitness across different environments in which the agent of stress is a pathogen (Lenski 1988) or a novel diet (Pashley 1988; Via 1991). It is commonly believed that evolution of resistance to insecticides also involves such trade-offs. Alleles conferring resistance are rare before exposure to insecticide and have been presumed to exert negative effects on fitness in the absence of insecticide (Crow 1957; Uyenoyama 1986; Hoffman and Parsons 1991). However, fitness costs associated with resistance to conventional synthetic insecticides appear to vary considerably among species and insecticides (Roush and McKenzie 1987; Roush and Daly 1990; Denholm and Rowland 1992). Moreover, little is known about fitness costs of resistance to insecticidal proteins from Bacillus thuringiensis, a common soil bacterium (McGaughey and Whalon 1992; Tabashnik 1994). Toxins from B. thuringiensis kill susceptible insects by binding to and disrupting the integrity of the midgut epithelium (Gill et al. 1992). Reduced binding of toxins is a primary mechanism of resistance in the diamondback moth, Plutella xylostella (Ferre et al. 1991), a major pest of cruciferous vegetables (Talekar 1992). The normal function of toxin-binding sites is not known but seems to be important because such sites are found in numerous insects (Feitelson et al. 1992). Several authors have suggested that alteration of toxin-binding sites may interfere with normal physiological functions (Van Rie et al. 1990; MacIntosh et al. 1991). Instability of resistance to B. thuringiensis in the absence of selection

Roles of selection intensity, major genes, and minor genes in evolution of insecticide resistance.

Abstract A prominent hypothesis about insecticide resistance is that genesof major effect play a key role in field-evolved resistance because theintensity of selection is extremely high in the field. A corollaryhypothesis is that the lower intensity of selection in laboratoryselection experiments favors polygenic control of insecticideresistance. Contrary to these hypotheses, a literature review revealedthat the intensity of selection for insecticide resistance in the fieldvaries widely and overlaps broadly with selection intensities in thelaboratory. Also contrary to these hypotheses, results from simulationsof population genetic models suggest that selection intensities typicalof laboratory selection experiments favor resistance that is conferredby major genes. Major genes dominated responses to selection forresistance across a wide range of simulated selection intensities, withand without fitness costs and refuges. The simulation results alsosuggest that the intensity of selection, rather than the number of lociconferring resistance, is central in determining rates of resistanceevolution and effectiveness of refuges.

Instability of resistance to Bacillus thuringiensis

The continued efficacy of environmentally safe biopesticides derived from Bacillus thuringiensis (Bt) is threatened by the potential for development of resistance in pest populations. Instability of resistance is defined here as the tendency for the frequency of resistant genotypes to decrease in a population beyond effects directly attributable to immigration or emigration. Instability can be quantified as R, the average rate of change in the logarithm of the LC50 (concentration killing 50% of individuals) per generation, which is analogous to the average rate of response to selection per generation. In seven strains of Plutella xylostella, the first insect with field populations reported to be resistant to Bt, resistance declined when exposure to insecticide ceased (mean R = − 0.19). In four other pests, resistance to Bt declined slowly or not at all (mean R = − 0.02) in the absence of exposure to Bt Reduced biotic fitness associated with resistance is the most likely cause of instability of resistance ...

ASSOCIATION BETWEEN LATITUDINAL VARIATION FOR EMBRYONIC DEVELOPMENT TIME AND CHROMOSOME STRUCTURE IN THE GRASSHOPPER CALEDIA CAPTIVA (ORTHOPTERA: ACRIDIDAE)

From southeastern Queensland to southern Victoria, over a transect of 11° latitude, the Moreton taxon of the Australian grasshopper Caledia captiva exhibits a cline in chromosome structure that involves change from a metacentric to an acrocentric genome. In this study, we show that embryonic development time covaries with chromosome structure along the transect. Both development time and chromosome short arm length exhibit an overall negative correlation with latitude, but with maxima just south of the northern limit of the taxons distribution. Selection for such a pattern appears to arise from changes in voltinism along the cline in season length that exists along the transect. Populations with the highest temperature thresholds for avoidance of embryonic diapause also have the slowest development time and probably represent the northern extreme of a primarily univoltine life cycle. North of this region bivoltinism increases in frequency and, as expected from a split of the season length, development time decreases. Maximum chromosome short arm length occurs in the vicinity of the northern univoltine populations, rather than at the limit of distribution where bivoltinism prevails. We conclude that variation in chromosome structure could be contributing to the heritable variation for development time that forms the basis for adaptive change in this trait. These results provide justification for investigating causal relationships between chromosome structure and development time, with an ultimate aim of understanding the adaptive significance of chromosomal variation in C. captiva.

Oviposition preference of the diamondback moth (Plutella xylostelld) unaffected by the presence of conspecific eggs orBacillus thuringiensis

Neither toxins fromBacillus thuringiensis Berliner nor conspecific eggs deterred oviposition by the diamondback moth,Plutella xylostella (L.) (Lepidoptera: Plutellidae), in laboratory choice tests. The finding that toxins did not deter oviposition by moths from a susceptible line shows that oviposition preference and larval survival were not associated in this line. Selection for larval resistance to toxins did not significantly alter oviposition preference, which rules out a strong genetic correlation between larval performance and oviposition preference. Failure of conspecific eggs to deter oviposition may not represent lack of association of preference and performance because other evidence suggests that larval performance may not be greatly affected by larval density. These results suggest that the ability of refuges to slow evolution of physiological resistance toB. thuringiensis toxins will not be magnified by an oviposition preference for untreated foliage.

Heritability of wing length in nature for the milkweed bug, Oncopeltus fasciatus

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Concerted Patterns of Chromosome Variation in the Grasshopper Caledia captiva (F.) (Orthoptera: Acrididae: Acridinae): An Adaptive Response to Seasonality Changes Along a Latitudinal Gradient?

Chromosomal variation in the Moreton subspecies of the Australian grasshopper, Caledia captiva (F.), involves the transposition of the centromere from terminal to medial locations on all chromosomes within the genome (2n = 23 6 /24 Y). At each end of its distributional range - covering 1500 km along the east coast of Australia - populations show fixed differences in chromosome structure. Between these two extremes, populations are characterized by complex chromosomal polymorphisms that take the form of latitudinal clines along which the entire genome changes gradually and systemically from metacentric in northern populations to acro/telocentric in the south. To date over 600 different chromosomal rearrangements have been identified. Using a series of DNA and protein markers, we have shown that gene flow along the chromosomal clines is continuous, suggesting that the establishment and maintenance of the clines has involved selective rather than stochastic events. Moreover, we have also revealed that the concerted patterns of chromosome change are correlated with changes in development time. Development time is negatively correlated with changes in the number of degree days available for successful completion of the life cycle with increasing latitude and probably represents an adaptation to the gradual reduction in season length. Increased body weight is also correlated with acro/telocentric chromosomes and with a rapid development time. An explanation of this unusual relationship is presented, whereby it is proposed that the changes in the location of the centromeres on every chromosome lead to changes in nuclear volume by modifying the centromere-telomere disposition at telophase when the nuclear envelope is reassembled. This proposal is analogous to the known consequences of increasing nuclear DNA content which leads to larger nuclear volume, larger cell and body size, and reduced generation time.