“Life on Earth would cease to function without insects,” says University of Georgia entomologist Michael Strand, whose passion for his work is unmistakable as he explains the importance of invertebrates to ecosystems. Strand’s research shows how basic science is fundamental to understanding the roles insects play in agriculture and disease transmission. An insect physiologist, Strand works at the intersection of genomics, molecular biology, ecology, evolution, and pest management. He has characterized invertebrate immune responses, symbiotic polydnaviruses, and insect gut microbiota and reproduction. Elected to the National Academy of Sciences in 2017, Strand reports in his Inaugural Article (1) the discovery of a previously unknown mechanism underlying growth and metabolism in a model mosquito species. Strand’s findings could lead to methods to prevent mosquito larvae from maturing into disease-transmitting adults.
Michael Strand. Image courtesy of the Board of Regents of the University System of Georgia.
Image of an adult mosquito, Aedes aegypti. Image courtesy of Jena Johnson.
Born in Norfolk, Virginia, Strand spent most of his formative years in Texas. His father was in the army, and his mother managed the household before working as a dental office technician. Strand says, “I was always interested in the outdoors and natural history, which was not of interest to anyone in my family. But I had no vision at that time for what I ended up doing for a career.” He does, however, recall a fifth-grade trip to a local aquarium. “It was like a religious experience for me, seeing all of the beautiful fish and unusual critters.”
Strand attended Texas A&M University, where his vision for his future gained clarity. He says, “My entomology epiphany occurred in 1979–1980, when I took a course in entomology and was hired as an hourly worker in the lab headed by Brad Vinson, who worked on social insects and parasite–host interactions.” Strand’s job included collecting glands from the red imported fire ant. He developed an affinity for daily laboratory tasks, and coauthored a paper with Vinson and another colleague, identifying the ant’s trail pheromone (2). Strand enjoyed working with Vinson and his postdoctorates, who encouraged Strand’s growing interest in invertebrates. He says, “I developed a particular fascination about insects with parasitic lifestyles.”
Strand remained in Vinson’s laboratory for his doctorate. He and Vinson investigated numerous species, from parasitic wasps to carpenter bees. “Brad Vinson provided me a broad perspective about insect biology,” Strand says. Max Summers, also at Texas A&M University, was another early mentor. “Summers gave me my first introductions to virology and the study of insect–microbial interactions.”
Strand visited several laboratories in the Netherlands and the United Kingdom before accepting a postdoctoral fellowship, funded by a National Science Foundation–North Atlantic Treaty Organization at the University of London’s Imperial College, in 1985. He then returned to the United States to accept an assistant professorship in entomology at Clemson University. In 1987 he joined the faculty at the University of Wisconsin–Madison as an assistant professor of entomology, and was promoted to professor 8 years later.
Strand continued studying species of parasitic wasps that are considered beneficial to humans because they prey upon agricultural plant threats. Females among certain species of these insects provide a unique model because they reproduce by depositing their eggs and an immune system-inhibiting polydnavirus into other insect hosts, such as caterpillars, which provide food for their offspring. Polyembryony, in which multiple genetically identical larvae develop from a single egg, is also common in parasitic wasps. In addition, some species of polyembryonic wasps have evolved a caste system whereby offspring from the same egg develop into two morphologically distinct types of larvae, called soldiers and reproductives.
Strand and his colleagues found that soldier larvae in mixed broods from the species Copidosoma floridanum are predominantly female and that they bias the sex ratio toward sisters by killing brothers (3). The discovery underscores the significance of sibling conflict in the evolution of reproductive strategies. Conversely, soldier caste larvae also defend clonal siblings from competitors. Their aggression, Strand found, correlates with kinship but not resource competition (4). The evolution of a caste system in polyembryonic wasps also likely arose as a consequence of unique traits associated with clonal development.
Strand’s interests evolved toward studying parasite–host and host–microbial interactions with a biochemical and molecular perspective. He credits collaborations with biologists Debbie McKenzie and Judd Aiken, both now at the University of Alberta, for helping him improve his technical expertise. Strand and colleague Louis Pech analyzed moth hemocytes, immune cells that play essential roles in defending invertebrates against pathogens and parasites. They found that two distinct types of hemocytes are required for encapsulation and destruction of foreign organisms (5). The interactions usually result in localized adhesion of the insect’s immune cells to the intruders. The researchers note that the process parallels mammalian immune responses, which involve specific adhesion mechanisms.
Symbiotic polydnaviruses function differently from most other known viruses. Studies conducted over the last two decades by Strand and his colleagues show how these viruses have evolved into obligate mutualists of parasitic wasps (6, 7). “These large, complex viruses rely on wasps for replication and vertical transmission, while wasps rely on these viruses to successfully parasitize hosts,” Strand says. “The viruses suppress host immune defenses and alter host development in ways that wasp offspring require for survival.” Assessment of the genetics of the polydnavirus Microplitis demolitor revealed that its genome is dispersed within the genome of its parasitic wasp host, and is organized to facilitate mutualism. Some insects also harbor symbiotic bacteria and bacteriophages that protect them against parasites. Strand says, “This is essentially the reciprocal scenario of polydnavirus–parasite associations.” He and his team have studied the system in aphids, for example, finding that bacteriophage loss weakens their defenses (8).
Strand was strongly influenced by biochemist John Law, whom he got to know at the University of Georgia after accepting a position there in 2001. Strand also maintains ties with many other University of Georgia colleagues, such as entomologist Mark Brown, who specializes in the study of arthropods that transmit human diseases. Brown, Strand, and colleagues have been studying mosquitoes over a decade, focusing on hormonal mechanisms that allow mosquitoes to produce eggs. In 2008, they characterized the receptor for an insulin-like peptide known as ILP3, which along with ovary ecdysteroidogenic hormone (OEH), activates mosquito egg production (9). The same year, Strand was elected to the American Association for the Advancement of Science. The following year, he received the Entomological Society of America National Recognition Award in Physiology, Biochemistry, and Toxicology.
Strand, Brown, and Kevin Vogel also identified the OEH receptor. Because most hormones bind to a single receptor, they hypothesized that an OEH receptor should be found in the genome of mosquitoes and the fruit fly Drosophilia mojavensis, given that all of these insects produce OEH. However, the fruit fly Drosophilia melanogaster does not produce OEH. The researchers identified and compared the sequences of more than 400 receptors in the genomes of the two fruit flies and three mosquito species. This effort uncovered a single gene as a candidate OEH receptor, which was then experimentally confirmed (10). The identification of the OEH receptor, a tyrosine kinase, may lead to ways to hamper mosquito reproduction.
Another line of research on mosquitoes Strand and his colleagues pursued concerns the mosquito gut microbiota. Strand says, “We noticed that if mosquito larvae lacked a microbial community in their gut, they did not grow. This wonky result captured our attention.” The researchers studied the phenomenon in three mosquito species reared under identical conditions and found that all required a gut microbiome for development (11).
Strand and his team next analyzed both field and laboratory populations of three species of mosquitoes from the southeastern United States. They found that communities of bacteria in the mosquitoes’ guts were essential for development, but that the microbiota varied greatly, depending on the insects’ habitat (12). By comparing patterns of gene expression in mosquito larvae with and without gut microbiota, the researchers found that mosquitoes without a gut microbiota had genetic alterations consistent with defects in nutrient assimilation. (13) “We wondered what living organisms, the gut microbes, could provide that dead ones could not, and respiration came to mind,” Strand says. Experiments supported their hypothesis, as aerobic respiration by bacteria was identified as a previously unknown but essential process for mosquito growth (14).
Strand’s Inaugural Article presents experimental evidence that oxygen deficiency resulting from bacterial respiration activates hypoxia-induced transcription factors (HIFs) in mosquitoes (1). HIFs, in turn, affect pathways with essential metabolic and growth functions. The researchers chemically inhibited larval growth before reversing the process to permit development. The discovery of the new mechanism for growth and metabolism in mosquitoes holds promise for future mosquito control (15).
Strand’s wife, Jena Johnson, shares his enthusiasm for entomology. “We have worked together for 25 years,” Strand says. “She is the manager of our lab, and her work is essential to our team’s success.” Their interest in invertebrates extends beyond the laboratory, as one of Johnson's skills is photographing insects, capturing their characteristics in vivid detail. To date, 60 postdoctoral fellows and graduate students have chosen to work in Strand’s laboratory, benefiting from his mentorship, Johnson’s technical support, and the couple’s passion for their chosen field.
“I’m not translationally driven, but instead am motivated to solve problems and explore cool ideas, many of which come from the talented people in my lab,” Strand says. He indicates that being an entomologist is humbling work, given the impressive biomass and incredible diversity of insects. He says, “The more we know about insects, the better we can appreciate their contribution to the planet.”
This is a Profile of a member of the National Academy of Sciences to accompany the member’s Inaugural Article on page 457 in issue 3 of volume 115.
Published under the PNAS license.
