The extended phenotype: when parasite genes have phenotypic expression in their hosts
This week, an eclectic group of host-parasite interaction scientists gathered at the immaculate Janelia Research Campus for a conference titled “Mechanisms of Inter-Organismal Extended Phenotypes.” Here are the key takeaways.
The conference “Mechanisms of Inter-Organismal Extended Phenotypes,” which took place at the HHMI Janelia Research Campus on June 2–5, 2024, felt like a TV show, with talks that verged on the spectacular and awe-inspiring — akin to watching a David Attenborough nature documentary spliced with genomics and molecular biology.
The event stood out for its unique content, bringing together an eclectic mix of host-parasite interaction scientists who don’t typically converge. It was also timely, as this field — deeply rooted in spectacular biological processes — is entering a molecular mechanistic phase, fueled by the advent of accessible genomics and genetic manipulation tools. Additionally, there is a growing convergence between traditionally independent fields, such as ecological and behavioral studies with neuroscience.
The extended phenotype defined
The question arose more than once: what is the extended phenotype? The concept dates back to a 1982 book by Richard Dawkins. We, along with our colleagues Renier van der Hoorn and Ryohei Terauchi, revisited the concept in the context of pathogen virulence effectors in a review article 15 years ago:
“The concept of “extended phenotype” (i.e., “genes whose effects reach beyond the cells in which they reside”) put forward by Richard Dawkins in a classic book (Dawkins 1999) sums up perfectly this view of effectors. Effectors can be viewed as “parasite genes having phenotypic expression in host bodies and behavior” (Dawkins 1999). Indeed, effectors are the products of genes that reside in pathogen genomes but that actually function at the interface with the host plant or even inside plant cells, providing a vivid example of Dawkins’ extended phenotype.”
Zombie hosts serve as a quintessential example of extended phenotypes. Parasite-induced developmental structures are another example, leading to host phenotypes that would never occur in an uninfected host. Venom toxins and parasite effectors — functionally similar — are yet more clear examples of extended phenotypes. While the genes reside in the parasite genome, the products of these genes, often proteins, directly traffic inside a host cell and modify it.
Indeed, venom toxins and virulence effectors represent the molecular expression of the extended phenotype. It’s a truly spectacular concept to consider: the product of a gene encoded in the genome of one organism functions within another. This notion has profound conceptual implications. Consider, for instance, that the products of effector genes of plant pathogens function in fact as plant proteins since they operate inside plant cells. We like to say that the most amazing plant proteins are actually encoded by pathogen genes.
Content
• Mozzie’s saliva: the vampire’s kiss of disease
• Mozzies and their microbes: the good, the bad and the colorful
• Wasp tricks caterpillar’s neural circuitry
• Ampulex: the cockroach terminator
• Carpenter ants hijacked by a fungus
• Ants calling in sick
• Beetles squatting in ant nests
• Plastic true bugs
• Stinkbug symbiotic organ
• Scheming aphids alter plant responses
• Ecosystem engineers: plant galls and consequences
• Cynipid wasp galls: timing is everything
• Aphids on BICYCLES: mechanisms of gall formation
• How to build a grape leaf gall
• Clubroot chronicles: the root-bending microbe
• Phytoplasma tricking hoppers: bacterial matchmakers
• Zombie flies: the last of (fung)us
• Bonded for life: a beetle-fungus symbiosis
• Fruit flies: wine connoisseurs
• Insane in the brain: a virus zombie story
• Best of the rest
Mozzie’s saliva: the vampire’s kiss of disease
Blood-sucking insects — miniature vampires — like mosquitoes and sand flies carry proteins in their saliva that are injected into the host’s feeding site to prevent hemostasis and inflammation. Karin Peterson and Eric Calvo, both researchers at the National Institutes of Health/NIAID, delivered talks on various aspects of how mosquito saliva impacts the human host. Notably, the D7 salivary proteins, part of the insect odorant-binding protein (OBP) family, modulate blood feeding and parasite infection and have independently evolved in different blood-feeding insects, such as mosquitoes and sand flies.
Mozzies and their microbes: the good, the bad and the colorful
We all know about mosquitoes. They’re not just annoying; they can transmit serious diseases like malaria and Dengue fever. What is perhaps less known is their interactions with a variety of microbes, some of which may temper their capacity to transmit diseases, while others enhance it. This is the focus of research by Johns Hopkins’ @MosquitoExpert, George Dimopoulos. His team has identified bacteria that inhibit the malaria parasite Plasmodium and various arboviruses in mosquitoes.
However, not all the microbes carried by mosquitoes are beneficial. Don’t be misled by the colorful Talaromyces fungus. Dimopoulos and his colleagues have shown that one strain of this fungus, associated with the Aedes aegypti vector, actually increases susceptibility to the Dengue virus by modulating gut trypsin activity. Moreover, the fungus Penicillium chrysogenum down-regulates the mosquito midgut nitric oxide response, thereby enhancing mosquito susceptibility to Plasmodium parasites and possibly malaria transmission. Yet, much remains unknown about the interactions between fungiand mosquitoes, especially its prevalence among disease-carrying mosquito vectors.
Wasp tricks caterpillar’s neural circuitry
Shelley Adamo from Dalhousie University shared insights on how the parasitic wasp, Cotesia congregata, employs multiple mechanisms to manipulate the brain and behavior of its caterpillar host, Manduca sexta, to inhibit its feeding. The wasp intricately interacts with the complex neuronal circuitry of its host. Discover more about this fascinating case of parasitisim in this video from the Adamo Lab.
Adamo brought up the question of parasites driving the evolution of complexity in their hosts. To what extent does this happen? In this 2008 study, Marcel Salathé and Orkun Soyer discussed increased robustness in biological networks under parasite interference. They proposed that a high degree of redundancy in these networks may reflect historical interference by parasites.
Ampulex: the cockroach terminator
Frederic Libersat from Ben-Gurion University shared the incredible case of the Ampulex wasp, famously known for preying on the hapless Periplaneta cockroaches to nourish its larvae. The wasp skillfully severs one of the cockroach’s antennae, drinks the exuding hemolymph to gain energy and then ingeniously utilizes the detached area on the head as a leverage point to haul the significantly larger cockroach towards its underground burrow. Make sure to watch the horror movie below — unless you’re a cockroach, of course!
Libersat revealed how the wasp cunningly uses mimics of neurochemicals like GABA, Dopamine, and opiates to control its cockroach host. By immitating fundamental neurochemicals integral to the host’s physiology, the wasp effectively stalls the evolutionary arms race. However, the battle isn’t over; Ampulex specializes and only preys on certain cockroach species and who knows there might be some roaches that are immune to the wasp. The mechanical constraints of the stinger could hinder the wasp’s ability to prey on other insect species.
The saga of the Emerald Cockroach Wasp (Ampulex) and its zombie cockroach never ceases to amaze. Here’s another video showing the doomed roach being led to the wasp’s burrow.
Carpenter ants hijacked by a fungus
In another riveting talk on zombie hosts, Charissa de Bekker @CharissaB from Utrecht University, delved into the numerous ways the fungus Ophiocordyceps manipulates the behavior of its carpenter ant host. Intriguingly, microbes might also play a significant role. In a very recent study, de Bekker and her colleagues demonstrated that ants infected with Ophiocordyceps and exhibiting altered behaviors show significant dysbiosis in their gut bacterial and fungal microbiota. This imbalance may contribute to the host behavioral changes, such as “summiting,” where the ant climbs to an elevated position and anchors itself in place with its mandibles.
For more fascinating insights into zombie ants, check out the “Teaching and Outreach” section on the fantastic de Bekker Lab website. There is even a Virtual Reality game called the “Zombie Ant Experience”.
Ants calling in sick
Ants, those ubiquitous social insects, continue to mystify biologists. Yuko Ulrich at the Max Planck Institute for Chemical Ecology has set up her research lab around the raider ant Ooceraea biroi. Thanks to its relatively simple colony structure —it doesn’t have a queen caste — and due to its clonal nature, this ant species is readily maintained in the laboratory and highly amenable to experimentation.
How do parasites affect the social behavior of their hosts? Ulrich and her team have explored this question by studying a nematode parasite that infects the ‘social’ gland in the pharyngeal region of the brain. They discovered that infection by this parasite significantly alters the ants’ social behavior. Essentially, the infected worker ants ‘call in sick’ — staying within the nest — which may not only impact the colony’s social organization, but could also promote nematode spread within the colony. This intriguing finding highlights how infections can subtly but profoundly affect social structures in insect societies.
Beetles squatting in ant nests
Joe Parker @Pselaphinae, a researcher at Caltech, has had a passion for rove beetles (Staphylinidae) since his teenage years. Even by beetle standards, rove beetles are remarkably diverse, with the 200 million-year-old lineage comprising about 65,000 species across thousands of genera. Parker focuses on myrmecophiles among the rove beetles — species that have convergently evolved as obligate symbionts living in ant nests. This evolutionary shift from a free-living to a symbiotic lifestyle has led to extreme adaptations, including dramatic changes in social behavior and chemical communication, allowing these beetles to integrate into the social structures of their ant hosts.
Parker explored the parallel evolutionary paths of rove beetle myrmecophiles that have occurred alongside the rise of ants over the past approximately 100 million years. Particularly striking are the various ways these beetles circumvent ant aggression through sophisticated chemical and behavioral adaptations. Sceptobius beetles exhibit remarkable integration into velvety ant nests by secreting a chemical that pacifies the ants, facilitating the beetles’ grooming and theft of their cuticular hydrocarbons. Paradoxically, the beetles rely on their ant hosts for survival, creating a symbiotic deadlock — a catch-22 scenario — that tightly binds the beetles to an interdependent lifestyle with the ants.
Watch Joe Parker’s fascinating talk on how beetles deceive ant societies.
Plastic true bugs
At the University of California, Riverside, Kerry Mauck investigates the world of Hemiptera, or true bugs, a large order of insects that includes many agronomically important pests such as aphids, whiteflies, and leafhoppers. Mauck is captivated by the ability of many species to switch from one host to another, a phenomenon known as phenotypic plasticity. Her lab is exploring how associations with symbiotic bacteria may influence host-vector interactions, particularly the capacity to switch hosts. In another study, Mauck’s lab demonstrated how a symbiotic bacterium affects host and vector interactions and investigated the role of plant volatiles in mediating host choice.
Stinkbug symbiotic organ
The complex relationship between hemipteran insects and bacteria presents another fascinating example of symbiosis. At Tsukuba, Takema Fukatsu @fkttkm showcased the sophisticated bacterial symbiosis of plant-feeding stinkbugs (Hemiptera: Pentatomoidea). Most stinkbugs possess a symbiotic organ in their midgut, which hosts a dense population of specific symbiotic bacteria. Fukatsu and his team have explored the diversity and evolution of the bacteria residing in this organ, uncovering a dynamic pattern of recurrent gains and losses of symbionts.
The brown-winged green stinkbug Plautia stali typically harbors a bacterial symbiont associated with its essential gut symbiont, the Enterobacteria Pantoea sp. However, Fukatsu and his colleagues have demonstrated that Escherichia coli is also potentially capable of forming a symbiotic relationship with the stinkbug, even managing to colonize the symbiotic organ.
In an impressive tour de force, Fukatsu’s team utilized hypermutating strains of E. coli to independently evolve new mutualists that not only support higher levels of adult emergence of the insect host but also enhance its body color. This groundbreaking work establishes an experimental system for probing fundamental questions about how host-microbe symbioses evolve and allows for the testing of specific hypotheses concerning these intriguing bacterial-insect mutualistic interactions.
Stay tuned for more work on this and related topics. Be sure to check out the newly funded project of Takema Fukatsu and his colleagues, “Emergence of Extended Phenotype,” and follow their progress @extended_pheno.
Scheming aphids alter plant responses
In another insightful presentation on hemipteran bugs, Gustavo MacIntosh @DGusmac from Iowa State University explored how soybean aphids manipulate plant responses to induce susceptibility. Aphids, like other plant parasites, trigger immune responses in their hosts. Consequently, they have evolved mechanisms to suppress and evade these defenses. MacIntosh’s research employs omics technologies, alongside variations in host genetics, to delve into these complex interactions.
Ecosystem engineers: plant galls and consequences
Many parasitic organisms induce the formation of galls in their host plants. No fewer than 30,000 arthropod species, along with thousands of other parasites and microbes, induce galls. Galls are abnormal growths that can appear on leaves, stems, or roots. They can serve various functions for the invading organism, such as providing a protective environment that enhances the survival and growth of the parasite, a nutrient-rich site for feeding and many other adaptive functions.
The conference features several talks that delved into galls and other forms of developmental rewiring caused by parasites and pathogens. University of New Mexico’s Ellen Martinson @wasp_venom studies several aspect of insect-induced galls. She described how Aciurina galling flies (Diptera; Tephritidae) act as ecosystem engineers, fostering the establishment of a rich and multi-trophic community of arthropods.
Cynipid wasp galls: timing is everything
University of Edinburgh’s Graham Stone, who has spent a lifetime criss-crossing the globe studying wasp galls, emphasized the importance of developmental timing in the various extended phenotypes of oak gall wasps (Hymenoptera: Cynipidae). Stone, along with Jack Hearn and colleagues, has now embarked on the genomic dissection of the oak galling phenotype, revealing the complexity of molecular processes associated with these wasp-induced plant structures.
Stone’s research in China further showcases the incredible complexity of gall ecosystems. At the conference, he sported a T-shirt featuring the multitude of wasp species associated with just one type of gall found on Mount Emei in Sichuan, China, illustrating the rich biodiversity these galls support.
Aphids on BICYCLES: mechanisms of gall formation
What are the molecular mechanisms behind gall formation? Wasps and other hymenopteran insects boast sophisticated venom glands integrated into their stingers, which evolved from their ovipositors to aid in predation and defense. These venom stingers have driven the remarkable evolutionary success of these insects, including the evolution of social castes. However, the role of venom glands in gall induction, particularly in adult females, remains uncertain. Antoine Guiguet from Naturalis Biodiversity Center in Leiden, who attended the conference, described in a 2023 paper a significant expansion of secretory organs in gall-inducing wasps, suggesting their involvement in gall formation.
In contrast to hymenopteran insects, insects like flies, midges, and aphids lack stingers and venom glands, relying instead on their salivary secretions to induce galls and manipulate their hosts — similar to the mechanisms used by mosquitoes. The specific molecular signals that trigger gall formation in these insects are not well understood. A major breakthrough came from the work of HHMI’s David Stern, who hosted the conference at the Janelia campus. Stern and his team leveraged a color polymorphism (red vs. green) observed on the grounds of Janelia Campus in the galls induced by the aphid Hormaphis cornu on witch hazel (Hamamelis virginiana) leaves. Through genetic association studies, they identified an aphid gene — determinant of gall color (dgc) — that up-regulates seven host plant genes involved in anthocyanin synthesis, leading to the accumulation of these pigments in the galls.
The dgc gene encodes a small secreted protein that belongs to a very large and diverse family of salivary proteins Stern and colleagues named BICYCLE proteins based on the two conserved CYC motifs. They proposed that BICYCLE effectors act as bona fide effector proteins secreted by aphids to enable gall development and potentially other effector activities.
How to build a grape leaf gall
Paul Nabity from the University of Melbourne is investigating the genes of the aphid-like grape phylloxera Daktulosphaira vitifoliae (Hemiptera; Phylloxeridae). Grape phylloxera, a pest that devastated vineyards across Europe in the 19th century, dramatically altered viticulture practices and wine production worldwide. This hemipteran insect induces the formation of leaf galls on native American Vitis species. Armed with the D. vitifoliae genome sequence, Nabity now has access to a comprehensive list of candidate secreted proteins, which he is actively assaying to identify their potential effector activities.
Clubroot chronicles: the root-bending microbe
Microbes can also cause galls and other deformations in their host plants. Edel Pérez-López @Edel_PLopez at the University of Laval introduced Plasmodiophora brassicae, a soil-borne protist in the Rhizaria lineage that forms galls or club-like structures, disrupting the normal root development of its host plants in the Brassicaceae (cabbage) family. Equipped with a telomere-to-telomere genome sequence of P. brassicae, Pérez-López has been actively expressing candidate effector genes in plants, leading to a variety of developmental changes observed in the host plant.
Phytoplasma tricking hoppers: bacterial matchmakers
Saskia Hogenhout @SaskiaHogenhout from the John Innes Centre reviewed the world of phytoplasma extended phenotypes. These bacteria transform their host plants into sterile “zombie” plants by inducing profound developmental changes through small secreted effector proteins known as SAPs. These effectors not only alter the development of their host plants but also extend their influence to modify interactions between the infected plant and the leafhopper insect vector. For example, SAP effectors can trigger leaf-like tissue proliferation, delay plant aging, and attract more insect vectors.
In a recent study, Hogenhout and her team reported that the Aster leafhopper, Macrosteles quadrilineatus, shows a preference for ovipositing on plants with leaf-like flowers. A single phytoplasma gene, encoding for the SAP54 effector, is enough to induce leaf-like flowers, a process involving the degradation of multiple MADS-box transcription factors via the 26S proteasome. Moreover, the SAP54 effector makes the host plant more attractive to the leafhopper. Intriguingly, the leaf-like flowers are not required for the leafhopper attraction. Instead, modulation of leaf processes by the MADS-box transcription factor SVP is involved.
Moreover, phytoplasmas also play the role of insect matchmakers; their infected plants enhance insect vector colonization through a male-dependent attraction mechanism of females. This research highlights the long reach of a single parasite genes and paves the way for new insights into how transcription factors directly or indirectly regulate plant-insect interactions.
Zombie flies: the last of (fung)us
The manipulation of host behavior by parasites was a recurring theme at the conference. However, understanding the molecular mechanisms behind this requires a tractable host system. Effector biology is host biology. Similarly to how the clubroot and phytoplasma studies benefited from a manageable model host plant like Arabidopsis, Carolyn Elya @homiec17 from Harvard University has developed a laboratory system for studying zombie insects that is conducive to molecular experimentation.
Elya discovered in her backyard that some fruit flies, Drosophila melanogaster (Diptera; Drosophilidae), were exhibiting symptoms indicative of a fungal infection, essentially turning them into zombies. This observation led to the establishment of a fruit fly model system with the fungal pathogen Entomophthora muscae, which now underpins her lab’s research at Harvard. Check these spectacular videos documenting the fruit fly zombie behavior.
Elya and her colleagues recently completed sequencing the genome of Entomophthora muscae, which turned out to be a massive ~1 Gb in size, as a result of an extensive Ty3 retrotransposon activity. Stay tuned for some exciting findings on how this fungus modifies the behavior of its insect host.
And make sure to check out Carolyn Elya’s lab cool website and videos. They’re a bit obsessed with zombies — and rightly so!
Bonded for life: a beetle-fungus symbiosis
Fungi don’t always spell doom for their insect hosts. At the University of New Mexico, Vince Martinson — aptly known on X/Twitter as @bugs_in_bugs — has revived interest in a beetle-fungus symbiosis that was overlooked for 40 years.
The Drugstore beetle and the Cigarette beetle (Coleoptera; Anobiidae) harbor specialized cells (mycetocytes) in their digestive tracts that are colonized by symbiotic fungi (Symbiotaphrina; Pezizomycotina), which provide essential B vitamins and sterols to their beetle hosts. Unlike most symbiotic relationships where transmission is intracellular, Symbiotaphrina are transmitted extracellularly across generations on the surface of eggs, yet they live intracellularly in larvae and adults. Each generation, beetle mycetocytes are newly infected by the symbiont. This unique transmission mode allows both beetles and fungi to be grown in axenic culture, facilitating the experimental establishment of novel host-symbiont pairs. This distinctive feature of beetle-fungus mutualisms made them pivotal in the early days of symbiosis research. Stay tuned for some cool extended phenotype stories on this unique experimental system.
Fruit flies: wine connoisseurs
Sophie Caron @sophiejccaron at the University of Utah studies the world of the fruit fly Drosophila melanogaster, known for its penchant for wine and other yeast-fermented products. In her innovative research, Caron explores how fruit flies eavesdrop on yeast quorum-sensing signals to pinpoint their food sources. It’s fascinating to consider how the extended phenotype of winemaking yeast extends its influence far and wide, even impacting the behaviors of these tiny, wine-loving insects.
Insane in the brain: a virus zombie story
The best way to wrap up these vignettes is with a tale about viruses, the ultimate host manipulators, living directly inside host cells. Vera Ros @VeraIDRos gave an engaging talk on how baculoviruses invade caterpillar brains to alter their behavior. These viruses induce the wonderfully named Wipfelkrankheit syndrome — German for “tree top disease.” Ros and colleagues have pinpointed the specific viral genes that modify various caterpillar processes. It turns out that baculoviruses employ multiple mechanisms to alter host behaviors, leading to symptoms like hyperactivity, altered phototaxis, and an inclination to move towards elevated positions.
Best of the rest
There were many other fascinating talks at the conference, but alas, we couldn’t cover them all. For those eager to delve deeper, check out the X/Twitter hashtag #ExtendedPhenotypes2024 and follow the link to the conference program for the complete list. Highlights included engaging discussions on male killer genes, plants that kill other plants, “Russian dolls,” lady beetles, choanoflagellates, daphnia, and much more. Given the rapid progress in the field, it’s clear there’s a strong case for reconvening in a couple of years to catch up on the latest developments.
Acknowledgments
A big thank you to David Stern, who came up with the vision for the event and thanks to his wide-reaching connections brought together this disparate mix of scientists, and to the other organizers and participants for their positive spirit and great presentations and discussions that inspired this article. Thanks also to Antoine Guiget and other participants for clarifying a number of points. Apologies to those whose work we were unable to cover. The article was written with assistance from ChatGPT.
This article is available on a CC-BY license via Zenodo.
Cite as: Kamoun, S., & Hogenhout, S. (2024). The extended phenotype: when parasite genes have phenotypic expression in their hosts. Zenodo https://doi.org/10.5281/zenodo.11541224
