Evolution of anaerobic ciliates from the gastrointestinal tract: phylogenetic analysis of the ribosomal repeat from Nyctotherus ovalis and its relatives

The 18S and 5.8S rDNA genes and the internal transcribed spacers ITS-1 andITS-2 of ciliates living in the hindgut of frogs, millipedes, andcockroaches were analyzed in order to study the evolution of intestinalprotists. All ciliates studied here belong to the genus Nycrotherus.Phylogenetic analysis revealed that these ciliates from a monophyleticgroup that includes the distantly related anaerobic free-livingheterotrichous ciliates Metopus palaeformis and Metopus contortus. Theintestinal ciliates from the different vertebrate and invertebrate hostsare clearly divergent at the level of their rDNA repeats. This argues forthe antiquity of the associations and a predominantly verticaltransmission. This mode of transmission seems to be controlled primarily bythe behavior of the host. The different degrees of divergence betweenciliates living in different strains of one and the same cockroach speciesmost likely reflect the different geographical origins of the hosts. Inaddition, host switches must have occurred during the evolution ofcockroaches, since identical ciliates were found only in distantly relatedhosts. These phenomena prevent the reconstruction of potential cospeciationevents.