Current Research Projects
Bt Toxins Resistance in Plutella xylostella, the Diamondback Moth
Bt toxins were first identified in the bacterium Bacillus thuringiensis, and are widely used to control agricultural insect pests by crop sprays, or expressing toxin encoding genes in transgenic plants. These toxins show specificity to targeted pests, while remaining harmless to most non-target insects and vertebrates, making Bt a highly valued agricultural tool. To kill targeted pests, Bt toxins must be ingested, activated by proteases and bind to targets in the midgut where form pores which ultimately causes death.
- Project 1. How do Bt Resistant Insects Survive with Recessive Mutations?
Bt Cry1A toxin resistance is associated with mutations in an ABC transporter in several lepidopteran species. This project uses microarray analysis to compare midgut gene expression patterns of diamondback moth strains that are either resistant or susceptible to Bt toxins. The aim is to determine how resistant insects midgut cells compensate for deleterious mutations in ABCC2..
- Project 2. Is There a Fitness Cost Associated with Bt Resistance?
Measuring fitness costs associated with Bt resistance can be difficult, as different insect strains can produce opposing results. To address this question we are measuring the changes in allele frequency over time, using long term population cages. The founding population contained 50% Bt-resistant and 50% Bt-susceptible ABCC2 alleles and subsequent generations are being maintained around 200-500 adults per cage. The aim is to model the rate at which resistance may be removed from populations in the field, based on these results
- Project 3. Sequencing the diamondback moth genome
Diamondback moth has a relatively small genome size of around 339 Mb(3), and contains high levels of polymorphic variation. Generating inbred strains with low levels of heterozygosity for whole genome sequencing has been challenging. We are currently working towards a high quality diamondback moth genome sequence with collaborators in China.
- Project 4. Use Targeted Genome Squencing for Population Genetic Analysis
One of the central questions of insecticide resistance evolution concerns the origin of mutations. Does insecticide resistance occur through new adaptive mutations that arise in insect populations, or through selection on pre-existing (standing) genetic variation? Through analysing DNA sequence variation around genes that cause insecticide resistance, we are aiming to estimate the age resistance alleles.
- Project 5: Diamondback Moth Insecticide Resistance in Hawaii
Insecticide application can provide the ultimate environmental selective pressure on targeted pests: death upon exposure or survival through the evolution of resistance. The diamondback moth is a worldwide pest of brassica crops, including cabbage and canola, and is often the first species to evolve resistance to insecticides in the field (eg. DDT, Bt toxins, Spinosad)(4). Identifying genes and mechanisms causing resistance in diamondback moth can be extremely useful for predicting the evolution of resistance mechanisms in unrelated species.
This project focuses on the genetic basis for resistance to insecticides in the "Waipio" strain, collected from Hawaii. Waipio is highly resistant (324 fold) to the chemical indoxacarb compared to laboratory control strains, and insect crosses have predicted that resistance is controlled by a single autosomal gene. Furthermore, Waipio is cross-resistant to a second class of synthetic insecticide, cypermethrin, a pyrethroid (>4000 fold resistant). To identify and characterize resistance mechanisms, Illumina based genome sequencing methods will be applied. First, insect crosses will be analysed with "RADseq", a method for sequencing thousands of markers across a genome, to identify loci linked with insecticide resistance (5). Second, whole genome re-sequencing of the Waiin will be performed to search for resistance candidates and gene mutations that may be causing resistance. Third, gene expression analysis of resistance candidate genes will be compared between the Waipio strain and insecticide susceptible strains.
Please contact Dr. Simon Baxter to discuss any of these projects further.