Parasitic The Walk Ciliates Taxonomic And Molecular Studies

Mobilids are one of the most common parasitic protozoans,which have special structure of adhesive disc,and can swim freely.Based on the characteristic of the oral apparatus,mobilids are assigned to the order Mobilida within the subclass Peritrichia. It is widely accepted that Mobilida evovled from telotroch-like stage of sessilida,the other order within subclass Peritrichia.Following the application of molecular methods,the phylogenical position of the mobilids and the monophyly of Peritrichia have recently been questioned.In this dissertation,we studied the morphology and the molecular phylogeny of mobilids.1.The mobilids is a ciliate group commonly parasitized on marine invertebrates, especially molluscs.Five species of mobilids,isolated from the gills of five marine invertebrates from aquaculture beds off the coast of Shandong Province,China,were studied using live observation and protargol-impregnation.These are Leiotrocha serpularum from Serpula sp.,Ureceolariapatellae from Saxidomus purpuratus, Urceolaria korschelti from Solen strictus,Urceolaria urechi from Urechis unicinctuswere,and Trichodina sinonovaculae from Sinonovacula constricta.Former four species are the first records in China.Comparisons with related species/populations are provided.2.Phylogenetic relationships between mobilids and sessilids based on SSrRNA gene and alpha-tublin gene sequences.Based on the characteristic of the oral apparatus, mobilids are assigned to the order Mobilida within the subclass Peritrichia.It is widely accepted that Mobilida evovled from telotroch-like stage of sessilida,the other order within subclass Peritrichia.Following the application of molecular methods,the monophyly of Peritrichia has recently been questioned.We investigated the phylogenetic relationships of the peritrichous ciliates based on seven further complete small subunit ribosomal RNA sequences of mobilids and three alpha-tublin gene sequences.In all phylogenetic trees,the mobilids never clustered with the sessilds.We therefore formally separate the mobilids from the sessilids(Peritrichia sensu stricto) and establish a new subclass,Mobilia(Kahl,1933) n.subcl.,which contains the order Mobilida Kahl,1933.We argue that the oral apparatus in the mobilians and sessilid peritrichs is a homoplasy,probably due to convergent evolution driven by their similar life styles and feeding strategies.Morphologically,the mobilians are distinguished from all other oligohymenophoreans by the presence of denticles in the adhesive disc, this character being a synapomorphy for the Mobilia.