The free-living generation of the nematode Strongyloides papillosus undergoes sexual reproduction

The nematode genus Strongyloides consists of parasites that live as parthenogenetic females in the small intestines of their hosts. They can also form a facultative free-living generation with males and females. Recently, research on Strongyloides cellular and molecular biology has concentrated on Strongyloides ratti and Strongyloides stercoralis. We propose that the related nematode Strongyloides papillosus, a common parasite of ruminants, is well suited for comparative and evolutionary studies and we show that it is phylogentically basal to S. ratti and S. stercoralis. Based on cytological observations several reports have proposed that Strongyloides males do not contribute genetically to the next generation, leaving open the question of why males still exist. In contrast, the only study employing molecular markers showed that S. ratti males do pass on genetic material. Here, we demonstrate that in S. papillosus males also contribute molecular genetic markers to the next generation. This is interesting for two reasons. First, it shows that S. papillosus is amenable to genetic analysis and second, it indicates that sexual reproduction is more common in Strongyloides than previously assumed.