Windfall apples, full of worms Few researchers would need much persuading to take a break from the lab or office on a late summer afternoon, and picnic in an idyllic, ancient orchard in the quiet countryside near Paris. What motivated Marie-Anne Félix’s visits to the orchards of Orsay and Santeuil,
however, was the opportunity to track for the first time robust, proliferating populations of C. elegans (and the related Caenorhabditis briggsae) in the wild. She and her lab team returned to survey the orchards for four seasons, and also examined additional rotting fruit and plants for Caenorhabditis across France, and are now able to report on the ecology of these populations [4]. C. elegans was found in rotting fruits of many kinds, and also in the rotting stems of herbaceous plants. It was frequently found with C. briggsae, and both species could be found in the same fruit. The nematodes were surprisingly common (20% of rotting apples from Orsay harbored nematodes) and showed reproducible seasonal changes in abundance. The rotting fruit and stems often contained many hundred animals, and all life stages were present, including rare males. As might be predicted from laboratory knowledge of the temperature preferences of the two species, C. briggsae was commoner in the warmth of the summer, and C. elegans in the cooler autumn. Félix and Duveau carried out competition experi ments to show that indeed Orsay and Santeuil C. briggsae outcompeted co-isolated C. elegans at 25°C, while the situation was reversed at 15°C. Caenorhabditis were not detected in soils, other than under the rotting fruit, or from rotting fruit yet to fall from the trees, but were isolated from molluscs and arthropods associated with the rotting food sources. It remains to be tested whether these are transport hosts for the nematodes. Importantly, it was possible to return to the same site repeatedly and recover animals each time. The world of, and in, the worm So, rather than the monoxenic environment of the agar plate, it is rich habitats like a rotting apple that C. elegans’ chemosensory system has evolved to process. C. elegans lives in a complex ecosystem of bacteria, fungi, slime moulds, hexapod arthropods (adults and larvae), mites, isopods, millipedes, pulmonate molluscs, lumbricid earth worms and other nematode species exploiting this seasonal resource. C. elegans has a fully functioning immune system, with both anti-cellular (bacterial, fungal) and anti-viral arms intact. In the laboratory, in the absence of knowledge of natural pathogens, these systems have been challenged with exotic ones, such as the agents of human disease. C. elegans can be killed by many bacterial species, sometimes through direct toxic effect but also via interference with efficient processing of food [8]. In the laboratory one of the hallmarks of pathogenic interaction between a bacterium and C. elegans is the proliferation of bacteria within the gut. The species that do this avoid lysis in the nematodes’ pharyngeal grinder, and are resistant to digestion. Interestingly, Félix and Duveau [4] found many instances of apparently healthy nematodes with distended
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intestines full of bacteria and yeasts. Whether this is a nutritive (C. elegans is deficient in sterol synthesis and must obtain sterols from food: this may be derived from yeasts) or a morbid interaction remains to be tested. Howe ver, fungal pathogens were detected, including some species that produce invasive spores, and others that make nematode-trapping rings and adhesive hyphal traps. As previously described by Félix and colleagues, many nematodes were infected with microsporidia [9], and the first-ever nematode viruses were described from these orchards only last year [10]. Killer bacteria were also isolated, including strains that can digest even the resistant cuticle of the nematodes.