Mutual exclusion of congeneric monogenean species in a space-limited habitat.

Adults of the monogenean genus Protopolystoma infecting Xenopus species occur in an extremely space-limited habitat, the urinary bladder. Xenopus wittei, from a population in Rwanda naturally infected with Protopolystoma fissilis and Protopolystoma simplicis, were exposed to reinfection in captivity (for 1-3 months post-capture) and then monitored in the laboratory for up to 5 months in transmission-free conditions. The two parasites co-occurred in individual bladders less frequently than expected if they were dispersed randomly. Distribution of bladder infections was significantly non-independent (n = 157) and gravid worms of both species were never found in the same host. This pattern might be explained by interference competition between the parasites or by genetic differences in susceptibility within the host species, which is of allopolyploid origin. Other distributional data for sympatric polystomatid species pairs, including P. fissilis and P. ramulosus, show concurrent infections at frequencies consistent with random distributions (i.e. no evidence of interspecific competition or variability in species-specific susceptibility of the hosts). Interference between P. fissilis and P. simplicis (assuming host genetic factors are not involved) may therefore result from a mechanism specific to this species pair. Observations on infection turnover in captive hosts suggest that loss of adult worms may be related to the arrival of juveniles (of either species) in the urinary bladder. Ectopic infection of the host urinary ducts by adult and subadult P. fissilis was observed in some single-species infestations and may be density related. However, the use of an ectopic-site 'refugium' has never been observed in concurrent polystomatid infections.

Action of babycurus-toxin 1 from the east African scorpion Babycurus centrurimorphus on the isolated cockroach giant axon.

A toxin named babycurus-toxin 1 (mol. wt 8191), from telson extracts of the scorpion Babycurus centrurimorphus, was found to depolarize the cockroach giant axon. It progressively blocked the evoked action potentials after a short period of limited repetitive activity and after 30 min of toxin action it became impossible to evoke responses to current stimulations. Voltage-clamp experiments on the sodium current indicated that the toxin in micromolar concentrations progressively decreased the transient inward peak sodium current, but also slowed the activation phase of this sodium current and maintained an inward current during the voltage pulses, which deactivated slowly. The toxin also induced in the insect axon a slowly activating-deactivating component of the sodium current. This suggests that the toxin modifies both activation and inactivation mechanisms of sodium channels. Thus there is some similarity in the electrophysiological effects between BcTx1 and the beta-toxins active on mammals.