Parasitic Relationships: Trapped in time

Analysis of specimens preserved in amber from the Cretaceous period suggests that nematodes changed their host preference towards insects with a complete metamorphosis more recently.
  1. Kenneth De Baets  Is a corresponding author
  2. Karina Vanadzina
  3. James Schiffbauer
  1. Institute of Evolutionary Biology, Faculty of Biology, University of Warsaw, Poland
  2. Department of Geological Sciences, University of Missouri, United States

Fossils trapped in amber can reveal astonishing insights about the lives of ancient organisms. Amber can preserve life forms in incredible detail, from their three-dimensional anatomy down to cellular level. It also often catches animals in the middle of an action, like copulation, feeding or pollination, thus providing valuable information about the behavior of a species.

Fossils may also reveal insights into the relationship between parasites and their hosts and how they coevolved. Parasitic relationships, where one species benefits from another – and usually to the other’s detriment – are widespread and crucial for ecosystems. A better understanding of how these relationships have changed over time is thus vital for predicting future changes to diversity (Farrell et al., 2021).

Scientists often rely on phylogenetic analysis and host-distribution data to study these interactions. However, the potential of fossils has so far been overlooked when studying the association between parasites and their hosts (De Baets et al., 2021). Now, in eLife, Bo Wang and colleagues from various research institutes in China and the United States – including Cihang Luo as first author – report how they used fossil records to study the coevolution of nematodes and their invertebrate (Luo et al., 2023).

Nematodes, also known as roundworms, can be found in most habitats across the globe and multiple lineages have independently acquired a parasitic lifestyle. Some parasitic roundworms using plants as hosts have existed at least since the Devonian period around 408 million years ago (Figure 1). The oldest lineages exploiting animals can be found as eggs in Triassic vertebrate coprolites, which may indicate that lineages exploiting invertebrates might have evolved by that time. However, fossils of intact soft-bodied worms are rare, as they usually decay before fossilization sets in. Luo et al. therefore decided to focus their attention on the Mermithidae family, a group of nematodes that eventually kill their insect hosts once they emerge, making it easy to study their life histories.

Number of reported parasitic and nonparasitic nematode fossil species from the Devonian through to Holocene.

Nematodes are roundworms that can form parasitic relationships (solid bars) with their hosts. One of the oldest known examples – nematodes that parasitize plants – dates back to the Devonian period, around 408 million years ago (ma). The figure shows reconstructions (not to scale) of the fossil nematodes Palaeonema phyticum (left) from early land plants found in well preserved fossil beds known as Lagerstätte (purple bars); Ascarites priscus (middle) preserved in Cretaceous vertebrate coprolite ca. 125 ma (red bars); and Cretacimermis incredibilis (right) exiting a bristletail preserved in Cretaceous amber described by Luo et al., 2023 (orange bars). Data updated from De Baets et al., 2021. Abbreviations: D: Devonian, C: Carboniferous, P: Permian, T: Triassic, J: Jurassic, K: Cretaceous, PE: Paleocene, N: Neogene, Q: Quaternary, Plio: Pliocene, Pleis: Pleistocene, Holo: Holocene.

Illustrations provided by Franz Anthony (CC BY 4.0).

By analyzing several fossil samples, the team could augment the number of described mermithid species from Cretaceous amber to 13, identifying nine species of mermithids with an age around 100 million years which had not been previously recorded. This doubles the currently known diversity of parasitic nematodes alive during the Cretaceous from nine to 18 species and suggests that parasitism by mermithids was rather widespread during this time, which probably helped regulate insect populations.

Luo et al. further provided the first fossil records of seven host associations, including three arthropod groups that are not recognized hosts today: bristletails, bark lice and extinct planthoppers. Additional amber samples further revealed four modern mermithid-host associations not yet known from the fossil record including dragonflies, earwigs, crickets and cockroaches.

To further explore the evolutionary relationship between nematodes and their hosts, Luo et al. looked at data records from three well-known amber periods: the mid-Cretaceous, the Eocene and the Miocene. This revealed a major shift in host preference over time. Nematodes in the Cretaceous period preferred insects that lacked complete metamorphosis, while roundworms from the Eocene and Miocene mainly used insects with a complete metamorphosis (80%).

Similar to many other parasitic worms, mermithids prefer aquatic or moist environments but have evolved a strategy to infect land-living arthropods (which would also frequent water sources) as juveniles; the nematodes then kill their hosts when approaching adulthood, so they can continue developing and reproducing in aquatic or humid soil environments (Ni et al., 2021). Due to this parasitoid strategy, mermithids have a tight relationship with their host, but their initial host preferences remain poorly known.

The study of Luo et al. adds to previous research indicating important changes in the diversity of strategies of parasitoids exploiting insects from the Cretaceous period to present day (Labandeira and Li, 2021). It is tempting to attribute this shift to a change in the diversity of host species, which saw some major arthropod lineages becoming extinct and ones appearing during the Cretaceous and the Paleogene period. However, several other factors could also contribute to host preferences, including the variety of sampled paleoenvironments or differences in the quality of preservation or collection practices (Penney, 2016; Solórzano Kraemer et al., 2018).

In the past, researchers have mainly studied amber fossils with the aim of discovering new insect species, leaving nematodes – including mermithids – and other potential hosts undersampled or understudied (Schmidt et al., 2010; Stilwell et al., 2020; Košulič and Mašová, 2019). Further investigations will require systematic sampling and global collaboration among all relevant communities and research institutions, along with sufficient funding. Moreover, since nematodes are less likely to remain fully accessible in amber, it is challenging to study morphological traits that could identify different species. Recent advancements in microscopy and tomography techniques offer promising opportunities to overcome this challenge and to visualize the full morphology of nematodes (Penney, 2016).

Mermithid nematodes, with their distinct physique and the characteristic damage they cause when exiting their hosts, serve as an intriguing model to study evolutionary host-parasite dynamics. Luo et al. show the potential of studying amber deposits to document ancient shifts in host preferences, from the Mesozoic to the present, and demonstrate the advantages of investigating ancient host-parasite relationships to better understand evolutionary patterns and estimate the extinction risk of modern species (Ni et al., 2021; Mulvey et al., 2022).


Article and author information

Author details

  1. Kenneth De Baets

    Kenneth De Baets is in the Institute of Evolutionary Biology, Faculty of Biology, University of Warsaw, Warsaw, Poland

    For correspondence
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-1651-321X
  2. Karina Vanadzina

    Karina Vanadzina is in the Institute of Evolutionary Biology, Faculty of Biology, University of Warsaw, Warsaw, Poland

    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-7952-2658
  3. James Schiffbauer

    James Schiffbauer is in the College of Arts and Science, Geological Sciences, University of Missouri, Columbia, United States

    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-4726-0355

Publication history

  1. Version of Record published: July 14, 2023 (version 1)


© 2023, De Baets et al.

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.


  • 247
    Page views
  • 38
  • 0

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Kenneth De Baets
  2. Karina Vanadzina
  3. James Schiffbauer
Parasitic Relationships: Trapped in time
eLife 12:e90008.
  1. Further reading

Further reading

    1. Ecology
    2. Genetics and Genomics
    Franziska Grathwol, Christian Roos ... Gisela H Kopp
    Research Advance

    Adulis, located on the Red Sea coast in present-day Eritrea, was a bustling trading centre between the first and seventh centuries CE. Several classical geographers--Agatharchides of Cnidus, Pliny the Elder, Strabo-noted the value of Adulis to Greco--Roman Egypt, particularly as an emporium for living animals, including baboons (Papio spp.). Though fragmentary, these accounts predict the Adulite origins of mummified baboons in Ptolemaic catacombs, while inviting questions on the geoprovenance of older (Late Period) baboons recovered from Gabbanat el-Qurud ('Valley of the Monkeys'), Egypt. Dated to ca. 800-540 BCE, these animals could extend the antiquity of Egyptian-Adulite trade by as much as five centuries. Previously, Dominy et al. (2020) used stable istope analysis to show that two New Kingdom specimens of P. hamadryas originate from the Horn of Africa. Here, we report the complete mitochondrial genomes from a mummified baboon from Gabbanat el-Qurud and 14 museum specimens with known provenance together with published georeferenced mitochondrial sequence data. Phylogenetic assignment connects the mummified baboon to modern populations of Papio hamadryas in Eritrea, Ethiopia, and eastern Sudan. This result, assuming geographical stability of phylogenetic clades, corroborates Greco-Roman historiographies by pointing toward present-day Eritrea, and by extension Adulis, as a source of baboons for Late Period Egyptians. It also establishes geographic continuity with baboons from the fabled Land of Punt (Dominy et al., 2020), giving weight to speculation that Punt and Adulis were essentially the same trading centres separated by a thousand years of history.

    1. Ecology
    Jingxuan Li, Chunlan Yang ... Zhong Wei
    Research Article Updated

    While bacterial diversity is beneficial for the functioning of rhizosphere microbiomes, multi-species bioinoculants often fail to promote plant growth. One potential reason for this is that competition between different species of inoculated consortia members creates conflicts for their survival and functioning. To circumvent this, we used transposon insertion mutagenesis to increase the functional diversity within Bacillus amyloliquefaciens bacterial species and tested if we could improve plant growth promotion by assembling consortia of highly clonal but phenotypically dissimilar mutants. While most insertion mutations were harmful, some significantly improved B. amyloliquefaciens plant growth promotion traits relative to the wild-type strain. Eight phenotypically distinct mutants were selected to test if their functioning could be improved by applying them as multifunctional consortia. We found that B. amyloliquefaciens consortium richness correlated positively with plant root colonization and protection from Ralstonia solanacearum phytopathogenic bacterium. Crucially, 8-mutant consortium consisting of phenotypically dissimilar mutants performed better than randomly assembled 8-mutant consortia, suggesting that improvements were likely driven by consortia multifunctionality instead of consortia richness. Together, our results suggest that increasing intra-species phenotypic diversity could be an effective way to improve probiotic consortium functioning and plant growth promotion in agricultural systems.