The enemy of my enemy is my friend: Nematode infection of non-pollinating fig wasps in Ficus petiolaris

This blog post is provided by Justin Van Goor, Finn Piatscheck, Derek Houston, and John Nason and tells the #StoryBehindThePaper for their article Differential effects of nematode infection on pollinating and non-pollinating fig wasps: can shared antagonism provide net benefits to a mutualism?“, which was recently published in the Journal of Animal Ecology.
Justin Van Goor completed his PhD at Iowa State University studying fig nematodes in 2018. He is currently a postdoc studying sperm competition in Caenorhabditis nematodes with Eric Haag at the University of Maryland College Park. Finn Piatscheck was also a PhD student during the data collection for this manuscript and is now a postdoc working with Allen Herre at the Smithsonian Tropical Research Institute. Derek Houston was a postdoc during this data collection and is now the Thornton Chair of Biology at Western Colorado University. John Nason is a professor and Chair of the Department of Ecology Evolution and Organismal Biology at Iowa State University.
A Ficus petiolaris tree near La Paz, Baja California Sur, Mexico. Like many of its tropical relatives, F. petiolaris is a “strangler” fig, although it strangles rocks rather than other trees. (Photo Credit: John Nason)

Individual species are universally part of broader organismal community networks. The organisms within communities constantly interact with one another via ecologies ranging from obligately mutualistic to strongly antagonistic. These interactions have undoubtedly shaped the evolution of all community associates, but untangling the roles that individual species play on the complicated network with which they interact is very challenging for many systems. Developing knowledge of this complex context would allow for greater insight to how whole communities have co-evolved, and is therefore quite rewarding.

One model system capable of addressing and investigating this context is the fig-fig wasp mutualism. Most of the 750+ fig (Ficus) species worldwide maintain obligate mutualistic associations with a pollinating fig wasp species. Additionally, each fig species is associated with at least one, if not many non-pollinating fig wasps (NPFWs). The ecologies of these NPFWs vary based on species, and can function simply as freeloaders (commensals) or can directly compete with or exploit their host fig and/or pollinating wasp (antagonists). Pollinators, NPFWs, and fig seeds all grow in shared reproductive space (the fig interior), and have tightly synchronous life histories.

Idarnes flavicollis (center-right, ovipositor up) and Idarnes carme non-pollinating fig wasps on a receptive-phase Ficus petiolaris fig. (Photo Credit: John Nason)

Each fig pollinator is also associated with at least one nematode species. Depending on the genus, these nematodes can also range from commensalistic to parasitic. These nematodes share the same reproductive space as pollinators and NPFWs, and latch onto hosts as they are exiting a natal fig. Importantly, pollinators are the only wasp in most fig communities that directly enter a fig and can provide nematodes with nutrition and access to their own reproductive space. Conversely, all Neotropical NPFWs oviposit their eggs from the fig exterior, precluding nematode access to successful reproduction and subsequent dispersal. As such, there should be strong selection against nematode infection of NPFWs in favor of the “proper” pollinating wasp host.

Many adult Parasitodiplogaster nematodes forming mating aggregates outside of 10-15 dead Pegoscapus pollinating foundress wasps. This many wasps/nematodes inside of one fig is uncommon, generally each F. petiolaris fig is only visited by a single pollinating foundress with a ~40% chance of being infected with nematodes. (Photo Credit: Justin Van Goor)

Ficus petiolaris is the only endemic fig species in Mexico’s Baja California Peninsula. Along with its mutualistic Pegoscapus pollinating wasp species, F. petiolaris is associated with at least nine NPFW species. The pollinator is also subject to parasitism by one undescribed Parasitodiplogaster nematode species. The original intent of this work was to describe the effect of nematodeinfection on fig and pollinating fig wasp fitness over geographic space and time. This community context could be applied to better understand how mutualisms evolve in the face of ever-present antagonism.

Vista from F. petiolaris collection site 158, closest to El Crucero, Baja California. (Photo Credit: John Nason)

To address this (and other projects), our lab would drive a pickup truck laden with camping and sampling gear from Iowa to the states of Baja California and Baja California Sur, Mexico, where we repeatedly visited nine GPS-mapped F. petiolaris populations spanning a nearly 750km longitudinal gradient of the peninsula. Collectively, we wound up completing four of these collection trips. When sampling, we would travel to F. petiolaris populations of interest, which were often located on rancher-owned lands hours away from the closest town. We would camp out at these populations for one or two days while sampling, before driving to the nearest town where we would set up a makeshift “laboratory space” in roadside motels for observations/sample processing (much to the amusement/chagrin of the owners/housekeeping staff).

John Nason and a typical F. petiolaris arroyo habitat near Canipolé, BCS. (Photo Credit: Derek Houston)

I remember quite vividly the moment that this manuscript “came to life”. It was May 2013, and I was assessing nematode infection in mature, wasp rearing figs using my dissection microscope in our hotel room at the Malarrimo Hotel in Guerrero Negro, BCS (great food, by the way). As wasps were emerging from their natal galls, I watched juvenile, infectious-stage Parasitodiplogaster nematodes performing nictation behavior to contact wasp hosts. Surprisingly, I saw a nematode latch onto an Idarnes flavicollis female wasp. Being a NPFW, I “knew” that this was not supposed to happen, and thus I wrote it off as a fluke (or a very confused nematode). However, afterward I began to see this type of infection occurring more frequently. It seemed to be happening quite regularly, and seemingly to all NPFWs the community had to offer. I even saw juvenile nematodes infecting male NPFWs, which really shouldn’t happen considering males in most species will never leave a natal fig in their lifetime. Infection of any NPFW (male or female) constituted a reproductive dead-end for the nematodes, and yet I was seeing this behavior consistently. Why was this happening? What are the community-level consequences for this? Could nematodes be helping the fig-fig wasp mutualism by infecting antagonistic NPFWs?

Justin Van Goor examining wasp-rearing figs and nematodes in a hotel “laboratory” in Guerrero Negro, BCS. (Photo Credit: Finn Piatscheck)

To address this, we needed to first assess which NPFWs acted antagonistically with the mutualism. Through our years of collections we were able to amass the wasp contents of over 2400 figs, allowing us to make inferences through correlations. We were then able to determine how frequently wasps were infected by cutting open thousands of tiny (generally <2mm) wasps and counting the numbers of nematodes inside, if infected. By rearing out wasps in “longevity vials” we were able to determine if there was an effect of nematode infection on wasp lifespan, as well as the numbers of nematodes found inside “departing” hosts. While the nematode number in successfully dispersed pollinators is easy to deduce (because they spill out into the fig interior), we needed to be a bit more creative to find out how many nematodes were in successful NPFWs. This involved spending hours standing around receptive figs, aspirators in-hand, hoping to see NPFWs arriving so that we could collect them.

Vials used to collect F. petiolaris-associated fig wasps. (Photo credit: Derek Houston)

Ultimately, through this work and its direct predecessor (see Van Goor et al 2018); we found that while Parasitodiplogaster infection of F. petiolaris pollinators has a benign effect on wasp (and therefore mutualism) fitness, infection of NPFWs may have drastically stronger effects. Nematode infection appears to significantly limit NPFW dispersal ability, and therefore fitness. Thus, far from being “simple” parasites, nematodes in this system may provide indirect mutualistic benefits to their wasp and fig hosts.  Further, this could represent a more ecologically common phenomenon than previously described, where communities are modulated around shared reproductive, energetic, and infectious sources, especially in invertebrate-rich assemblages.

Left to right: Finn Piatscheck, Justin Van Goor, and Derek Houston during field collections at F. petiolaris site 172, closest to La Lagunita, BC. (Photo Credit: Finn Piatscheck)
Read the paper

Read the full paper here: Van Goor, J., Piatscheck, F., Houston, D.D. and Nason, J.D. (2021), Differential effects of nematode infection on pollinating and non‐pollinating fig wasps: can shared antagonism provide net benefits to a mutualism?. Journal of Animal Ecology. Accepted Author Manuscript.

Too many, too few, or empty: The number of passengers determines whether nematodes will hitchhike on a vehicle

This blog post is provided by Satyajeet Gupta and Renee M Borges from the Centre for Ecological Sciences, Indian Institute of Science, Bangalore and tells the #StoryBehindThePaper for the article “Hopping on: Conspecific traveller density within a vehicle regulates parasitic hitchhiking between ephemeral microcosms, which was recently published in the Journal of Animal Ecology.

Tiny organisms that are unable to move far by themselves or assisted by water or wind may hitch rides on other animals to disperse out of habitat patches that become unsuitable due to diminishing food resources, overcrowding or unavailability of mates. The phenomenon of using other animals to disperse is referred to as phoresy and such organisms are called phoretic organisms. Although phoresy is one of the most understudied modes of dispersal, its prevalence has been observed across highly diverse taxa. Among the classic examples of phoresy are ostracods travelling on tree frogs, pseudoscorpions hitching rides on beetles and flower mites using hummingbirds as vehicles. Similar vehicle–passenger interactions have also been observed in the 75+ million-year-old fig–fig wasp brood-site pollination mutualism.

The fig tree Ficus racemosa is a common tropical keystone plant species which shares a mutualistic association with its pollinator fig wasp Ceratosolen fusciceps. Each fig inflorescence acts as a developmental microcosm for pollinator wasps as well as three hitchhiking genera of nematodes, i.e., the herbivorous Schistonchus, the carnivorous Teratodiplogaster and the omnivorous Pristionchus. These nematodes have an obligate vehicle–passenger relationship with the pollinator wasps wherein they enter the abdomen of wasps and use them as vehicles to disperse from one fig inflorescence to another. This hitchhiking relationship is estimated to be 15–45 million years old. The pollinator wasps carry juvenile nematodes to other figs where the nematodes will mature and mate, and their offspring in turn will use the wasps to disperse out of their fig nurseries.

Pollinator fig wasp on the fig syconium surface. Photo credit: Nikhil More

In a previously published study, we showed by experimentally loading different numbers of nematodes onto wasp vehicles that the hitchhiking nematodes behave as harmless commensals at lower numbers while at higher numbers they act as parasites; such parasites negatively affect the survival and reproduction of both their fig wasp vehicles as well as of the host fig trees. These experimental results together with the natural low abundance of nematodes on wasp vehicles that successfully reach their destination suggested that the nematodes might be able to differentiate between wasp vehicles loaded with lower or higher numbers of co-passenger nematodes. We therefore asked the following questions. Do the nematodes estimate the numbers of co-travellers on vehicles before embarking on them? If yes, what are the cues involved? Do they differentiate between co-passengers of the same species (i.e., conspecifics) from co-passengers belonging to different species (i.e., heterospecifics)? Does this discrimination ability differ between carnivorous and herbivorous nematodes?

Adult diplogasterid nematodes under the microscope. Photo credit: Satyajeet Gupta

To answer these intriguing questions, we offered individual nematodes a choice between live fig wasps loaded with varying numbers of conspecific nematodes. As expected, both Schistonchus and Teratodiplogaster avoided overcrowded vehicles and chose to embark onto vehicles with lower numbers of conspecific nematodes. Since vehicles contain potential mates, therefore the presence of conspecifics as co-passengers is vital for reproduction at their destination. Surprisingly, the herbivorous nematodes preferred an empty vehicle while the carnivorous ones did not. The choice of an empty vehicle might sound absurd at first, but when we observed Schistonchus nematodes boarding the empty vehicle “in pairs”, unlike Teratodiplogaster, then we were assured that the choice is correct indeed! Hitchhiking as a pair ensured that the juvenile phoretic nematodes always had a partner to mate with at their final destination. So far, we have found nematodes of both sexes, and no hermaphrodites, but we do not know how the pre-boarding pairing occurs.
Both nematode types were unable to discriminate between live fig wasps loaded with varying numbers of heterospecific nematodes. It appears that the nematodes fail to recognize the presence of heterospecific co-passengers and treat them as empty vehicles.

Well then, how do the nematodes discriminate between vehicles carrying different numbers of conspecifics? Nematodes are known to use a combination of different types of host- specific cues such as chemical and vibrational cues that might play a role in directing them towards their carriers. Since vibrational cues potentially emitted by wasps loaded with different numbers of passengers were hard to mimic, we tested if chemical cues were responsible for the discrimination. Nematodes gather host-specific chemical information by standing on their “tails” and waving their front ends that are laden with sensory structures. This behavior is referred to as nictation. By offering individual nematodes a choice between volatiles emitted by wasps carrying a low or high number of conspecific passengers, we showed that both types of nematodes chose volatiles emitted by wasps carrying fewer passengers. This suggests that hitchhiking nematodes use olfactory cues from their vehicles and co-passengers to choose their vehicles.

The study system – fig plant, fig wasp and nematode parasites. Nematodes prefer to hitchhike on wasps with fewer conspecifics. Photo credit: Satyajeet Gupta

Picking a wasp vehicle with a large number of conspecifics on it might seem lucrative due to the promise of obtaining multiple partners to mate with later, upon reaching the destination site. However, overloaded vehicles have a lower probability of reaching their destination as we have shown before. Therefore, it is essential to choose fig wasp vehicles with low conspecific numbers of nematodes. Through this exciting study, we have thus been able to pinpoint two opposing forces of natural selection – overcrowding and the presence of conspecifics that dictate vehicle choice. The trade-off between these two factors determines which vehicle the hitchhikers will select for dispersing from one host plant to another.

Read the paper

Read the full paper: Gupta, S. and Borges, R.M. (2020), Hopping on: Conspecific traveller density within a vehicle regulates parasitic hitchhiking between ephemeral microcosms. J Anim Ecol. Accepted Author Manuscript.

Volume 85:6 a slideshow


Male Montagu’s Harrier Edwin on the hunt for grasshoppers near Djilas, Senegal. Ellinor Schlaich et al.

Issue 85:6 is now online and for the first time we have two In Focus papers in the issue as we no longer want to limit ourselves to championing only one great paper!

The First is by Pedro Jardano and takes a look at the paper by Sazatornil et al. on morphological matches and the assembly of mutualistic hawkmoth–plant networks. The second is by Shawn Wilder and Punidan Jeyasingh and they review the paper by Zhang et al. on how warming and predation risk shape stoichiometry.

To make the most of all the great photos from our authors we have included a slideshow of the best images.

Read the full November 2016 issue here.

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