Red milkweed beetle genome offers insight into plant-insect interactions

Although the relationship between the red milkweed beetle and milkweed plants has been studied for nearly 150 years, an Arkansas Agricultural Experiment Station scientist joined colleagues at the University of Memphis and the University of Wisconsin Oshkosh to do what no one else has done -- curate the beetle's genome and its arsenal of genes related to plant-feeding and other biological traits.

With support from the National Science Foundation, they sequenced and assembled the entire genome of the host-specialist milkweed beetle (Tetraopes tetrophthalmus). They then compared aspects of genome biology to a relative, the host-generalist Asian longhorned beetle (Anoplophora glabripennis), which is an invasive exotic species that feeds on a variety of trees important to forestry.

Their study, "Functional and evolutionary insights into chemosensation and specialized herbivory from the genome of the red milkweed beetle," was published in the Journal of Heredity by the American Genetic Association this summer.

"From a biological standpoint, there is a lot of correspondence that suggests that longstanding interactions between milkweed beetles and their toxic milkweed hosts should influence the biology of both interacting partners," said Rich Adams, a lead author of the study. "But, to date, no one had assembled a milkweed beetle genome, which opens the door for targeting a lot of interesting questions at the interface between insect and plant."

Adams is an assistant professor of agricultural statistics in the department of entomology and plant pathology for the University of Arkansas System Division of Agriculture. He is also a member of the Center for Agricultural Data Analytics, a new initiative of the experiment station, and he teaches statistics courses in the Dale Bumpers College of Agricultural, Food and Life Sciences.

Scientific development

Milkweeds and milkweed beetles (genus Tetraopes) have been studied as valuable models for over a century of research into ecology, evolution, developmental biology, biochemistry of toxins and more, Adams said. They are also providing an interesting and compelling case of co-divergence patterns between insect and plant -- meaning the plants and insects share similarities in the timing of co-evolution across their histories of interaction, Adams explained.

The research team showed that the red milkweed beetle has an apparent expansion of genes from the ABC transport family, which may help them feed on milkweeds and sequester its toxins inside beetle tissues. Milkweeds are renowned for their toxic latex cocktails, which affect the balance of sodium, calcium and potassium that keeps heart cells pumping. Adams said this genome provides insights into the genes the beetle has evolved to safely interact with its toxic milkweed hosts.

"Milkweeds produce a particularly nasty type of toxin called cardiac glycosides alongside other types of toxins that come with it," Adams said. "For many insects that eat it, the toxin will block their sodium-potassium pumps. But this beetle developed a way to not only resist the toxin, but also sequester it, hold on to it, to keep the beetles themselves safe from would-be predators."

The study also pinpointed differences in genes responsible for smell, taste and metabolic enzymes that degrade the plant cell well. Adams said it provides a new vantage point for exploring the ecology and evolution of specialized plant-feeding in longhorned beetles, and other plant-eating beetles.

Applications in agriculture, human health

These findings may help us understand and identify the genetic factors that shape agricultural and forestry pests and allow them to successfully feed on plants, as well as evade control efforts. Most animals that can digest woody plant material depend on microbes in their gut to break down plant cell walls; however, many plant-eating beetles do not.

Adams said many plant-feeding beetles, including longhorn beetles, acquired the ability to break down plant cell walls through horizontal gene transfers from microbes. By looking at the diversity of proteins encoded within beetle genomes, he said scientists can learn about the genomic basis of beetle biology, evolution and diversity, as well as their propensity for interactions with plants.

"Nature has made an incredible diversity of genes and genomes already out there that we have not yet deciphered," Adams said. "Understanding this diversity holds great promise for informing agriculture, forestry and human health. Herbivorous beetles would have a difficult time feeding on plants without their metabolic enzymes, because they can't eat effectively without them."

In addition to studying the genomic DNA of the milkweed beetle, the team collected RNA from male and female red milkweed beetle antennae to learn more about how they seek out mates and food through chemosensation.

"Learning more about chemosensory biology -- how an organism senses its environment, like sensing a host plant or reproductive partner -- has broad relevance for understanding insect-plant interactions, which is intensively relevant to agriculture and forestry," Adams said.

The RNA profile provided the first transcriptomic resource for Tetraopes. A transcriptome contains a range of genes that are transcribed into RNA molecules an organism expresses in a tissue or set of cells.

The DNA provides a gene sequence, the RNA offers "a better resolution of the gene and its expression, including how often the gene is getting made," Adams explained.

Co-authors of the study included Terrence Sylvester (also a lead author) and Rongrong Shen, postdoctoral researchers at the University of Memphis with Duane D. McKenna, William Hill Professor in the department of biological sciences and director of the Center for Biodiversity; Matthew A. Price, formerly with the University of Wisconsin Oshkosh and now with the University of Hawaii at Manoa; and Robert F. Mitchell, formerly at the University of Wisconsin Oshkosh and now associate professor in the department of entomology at Pennsylvania State University.

The study was funded by National Science Foundation grants DEB-1355169 and DEB-2110053.