The XVIIIth (New) International Congress of Zoology

Le XVIIIème (Nouveau) Congrès International de Zoologie

Due to the poor fossil record, the analysis of the protozoan- metazoan boundary has to rely predominantly on the phylogenetic analysis of DNA sequences of extent protozoa and animals. The deep history of this event, however, might flaw such an analysis. A phylogenetic analysis of the small subunit of the ribosomal genes, for example, can only provide a rather general view because of the limited information content of the gene. Protein phylogenies, on the other hand, might lead to rather contrasting views, since gene duplications, lateral gene transfer, and a changing compartmentalisation of metabolic pathways can blur the view on the evolutionary history. Whole genome studies will be much more informative, but also rather expensive and, consequently, restricted to a few model organisms. There is a certain commonsense behind the assumption that the increasing availability of oxygen, and concomitantly, of ATP, has been the driving force behind the evolution of metazoans. However, the mitochondria of protists and metazoans alike can provide comparable amounts of ATP. Moreover, in both protozoa and metazoans, secondary evolutionary adaptations to anaerobic niches occurred. Such adaptations involved the loss of mitochondria, the evolution of mitochondria that are able to ?respire? under anaerobic conditions, and, lastly, the evolution of hydrogenosomes. Therefore, the adaptation of ciliates, fungi, molluscs, and worms to anaerobic niches will be discussed, with particular emphasis on the evolution of symbiotic associations with prokaryotic organisms.