Field studies reveal a close relative of C. elegans thrives in the fresh figs of Ficus septica and disperses on its Ceratosolen pollinating wasps

Gavin C. Woodruff, Patrick C. Phillips

Biotic interactions are ubiquitous and require information from ecology, evolutionary biology, and functional genetics in order to be understood. However, study systems that are amenable to investigations across such disparate fields are rare. Figs and fig wasps are a classic system for ecology and evolutionary biology with poor functional genetics; Caenorhabditis elegans is a classic system for functional genetics with poor ecology. In order to help bridge these disciplines, here we describe the natural history of a close relative of C. elegans, Caenorhabditis inopinata, that is associated with the fig Ficus septica and its pollinating Ceratosolen wasps. To understand the natural context of fig-associated Caenorhabditis, fresh F. septica figs from four Okinawan islands were sampled, dissected, and observed under microscopy. C. inopinata was found in all islands where F. septica figs were found. C.i nopinata was routinely found in the fig interior and almost never observed on the outside surface. C. inopinata was only found in pollinated figs, and C. inopinata was more likely to be observed in figs with more foundress pollinating wasps. Actively reproducing C. inopinata dominated early phase figs, whereas late phase figs with emerging wasp progeny harbored C. inopinata dauer larvae. Additionally, C. inopinata was observed dismounting from Ceratosolen pollinating wasps that were placed on agar plates. C. inopinata was not found on non-pollinating, parasitic Philotrypesis wasps. Finally, C. inopinata was only observed in F. septica figs among five Okinawan Ficus species sampled. These are the first detailed field observations of C. inopinata, and they suggest a natural history where this species proliferates in early phase F. septica figs and disperses from late phase figs on Ceratosolen pollinating fig wasps. While consistent with other examples of nematode diversification in the fig microcosm, the fig and wasp host specificity of C. inopinata is highly divergent from the life histories of its close relatives and frames hypotheses for future investigations. This natural co-occurrence of the fig/fig wasp and C. inopinata study systems sets the stage for an integrated research program that can help to explain the evolution of interspecific interactions.