Introduction

Many scientists work in the field of entomology, few in the field of nematology and almost none in the area between. And while almost one million species of insects have been named so far, merely 20 000 nematode species have been formally described. The number of existing nematode species, however is predicted to be between one and ten million, which promises exciting findings yet to come. Sadly, mostly nematodes of financial importance (parasites of plants and animals) are being studied and the biodiversity, phylogeny and ecology of insect-associated nematodes (leaving the families Steinernematidae & Heterorhabditidae aside) is vastly unknown.

To get a better understanding of the model organism Pristionchus pacificus, we search for new strains and species in the genus but also in the nematode family Pristionchus belongs to. Thereby we also gain knowledge about the environment which the nematodes have to deal with in nature and their biology and ecology “in the wild”. This can help the colleagues studying phenotypic plasticity to judge the ecological relevance of morphological changes.

Nematodes outside Pristionchus but still inside the family Diplogastridae are an important bycatch, which help to expand phylogenies and increase our understanding of nematode biodiversity and ecology. On Reunion Island – as it harbors the world genetic diversity of P. pacificus- we built a small fieldstation for long term studies (every year since 2008) and experiments in nature.

Projects

Expanding the collection

Adding new strains, species and even genera of nematodes to our phylogenies is an indispensable requirement to get a better resolution and character polarization. Seventeen years ago, we started a long-term, large-scale screening for new nematode strains and species. Except for Australia, we have covered all continents and conducted well over 50 expeditions. We were able to add four unknown genera to the family of Diplogastridae, amongst them the outgroup to Pristionchus (Parapristionchus), the most basal genus (Leptojaccobus) and the only viviparous genus in the family (Sudhausia).

The number of described Pristionchus species is now at 51 and we are still far from saturation (Kanzaki et al. 2021). At the moment we focus on Asia for most of our collecting as it has proven to be a biodiversity hotspot for the genus Pristionchus. All new nematode strains and species established in the lab are being identified, if new to science characterized and properly described, and finally frozen in liquid nitrogen, able to be revived virtually forever. We built a large N₂ collection (ca. 40 000 samples) and provide service to the scientific community by providing strains on demand.

Molecular phylogeny of described Pristionchus species, excluding the fig associated ones

world wide collections of Pristionchus pacificus strains

Diplogastrid nematodes of insects

Presumably the whole nematode family Diplogastridae is associated with insects in a certain way. There are several examples of genera with species-specific bindings as for example Micoletzkya with bark beetles or Teratodiplogaster & Parasitodiplogaster with fig wasps. The genus Pristionchus is predominantly found on Scarabaeoid beetles but with a lower specificity. By revising genera and removing species that had been falsely grouped in “trashcan taxa” we start to understand the true nature of some nematode-insect associations. The host finding, synchronization with the host cycle and competition of nematodes are extremely complex mechanisms that have to be studied combining lab and field approaches. We were granted access to a property of the MPI for Ornithology, at only 20 min drive that consists of a large meadow surrounded by forest and a stream nearby. There we can locally set up larger field experiments, using native beetles and their Pristionchus nematodes.

La Rèunion field station

Since 2010 we have a small field station on La Réunion island in the Indian Ocean. There we can find six species of Pristionchus (including pacificus) in association with endemic and introduced beetles. All of these species are hermaphroditic. We isolated hundreds of strains of P. pacificus, which show that the island was colonized several times independently from different parts of the world. Also, the other Pristionchus species were introduced in multiple events from different areas and show that there is a clear advantage for hermaphroditic species to colonize new areas.

Being an oversea department of France (and therefore part of the EU) and harboring the world range of genetic diversity of P. pacificusplus other Pristionchus species, we can use this island as nematode source with high predictability and low administrative burden. We can also perform field experiments there as this is basically observing natural behavior of native animals and no release of foreign material. In recent experiments we studied nematodes on decaying beetles in their natural environment to elucidate phenotypic plasticity traits. We could show that all nematodes exit the dauer stage in the eurystomatous morph and that the dispersal from the beetle carcass happens in a biphasic way (Renahan et al. 2021).

References:

Kanzaki, N., Herrmann, M., Weiler, C., Röseler, W., Theska, T., Berger, J., Rödelsperger, C., Sommer, R.J. (2021) Nine new Pristionchus (Nematoda: Diplogastridae) species from China. Zootaxa 4943, 1-66.

Renahan, T., Lo, W.-S., Werner, M. S., Herrmann, M., Rochat, J., Sommer R. J. (2021) Nematode biphasic ‘boom and bust’ dynamics are dependent on host bacterial load while linking dauer and mouth-form polyphenisms. Environmental Microbiology, Feb 15. doi: 10.1111/1462-2920.15438.

Herrmann, M., Kanzaki, N., Weiler, C., Yoshia, K., Rödelsperger, C., Sommer, R. J. (2019) Two new species of Pristionchus (Nematoda: Diplogastridae) include the gonochoristic sister species of P. fissidentatus. J. of Nematology 51, 1-14.

Rödelsperger, C., Röseler, W., Prabh, N., Yoshida, K., Weiler, C., Herrmann, M., Sommer, R. J. (2018) Phylotransciptomics of Pristionchus nematodes reveals parallel gene loss in six hermaphroditic lineages. Current Biology, 28, 3123-3127.

Kanzaki, N., Herrmann, M., Yoshida, K., Weiler, C., Rödelsperger, C., Sommer, R. J. (2018) Samplings of millipedes in Japan and scarab beetles in Hongkong result in five new species of Pristionchus (Nematoda: Diplogastridae). J. of Nematology, 50, 587-610.

Yoshida, K., Herrmann, M., Kanzaki, N., Weiler, C., Rödelsperger, C., Sommer, R. J. (2018) Two new species of Pristionchus (Nematoda: Diplogastridae) from Taiwan and the definition of the pacificus species-complex sensu stricto. J. of Nematology 50, 355-368.

Herrmann, M., Kanzaki, N., Ragsdale, E. J., Sommer, R. J. (2017) Redescription of four species of the genus Pristionchus (Nematoda: Diplogastridae) from the United States of America. Dumerilia 7, 97-103.

Herrmann, M., Weiler, C., Rödelsperger, C., Kanzaki, N., Sommer R.J. (2016) Two new Pristionchus species (Nematoda: Diplogastridae) from Taiwan are part of a species-cluster representing the closest known relatives of the model organism P. pacificus. Zoological Studies 55: 48.

Meyer, J., Markov, G., Baskaran, P., Herrmann, M., Sommer, R. J., Rödelsperger, C. (2016) Draft genome of the scarab beetle Oryctes borbonicus on La Réunion Island. Gen. Biol. Evol. 8, 2093-2105.

Susoy, V., Herrmann, M., Kanzaki, N., Kruger, M., Nguyen, C. N., Rödelsperger, C., Röseler, W., Weiler, C., Giblin-Davis, R. M., Ragsdale, E. J., Sommer, R. J. (2016) Large-scale diversification without genetic isolation in nematode symbionts of figs. Science Advance 2: e1501031.

Herrmann, M., Neuner, J., Vötsch, M., Wörner, J., Kanzaki, N., Sommer, R. J. (2015) Pristionchus scratchpads - an online platform for taxonomy, systematics and phylogeny. Zootaxa 3949, 597-600.

Wegensteiner, R., Wermelinger, B., Herrmann, M. (2015) Natural enemies of bark beetles: Predators, parasitoids, pathogens and nematodes. In: Bark beetles: Biology and ecology of native and invasive species (eds. Vega F.E., Hofstetter R.W.). Academic Press, London, pp. 247-304.

Susoy, V., Herrmann, M. (2014) Preferential host switching and codivergence shaped radiation of bark beetle symbionts, nematode of Micoletzkya (Nematoda:Diplogastridae). Journal of Evolutionary Biology 27, 889-898.

Kanzaki, N., Ragsdale, E., Herrmann, M., Sommer, R. J. (2014) Two new and two recharacterized species resulting from a radiation of Pristionchus (Nematoda: Diplogastridae) in Europe. J. of Nematology 46, 60-74.

Morgan, K., McGaughran, A., Ganeshan, S., Herrmann, M., Sommer, R. J. (2014) Landscape and oceanic barriers shape dispersal and population structure in the Island nematode Pristionchus pacificus. Biol. Journal Linnean Soc. 112, 1-15.
Herrmann, M., Ragsdale, E. J., Kanzaki, N., Sommer, R. J. (2013) Sudhausia aristotokia n. gen., n. sp. and S. crassa n. gen., n. sp. (Nematoda: Diplogastridae): viviparous new species with precocious gonad development. Nematology 15, 1001-1020.

Kanzaki, N., Ragsdale, E. J., Herrmann, M., Susoy, V., Sommer, R. J. (2013) Two androdioecious and one dioecious new species of Pristionchus (Nematoda: Diplogastridae): new reference points for the evolution of reproductive mode. Journal of Nematology 45, 172-194.

Ragsdale, E. J., Kanzaki, N., Roeseler, W., Herrmann, M., Sommer, R. J. (2013) Three new species of Pristionchus (Nematoda: Diplogastridae) show morphological divergence through evolutionary intermediates of a novel feeding-structure polymorphism. Zool. J. Linn. Soc. 168, 671-698.

Kanzaki, N., Ragsdale, E. J., Herrmann, M., Roeseler, W., Sommer, R. J. (2013) Two new species of Pristionchus (Nematoda: Diplogastridae) support the biogeographic importance of Japan for the evolution of the genus Pristionchus and the model system P. pacificus. Zool. Sci 30, 680-692.

Kanzaki, N., Ragsdale, E. J., Herrmann, M., Roeseler, W., Sommer, R. J. (2013) Pristionchus bucculentus n. sp. (Rhabditida: Diplogastridae) isolated from a shining mushroom beetle (Coleoptera: Scaphidiidae) in Hokkaido, Japan. J. of Nematology 45, 78-86.

Susoy, V., Kanzaki, N., Herrmann, M. (2013) Description of the bark beetle associated nematodes Micoletzkya masseyi n. sp and M. japonica n. sp (Nematoda: Diplogastridae). Nematology 15, 213-231.

Susoy, V., Herrmann, M. (2012) Validation of the genus Rhabditolaimus Fuchs, 1914 (Nematoda: Diplogastridae) supported by integrative taxonomic evidence. Nematology 14, 595-604.

Kanzaki, N., Ragsdale, E. J., Herrmann, M., Sommer, R. J. (2012) Two new species of Pristionchus (Rhabditida: Diplogastridae) P. fissidentatus n.sp. from Nepal and La Réunion island and P. elegans n.sp. from Japan. J. of Nematology 44, 80-91.

Kanzaki, N., Ragsdale, E. J., Herrmann, M., Mayer, W. E., Tanaka, R., Sommer, R. J. (2012) Parapristionchus giblindavisi n. gen, n. sp. (Rhabditida: Diplogastridae) isolated from stag beetles (Coleoptera: Lucanidae) in Japan. Nematology 14, 933-947.

Kanzaki, N., Ragsdale, E. J., Herrmann, M., Mayer, W. E., Sommer, R. J. (2012) Description of three Pristionchus species (Nematoda: Diplogastridae) from Japan that form a cryptic species complex with the model organism P. pacificus. Zool. Science 29, 403-417.

Morgan, K., McGaughran, A., Villate, L., Herrmann, M., Witte, H., Bartelmes, G., Rochat, J., Sommer, R. J. (2012) Multi locus analysis of Pristionchus pacificus on La Réunion Island reveals an evolutionary history shaped by multiple introductions, constrained dispersal events and rare out-crossing. Mol Ecol. 21 (2), 250-266.

Herrmann, M., Sommer, R. J. (2011) The genome of Pristionchus pacificus and implications for survival attributes. In: Molecular and Physiological Basis of Nematode Survival (Eds.: R. Perry & D. Wharton). pp 86-98.

Herrmann, M., Kienle, S., Rochat, J., Mayer, W., Sommer, R. J. (2010) Haplotype diversity of the nematode Pristionchus pacificus on Réunion in the Indian Ocean suggests multiple independent invasions. Biol. J. Linn. Soc. 100, 170-179.

Mayer, W. E., Herrmann, M., Sommer, R. J. (2009) Molecular phylogeny of beetle associated diplogastrid nematodes suggests host switching rather than nematode-beetle coevolution. BMC Evolutionary Biology 9:212.

Hong, R. L., Svatos, A., Herrmann, M., Sommer, R. J. (2008) Species-specific recognition of beetle cues by the nematode Pristionchus maupasi. Evol. & Devel. 10, 273-279.

Rae, R., Riebesell, M., Dinkelacker, I., Wang, Q., Herrmann, M., Weller, A. M., Dieterich, C., Sommer, R. J. (2008) Isolation of naturally associated bacteria of necromenic Pristionchus nematodes and fitness consequences. J. Exp. Biology 211, 1927-1936.

Zauner, H., Mayer, W. E., Herrmann, M., Weller, A., Erwig, M., Sommer, R. J. (2007) Distinct patterns of genetic variation in Pristionchus pacificus and Caenorhabditis elegans, two partially selfing nematodes with cosmopolitan distribution. Molecular Ecology 16, 1267-1280.

Mayer, W. E., Herrmann, M., Sommer, R. J. (2007) Phylogeny of the nematode genus Pristionchus and implications for biodiversity, biogeography and the evolution of hermaphroditism. BMC Evol Biol 7:104.

Herrmann, M., Mayer, W. E., Hong, R. L., Kienle, S., Minasaki, R., Sommer, R. J. (2007) The Nematode Pristionchus pacificus (Nematoda: Diplogastridae) is associated with the Oriental Beetle Exomala orientalis (Coleoptera: Scarabaeidae) in Japan. Zoological Science 24, 883-889.

Herrmann, M., Mayer, W. E., Sommer, R. J. (2006) Nematodes of the genus Pristionchus are closely associated with scarab beetles and the Colorado potato beetle in Western Europe. Zoology 109, 96-108.

Herrmann, M., Mayer, W. E., Sommer, R. J. (2006) Sex, bugs and Haldane’s rule: the nematode genus Pristionchus in the United States. Frontiers in Zoology 3:14.

Non peer-reviewed articles
Herrmann, M. (2008) Eine Käferleiche zum Frühstück. GfBS Newsletter No.21
Herrmann, M., (2008) Worms on my scarabs. SCARABS. Occasional Issue Number 32; Print ISSN 1937-8343 Online ISSN 1937-8351