Saturday, October 12, 2024

Red milkweed beetle genome offers insight into plant-insect interactions

Genome of host-specialist red milkweed beetle compared to generalist relative

University of Arkansas System Division of Agriculture

Mating pair of Red Milkweed Beetles — image:
A mating pair of red milkweed beetles pictured in the spring of 2024 on the University of Arkansas campus in Fayetteville.
view more
Credit: U of A System Division of Agriculture photo by Rich Adams

FAYETTEVILLE, Ark. — Studying the secrets of how the common red milkweed beetle can safely feed on a toxic plant helps illuminate the ecological, evolutionary and economic impact of insect-plant interactions from a genomic perspective.

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.

To learn more about the Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website. Follow us on X at @ArkAgResearch, subscribe to the Food, Farms and Forests podcast and sign up for our monthly newsletter, the Arkansas Agricultural Research Report. To learn more about the Division of Agriculture, visit uada.edu. Follow us on X at @AgInArk. To learn about extension programs in Arkansas, contact your local Cooperative Extension Service agent or visit uaex.uada.edu.

About the Division of Agriculture

The University of Arkansas System Division of Agriculture’s mission is to strengthen agriculture, communities, and families by connecting trusted research to the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the nation’s historic land grant education system.

The Division of Agriculture is one of 20 entities within the University of Arkansas System. It has offices in all 75 counties in Arkansas and faculty on five system campuses.

The University of Arkansas System Division of Agriculture offers all its Extension and Research programs and services without regard to race, color, sex, gender identity, sexual orientation, national origin, religion, age, disability, marital or veteran status, genetic information, or any other legally protected status, and is an Affirmative Action/Equal Opportunity Employer.

# # #

Journal

Journal of Heredity

DOI

10.1093/jhered/esae049

Method of Research

Data/statistical analysis

Subject of Research

Animal tissue samples

Article Title

Functional and evolutionary insights into chemosensation and specialized herbivory from the genome of the red milkweed beetle, Tetraopes tetrophthalmus (Cerambycidae: Lamiinae)

Symbiotic bacterium Rickettsia affects the reproduction of a predatory insect, an effective biological control agent for agricultural pests

Peer-Reviewed Publication

National Agriculture and Food Research Organization

image:
The predatory mirid bug, *Nesidiocoris tenuis*, which preys on agricultural pests, is used as a biological control agent.
view more
Credit: National Agriculture and Food Research Organization (NARO)

Many insects are naturally infected with symbiotic bacteria, which are typically transmitted vertically from mother to offspring but are not transmitted horizontally. Understanding the effects of these symbionts is important in terms of insect pest management as they can significantly affect the biology and reproduction of insects. The predatory mirid bug, Nesidiocoris tenuis, which preys on agricultural pests such as whiteflies and thrips, is an important biological control agent. Although the symbiotic bacterium Rickettsia is often found in N. tenuis, its effects on the host have not been clarified.

A research team led by NARO and the University of Miyazaki has revealed that Rickettsia induces strong cytoplasmic incompatibility (CI) in N. tenuis. CI is a phenomenon where eggs laid by uninfected females fail to hatch when mated with infected males. In this study, mating experiments using Rickettsia-infected insects and antibiotic-treated uninfected insects demonstrated that eggs did not hatch, specifically in the combination of CI. This study newly adds Rickettsia to the list of CI-inducing symbiotic bacteria.

The genome analysis revealed that the Rickettsia strain in N. tenuis is closely related to the Bellii group, a group of symbiotic Rickettsia commonly found in insects. Notably, on the plasmids of this Rickettsia strain, there existed homologs of the CI factor gene (cif gene), which is known as the causal gene of CI in Wolbachia, another symbiotic bacterium known to induce CI. This suggests that the ability to induce CI may have been acquired through horizontal gene transfer between Wolbachia and Rickettsia, providing insights into the evolution of symbiont-induced reproductive manipulation in insects.

This discovery has significant implications for biological control which involves the reproduction of predatory insects that are used for pest management. If predatory insects do not reproduce as expected, CI could be the cause. Therefore, managing the infection status of symbiotic bacteria in the predatory insects could contribute to effective pest management in agriculture.

The research team highlighted the importance of assessing the frequency of CI in wild populations of N. tenuis. They also noted, “The wide distribution of N. tenuis and related species across Europe, Asia, and other regions offers potential for better use of predatory insects in agriculture and to explore the evolutionary origins of CI.”

Future research will focus on the mechanism of Rickettsia-induced CI, which is important for the effective management of N. tenuis as a biological control agent, as well as for a better understanding of host manipulation by symbiotic bacteria.

The mating experiment (IMAGE)

National Agriculture and Food Research Organization

Mating experiments were conducted with insects infected with *Rickettsia* and those cured of *Rickettsia* infection by antibiotic treatment. Among the four possible mating combinations, only the pairing of uninfected females and infected males resulted in 0% egg hatch rate.
Cytoplasmic incompatibility (CI) is a phenomenon where the offspring of insect species fail to develop properly when uninfected females mate with males infected with specific symbiotic bacteria.
Credit
National Agriculture and Food Research Organization (NARO)

About National Agriculture and Food Research Organization (NARO)
NARO is the core institute in Japan for conducting research and development in a wide range of fields, from basic to applied, for the development of agriculture and food industries.
For more information, visit https://www.naro.go.jp/english/index.html.