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Moth Genome Offers Clues into Herbivory, Pesticide Resistance

NEW YORK (GenomeWeb News) – The genome of the diamondback moth is giving researchers a glimpse into how the insect has evaded plant defenses to be able to eat them, an international team of researchers from China, Australia, Canada, and elsewhere reported in Nature Genetics yesterday.

The researchers, led by BGI-Shenzen's Jun Wang, also suggested that those same mechanisms may have allowed the moth to develop resistance to a number of insecticides.

The diamondback moth, or Plutella xylostella, feeds on crops like cabbage and cauliflower, and is resistant to many insecticides, including DDT and the Bt toxin. According to some estimates, global crop damage wrought by P. xylostella can cost between $4 billion and $5 billion a year.

"Remarkably, it appears that the very genetic adaptations that allow diamondback moth to cope with these natural compounds also allow it to detoxify the insecticides used against it," Geoff Gurr, a professor at Australia's Charles Sturt University one of the paper's authors, said in a statement.

To unravel the moth's genome, the researchers collected individuals from a strain found in southeastern China. Then, by combining whole-genome shotgun-based Illumina sequencing with the sequencing of 100,800 fosmid clones to a depth of 200x, the researchers assembled the moth genome into 1,819 scaffolds.

This draft P. xylostella genome, the researchers reported, is about 343 megabases large and is predicted to contain 18,071 protein-coding genes. Of those genes, 1,412 are thought to be specific to P. xylostella.

A number of the P. xylostella-specific genes appear to be involved in pathways related to processing environmental signals, metabolism, and chromosome replication and repair. Compared to similar species, the moth genome contains more coding genes than expected, which the researchers chalked up to expansions in certain gene families, such as sensory-related gene families.

Based on phylogenetic analysis, the researchers also identified the divergence time of insect orders to be approximately 265 million years ago to 332 million years ago, which, they noted, corresponds to about when monocotyledonous and dicotyledonous plants diverged. Additionally, they placed the divergence of P. xylostella from the other members of the order Lepidoptera — like the silkworm Bombyx mori and the monarch butterfly Danaus plexippus — at about 124 million years ago, again mirroring a divergence in plants, this time of the Cruciferae and the Caricaceae. P. xylostella, the researchers added, has been thought to have co-evolved with the crucifer plant family on which it feeds.

The researchers also examined the transcriptome of the moth and found more than 350 genes that are preferentially expressed during its larval stage, among them genes involved in sulfate metabolism. Glucosinolate sulfatase, or GSS, allows the moth to safely eat cruciferous plants as it catalyzes the conversion of plant defensive compounds into more innocuous ones.

However, in order to function, glucosinolate sulfatase has to be modified post-translationally by sulfatase-modifying factor 1. The researchers found that the SUMF1 gene is highly expressed during the larval stage at the same time that the GSS1 and GSS2 genes are.

"We propose that the coevolution of SUMF1 and GSS genes was key in P. xylostella becoming such a successful herbivore of cruciferous plants," the researchers wrote.

The moth genome also contains a large number of detoxification pathways and insecticide resistance-related genes —more than the silkworm has. Some of the gene families in those pathways, such as ATP-binding cassette transporter families and three metabolic enzyme families, appear to have also undergone expansions. And a number of transposable elements located near those detoxification genes have undergone expansions during the past 2 million years.

"The polymorphism within the P. xylostella genome might support adaptation to host plant defenses and insecticides by providing a repertoire of alternative alleles or cis-regulatory elements and genetic variations for gene expression," the researchers added.

"This project has helped identify the genes that make diamondback moth such a successful pest and will enable new insecticide resistance monitoring techniques and pest management strategies to be developed," the University of Adelaide Ramsay's Simon Baxter said.