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Genome advances peril for pests

Parasitic wasps' newly sequenced genomes open avenues for pest control; insights in evolution, genetics

Parasitic wasps kill pest insects, but their existence has been largely overlooked by the public – until now.

Four researchers from Arizona State University are among a consortium of 157 scientists (the Nasonia Genome Working Group) led by John Werren, a professor of biology at the University of Rochester, and Stephen Richards at the Genome Sequencing Center at the Baylor College of Medicine, who have sequenced the genomes of three parasitoid wasp species. The genomes reveal many features that could be useful in pest control, medicine and the understanding of genetics and evolution. The study appears in the Jan. 15 issue of Science.

Plaguing pests

Parasitoid wasps females are like "smart bombs" – they seek out specific insect, tick or mite hosts, inject venom and lay their eggs, with the wasp young emerging to devour the host insect; traits that make them valuable assets as agents for biological control.

"Parasitic wasps attack and kill pest insects, but many of them are smaller than the head of a pin, so people don't notice them or know of their important role in keeping pest numbers down," says Werren. "There are over 600,000 species of these amazing critters, and we owe them a lot. If it weren't for parasitoids and other natural enemies, we would be knee-deep in pest insects."

The three genomes sequenced are in the wasp genus Nasonia, which is considered to be the "lab rat" of parasitoid insects. The study's architects suggest that the genomes could enhance pest control by providing information about which insects a parasitoid will attack, the dietary needs of parasitoids (to assist in economical, large-scale rearing of parasitoids) and identification of parasitoid venoms. Because parasitoid venoms manipulate cell physiology in diverse ways, they may also provide an unexpected source for new drug development.

Genetic toolkit
In ASU's School of Life Sciences, Nasonia species have been utilized to conduct studies in genetics, epigenetics, male courtship behavior, evolution of speciation and social insect societies by consortium members Juergen Gadau, associate professor and associate dean for graduate studies; Stephen Pratt, assistant professor; Florian Wolschin, assistant research professor; and Joshua Gibson, doctoral student, who are also members of the Social Insect Research Group in ASU's College of Liberal Arts and Sciences. Gadau was one of eight researchers, including Werren and Richards, who developed the original Nasonia Whitepaper sent to the National Institutes of Health to encourage funding for the sequencing project in 2004.

Like the fruit fly Drosophila, a standard model for genetic studies for decades, Nasonia are small, can be easily grown in a laboratory, and reproduce quickly. However, Nasonia wasps offer an additional feature of interest: the males have only one set of chromosomes, instead of two sets like fruit flies and people.

"A single set of chromosomes, which is more commonly found in lower single-celled organisms such as yeast, is a handy genetic tool, particularly for studying how genes interact with each other," says Werren.

Unlike fruit flies, these wasps also modify their DNA in ways similar to humans and other vertebrates – a process called "methylation," which plays an important role in regulating how genes are turned on and off during development.

"In human genetics we are trying to understand the genetic basis for quantitative differences between people such as height, drug interactions and susceptibility to disease," says Richards. "These genome sequences combined with haploid-diploid genetics of Nasonia allow us to cheaply and easily answer these important questions in an insect system, and then follow up any insights in humans."

The wasps have an additional advantage in that closely related species of Nasonia can be cross-bred, facilitating the identification of genes involved in species' differences.

"Nasonia is currently the best genomic model system for understanding the genetic architecture of early speciation and complex phenotypes like behavior," says Gadau.

Mitochondrial messaging
"Because we have sequenced the genomes of three closely related species, we are able to study what changes have occurred during the divergence of these species from one another," says Werren. "One of the interesting findings is that DNA of mitochondria, a small organelle that 'powers' the cell in organisms as diverse as yeast and people, evolves very fast in Nasonia. Because of this, the genes of the cell's nucleus that encode proteins for the mitochondria must also evolve quickly to 'keep up."

It is these co-adapting gene sets that appear to cause problems in hybrids when the species mate with each other. Gadau's ASU team is among the research groups delving into these mitochondrial-nuclear gene interactions. Since mitochondria are involved in a number of human diseases, as well as fertility and aging, the rapidly evolving mitochondria of Nasonia and coadapting nuclear genes could be useful research tools to investigate these processes.

"Mitochondrial diseases in humans which have their origin in the malfunction of this interaction are the most frequent genetic disorders in humans," Gadau notes. "What we learn in Nasonia might help us to understand how these diseases work and may lead to cures."

Another startling discovery is that Nasonia has been picking up and using genes from bacteria and Pox viruses (relatives of the human smallpox virus). "We don't yet know what these genes are doing in Nasonia," says Werren, "but the acquisition of genes from bacteria and viruses could be an important mechanism for evolutionary innovation in animals, and this is a striking potential example."

Study springboard

A series of companion papers are set to be released, in addition to the Science study. One, published today in Public Library of Science (PLoS) Genetics, reports the first identification of the DNA responsible for a quantitative trait gene in Nasonia, and heralds Nasonia joining the ranks of model genetic systems. Eight more publications, authored by the ASU investigators and their colleagues, will soon follow, to be published in Heredity, Insect Molecular Biology and PLoS.

"Emerging from these genome studies are a lot of opportunities for exploiting Nasonia in topics ranging from pest control to medicine, genetics, and evolution," says Werren.

Margaret Coulombe | EurekAlert!
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