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Protopolystoma xenopodis and Protopolystoma orientalis are polystomatid monogeneans respectively specific to the parapatric anurans Xenopus laevis and Xenopus muelleri. Parasite larval stages may invade the kidneys of foreign Xenopus spp. but die before migration to the definitive urinary bladder site. Laboratory experiments to assess the effect of a primary incompatible kidney infection on a secondary compatible infection found: (1) a small, significant decrease in the survivorship of P. xenopodis kidney stages (23-37 days p.i. at 25 degrees C) in X. laevis laevis previously challenged with P. orientalis; (2) a significant effect of prior P. orientalis challenge on P. xenopodis development and establishment in the urinary bladder of X. laevis 100 days p.i. (at 21 degrees C); (3) no effect of prior P. xenopodis challenge on adult P. orientalis establishment in X. muelleri (at 21 degrees C), but a significant negative influence on reproductive output (days 0-50 post-patency). Partial cross-resistance to heterospecifics may therefore be induced by Protopolystoma spp. infections in the kidneys of an incompatible host, demonstrating that at least some elements of the host response are non-species specific. The effects observed were weak compared to the strong host resistance known to be generated by an established compatible primary infection with respect to conspecifics. This difference suggests that strong acquired resistance to Protopolystoma species is species-specific and/or induced only by older stages surviving in compatible hosts.

Immune responses in ectothermic vertebrates and the life-processes of their parasites are profoundly linked to ambient temperature, but the functional effect of thermal environment on infectivity and host-specificity in helminths from ectotherms is poorly known. Primary infection establishment of Protopolystoma xenopodis (Monogenea) in compatible hosts ( Xenopus laevis: Anura) is strongly modulated by ambient temperature. Significantly fewer worms survived to the urinary bladder stage at 25 degrees C than at 15 degrees C (relatively high and low temperatures for this system in nature). Pre-infection exposure of X. laevis to cold (10 degrees C) did not significantly affect parasite establishment in the urinary bladder (at 15 or 25 degrees C p.i.), nor shorter-term postlarval survival in the kidneys (at 20 degrees C p.i.), suggesting that residual immunosuppressive effects on host susceptibility are not important. Low temperatures had no permissive effect on the establishment of P. xenopodis in incompatible hosts (Xenopus wittei). The link between thermal conditions and parasitic infection of ectotherms is discussed.

Protopolystoma xenopodis is an oviparous monogenean occurring as an adult in the urinary bladder of the clawed toad Xenopus laevis. Oncomiracidia invade the host's kidneys where juveniles develop, subsequently migrating to the definitive site. In central Africa, the tetraploid X. laevis occurs in sympatry with octoploid congeners, including Xenopus wittei, believed to be the hybrid derivatives of X. laevis- and Xenopus fraseri-like parental lineages. Twenty laboratory-raised, naive specimens of an X. wittei-like species from southern Rwanda were each exposed to 30 embryonated P. xenopodis eggs (at 20 degrees C) and screened for parasite egg production until 9 months post-exposure. These toads failed to support the development of gravid parasites (comparable experimental procedures produce at least 35% prevalence of patent infection in the natural host X. laevis). Further X. wittei aff. (n = 26) and X. laevis (n = 17) were exposed to P. xenopodis oncomiracidia and dissected at variable times post-exposure: larvae were able to invade the kidneys of X. wittei aff. and began feeding and morphological development. Severe mortality of juveniles occurred in both natural and unnatural hosts between invasion and 39 days post-exposure. However, while small numbers of parasites persisted in X. laevis, no stages were found in X. wittei aff. beyond 39 days. Present data demonstrate the incompatibility of P. xenopodis with X. wittei aff. and are consistent with a hypothesis that specificity in Protopolystoma-Xenopus systems is determined primarily by the ability of juveniles to complete development in the host's kidneys.