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Yeasts and Frogs

The Interaction Between the Yeast Rhodotorula, Frogs & Mosquitos - A Case Study

Depending on your point of view, the interaction between fungi and their insect host can be seen as symbiotic if the fungus is found within the gastrointestinal tract, pathogenic if the insect appears to be deleteriously affected, or commensal if the insect appears unaffected. Determining the nature of the interaction becomes extremely complex when other organisms are involved. In this case study, one fungus possibly symbiotic with one host appears to be pathogenic with a second.

Alison Mokany, University of Sydney, examined the interactions between mosquitos and frogs in freshwater. During this research she observed the presence of the yeast Rhodotorula glutinis in her experiments. The fungus lives in the gut of its host frog Limnodynastes peronii where it is probably a beneficial symbiont. LINK The fungus is excreted in faeces into the water by the frog. When in water, the fungus has a direct effect on the rate of development of co-occurring larvae of the mosquito Culex quinquefasciatus. In the presence of excreta from the frog, but not the frog itself, fewer female larvae survive, fewer larvae pupate, and adult females are smaller. The observation indicates that either the yeast, or a metabolic product, acted on female, but not male, larvae of the mosquito. When fungicide was added to the water, female larvae grew normally, which indicates that the yeast may be acting directly. Alison speculated that the yeast causes low-level disease.

The basidiomycetous yeast, Rhodotorula glutinis is found widely in fresh and salt-water bodies, soil and oxygenated sediments, though in high densities only in nutrient-rich environments. Members of the genus are widely recognised as inhabitants of the phylloplane, especially in nectaries and on pollen, where they use simple carbohydrates exuded by their host. Some species are known to be non-fermentative spoilage organisms of stored foodstuffs, including molluscs and fish. Some species of Rhodotorula are reported to grow at very low pH indicating that they can preferentially occupy nutrient rich acid conditions, such as the gut of vertebrates and insects. As they are found in seawater and marine sediments, they can tolerate conditions of low osmotic potential. The gut of marine animals is normally colonised by Rhodotorula in their natural habitat. In their natural environments, Rhodotorula also appears to lack secondary metabolites associated with induction of disease. Rhodotorula can be associated with disease induced by other organisms, though it is unlikely to be the initial causative agent. Thus the aquatic association between larvae of mosquitos and Rhodotorula is very different to other associations of the yeast seen elsewhere.

The reduction in survival, growth and development of female but not male larvae indicates that a specific response to the presence of the fungus is triggered. In different environments, fungi are known to excrete specific signal molecules that are detected by other organisms. Being in water, the molecules would need to be extremely active at low concentrations thus accounting for the response in the experiments reported above, where few experimental animals and insects were used. It seems unlikely though, that males and females would have different responses to the same molecule. It is also possible that the larvae consumed the yeast cells, and that the response of the host was a more direct interaction. The difference between male and female might then be explained by different feeding behaviours: male larvae need not be attracted to the fungus, or may consume different foods. The interaction requires much more detailed examination.

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Reference

Mokany A. 2001. Ecological and behavioural interactions between tadpoles and mosquito larvae. Honours thesis, School of Biological Sciences, University of Sydney.

Spencer JFT, Spencer DM. (eds) Yeasts in Natural and Artificial Habitats. Springer.

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