The sporadotrichid ciliates are an especially diverse group. A number of investigators have studied the morphological, morphogenetic, and molecular relationships among members of this group. Despite this, a consistent classification is still lacking and several important questions about the phylogenetic relationships within this group remain unsolved. To improve our understanding of these relationships, we constructed phylogenetic trees using the nucleotide sequences of the small-subunit rRNA (SSrRNA) gene and amino acid sequences of actin I and α-tubulin. Analyses of SSrRNA gene sequences indicated that: 1) the Sporadotrichida sensu Lynn (2008) and the Oxytrichidae are polyphyletic; 2) the Uroleptus species, which are classified to urostylids, formed a sister group with the oxytrichids; 3) Halteria grandinella, which is grouped morphologically with oligotrich species, clustered within the oxytrichids. These results are congruent with previous studies based on SSrRNA gene sequences. However, the amino acid sequences of actin I and α-tubulin yielded different topologies. The main results are: 1) in all phylogenetic trees, the genus Oxytricha was paraphyletic; 2) Uroleptus was sister to a subset of Urostyla and Holosticha, albeit with low supporting values; 3) Halteria grandinella was separated distantly from the Oxytrichidae in trees inferred from actin I amino acid sequences but clustered with oligotrichids in the α-tubulin analysis. The inconsistency among the trees inferred from these different molecular markers may be caused by rapidly accumulated genetic characterizations of ciliates. Further studies with additional molecular markers and sampling of more taxa are expected to better address the relationships among sporadotrichids.
The ciliate genus Protocruzia belongs to one of the most ambiguous taxa considering its systematic position,possibly as a member of the classes Heterotrichea,Spirotrichea or Karyorelictea,which is tentatively placed into Spirotrichea in Lynn’s 2008 system.To test these hypotheses,multigene trees(Bayesian inference,evolutionary distance,maximum parsimony,and maximum likelihood) were constructed using the small subunit rRNA(SSU rRNA) gene,internal transcribed spacer 2(ITS2) and a protein coding gene(histone H4).All analyses agree that:(1) four morphotypes of Protocruzia from different geographical origins group together and form a monophyletic clade,which cannot be assigned to any of the eleven described ciliate classes;(2) it is invariably positioned on an isolated branch separated from the class Spirotrichea suggesting that this clade should be clearly removed from Spirotrichea;(3) this leads us to hypothesize that this taxon may indeed represent a lineage on a class rank.Based on the fact that it is,both morphologically and in molecular features,closely related to the heterotrichs,Colpodea and Oligohymenophorea,Protocruziida might be an ancestral form for the subphylum Intramacronucleata in the evolutionary line from the class Heterotrichea(subphylum Postciliodesmatophora) to higher taxa.
Thorsten STOECKShin Mann KyoonAl-Rasheid Khaled A. S.Al-Khedhairy Abdulaziz A.
