COLD TOLERANCE OF AN ENDOPARASITIC NEMATODE WITHIN A FREEZING-TOLERANT ORTHOPTERAN HOST

Wetanema sp. is a nematode parasite of the hind gut of the freezing-to lerant orthopteran Hemideina maori (a New Zealand alpine weta). The pr evalence and intensity of infection remains high throughout the winter , suggesting that the parasite can survive despite the regular freezin g of the host. In the laboratory, Wetanema has survived freezing withi n its host to temperatures as low as -61 degrees C, much lower than th e supercooling point or lower lethal temperature of the weta. The free zing tolerance of the parasite is therefore much greater than that of its host. Female worms survived lower temperatures better than males a nd juveniles. Parasites might assist the freezing tolerance of their h ost if they acted as endogenous ice nucleators. However, there was no relationship between the size of the worm burden and the supercooling point of the host and no significant difference between the supercooli ng points of infected and uninfected hosts. The freezing and subsequen t survival of isolated Wetanema was observed directly on a microscope cold stage. This parasite of a freezing-tolerant host is thus also fre ezing tolerant. There are few other reports of a parasite surviving fr eezing within a living host.