What is the phylogenetic position of Cryptosporidium?

The phylogenetic position of Cryptosporidium is elusive. Although previous studies based solely upon small-subunit (SSU) rRNA sequences suggested that the genus was an early emerging lineage among the Apicomplexa, bootstrap s upport for this placement was low. Here, the phylogenetic position of Crypt osporidium has been re-evaluated for SSU rRNA, fused SSU/large-subunit (LSU ) rRNA and six protein sequences using traditional distance-based neighbour -joining, maximum-parsimony and maximum-likelihood methods of phylogenetic reconstruction as well as the new Slow-Fast analysis, which focuses upon th e slowly evolving positions within sequences and is especially useful if a long-branch attraction (LBA) artefact is suspected. All the methods of reco nstruction indicated a trend for the early emergence of Cryptosporidium at the base of the Apicomplexa and showed that an LBA artefact plays no role i n this placement. Although the inclusion of additional numbers of neither s pecies nor genes has significantly enhanced the bootstrap support for this phylogenetic position, recent biochemical. molecular and ultrastructural da ta are congruent with it. Therefore, we favour a working hypothesis that th is genus constitutes an early emerging branch of the Apicomplexa.