Differences in biological characteristics of two strains of the cestode Hymenolepis diminuta.

Biological characteristics of infectivity, growth rate and fecundity of Hymenolepis diminuta isolated from wild Rattus rattus in Japan were compared with parasites of Texas origin maintained for several generations in this and many other laboratories in laboratory bred Rattus norvegicus. The timing of development and maturation was similar in parasites from both sources, but the mean parasite dry weight was less and the mean egg production lower in Japanese parasites in both single and multiple infections. The differences persisted over 10 weeks in single infections, and were unaffected by rat strain. In all experiments there was much greater variation and heterogeneity in the biological characteristics of the Japanese parasites. It was concluded that the Texas parasites were better adapted to R. norvegicus and in the course of adaptation had become more homogeneous, and that the difference between the parasite strains had a complex genetic basis. Some Japanese parasites of the early isolated generations failed to produce normal eggs, but the proportion of these decreased in later generations. However, selected individual Japanese parasites whose egg production was equal to that of parasites of Texas origin did not produce offspring of larger mean size or higher mean fecundity, and heterogeneity of these characteristics was maintained. In the intermediate host, the mean number of cysticercoids of the Japanese isolate per beetle was higher than that of the Texas strain in Tribolium confusum but lower in Tenebrio molitor. The greater heterogeneity exhibited by Japanese parasites and the genetic basis of the strain difference are discussed with particular reference to geographical isolation, differences in definitive hosts and co-evolution of wild and laboratory host and parasite populations.