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The entomopathogenic nematodes described so far belong to the order Rhabditida (phylum Nematoda) and involve two main genera, Heterorhabditis and Steinernema, which belong to phylogenetically distant clades (Blaxter et al, 1998). These two nematode lineages are thought to have emerged independently at the same time 375 million years ago (Askary and Abd-Elgawad, 2017). The first insects appeared 395 million years ago. The genus Heterorhabditis is thought to have originated from an ancestor living in the marine environment, while the genus Steinernema is thought to have originated from an ancestor living in the terrestrial environment.

Then, Heterorhabditis and Steinernema developed, from ancestral necrophagous nematodes and independently, convergent and original strategies in the fields of symbiosis and virulence towards insects within this phylum. These strategies have resulted in two particular associations Steinernema/Xenorhabdus and Heterorhabditis/Photorhabdus (Boemare, 2002; Chaston et al, 2011).

Bibliography

Askary, T.H., Abd-Elgawad, M.M.M. 2017. Beneficial nematodes in agroecosystems: A global perspective. In Biocontrol agents: Entomopathogenic and slug. Parasitic nematodes, Abd-Elgawad, M.M.M., Askary, T.H., Coupland, J. (Eds), pp 3-25. DOI : 10.1079/9781786390004.0003.

Blaxter, M.L., De Ley, P., Garey, J.R., Liu, L.X.,  Scheldeman, P., Vierstraete, A., Vanfleteren, J.R., Mackey, L.Y., Dorris, M., Frisse, L.M., Vida, J.T., Thomas  W.K. 1998. A molecular evolutionary framework for the phylum Nematoda. Nature, 392, 71–75. DOI : 10.1038/32160.

Boemare, N. 2002. Interactions between the partners of the entomopathogenic bacterium nematode complexes, Steinernema-Xenorhabdus and Heterorhabditis-Photorhabdus. Nematology, 4, 601-603. DOI : 10.1163/15685410260438863.

Chaston, J.M., Suen, G., Tucker, S.L., Andersen, A.W., Bhasin, A., Bode, E., Bode, H.B., Brachmann, A.O. et al. 2011. The entomopathogenic bacterial endosymbionts Xenorhabdus and Photorhabdus: Convergent lifestyles from divergent genomes. PLoS One, 6, e27909.  DOI : 10.1371/journal.pone.0027909.

Poinar, G. 2014. Evolutionary history of terrestrial pathogens and endoparasites as revealed in fossils and subfossils. Adv Biol, 2014, 181353. DOI : 10.1155/2014/181353.