Species-specific differences in heterogonic development of serially transferred free-living generations of Strongyloides planiceps and Strongyloides stercoralis

The direction of free-living development (homogonic vs. heterogonic) in Strongyloides stercoralis and Strongyloides planiceps was examined by successive transplantation of the uterine eggs of free-living females into a test tube culture system containing fresh feces. The eggs from the first-generation free-living females of S. stercoralis did not develop into second-generation free-living adult worms, but all developed into filariform larvae. However, the majority of S. planiceps eggs from the first-generation free-living females developed into second-generation free-living adults. By successive transfer of uterine eggs of each generation, 9 generations developed, and in every cycle more adult worms developed than filariform larvae. However, the number of free-living generations was not infinite; in experiments repeated twice, the number of worms developing from the eggs of eighth or ninth generations was too small to continue further culture. These findings indicate that the pattern of free-living development is different between the 2 species.