The Concept of Pelobionts (Class Peloflagellatea): Brief History and Current State

摘要

Pelobionts are a specific group of amitochondrial amoeboid protists possessing flagella. They are free-living microaerobic or anaerobic organisms. The ideas concerning the nature, composition, and taxonomic status of pelobionts have changed repeatedlyand radically. The modern concept of pelobionts actually originated from a paper by Griffin (1988), although most species comprising this group were known long ago. Pelomyxa palustris, the type species and genus of the group, was described by R. Greeffin 1874. Other pelobionts, traditionally referred to as mastigamoebids, belong to three nominal genera: Mastigamoeba, Mastigina, and Mastigella. They have a number of similar features of cell organization (unique in their combination) and represent a most homogeneous group of apparently related organisms. Pelobionts are united into the class Peloflagellatea with the only order Pelobiontida. The results of the few molecular and biological studies ofphy-logenetic relationships between pelobionts and otherprotists are controversial. However, they provide conclusive evidence for the secondary loss of mitochondria by pelobionts in the course of their evolution. The atypical (deviating from the standard microtubular pattern) structure of the axoneme and flagellar basal body in some pelobionts is also attributed to the loss of the initial locomotor function of the flagellar apparatus in the course of evolutionary transition to a different type of cell organization. The species composition of pelobionts hasbeen poorly studied. Therefore, both taxonomic identification of species and determination of their relation to this group of protists are unreliable. Among the approximately 200 nominal pelobiont species described in the late 19th and early 20th centuries, less than ten species have been reisolated and redescribed using modern research methods. Pelomyxa palustris, the best known and the only valid species in this genus, is proposed to be a complex of closely related independent species.