THIEL, V.; W�RHEIDE, G.; BLUMENBERG, M.; PAPE, T.; MICHAELIS, W.; University of G�ttingen, Germany; University of G�ttingen, Germany; University of Hamburg, Germany; University of Hamburg, Germany; University of Hamburg, Germany: Lipid biomarkers: an unconventional approach to resolve sponge interclass relationships

During the last decade, molecular techniques have contributed significantly to understanding sponge phylogeny. However, conflicting data have not resolved early metazoan evolution, and the interclass relationships of the Porifera are still debated. Complementary to the classical morphological and molecular biological approaches, the biomarker concept focusses on the occurrence and distribution of specific organic molecules, commonly lipids. These lipids reveal information on the biological source (sponge, associated microbes) and the biosynthetic and metabolic pathways involved in the formation of the respective compounds. Demosponges, best studied among the Porifera, are known to possess unusual membrane lipids, such as exotic sterols and the so-called �demospongic� fatty acids. These compounds are indicative for specific enzyme systems controlling biosynthetic properties like carbon chain elongation and the introduction of characteristic double bonds. Thus, they provide excellent targets for chemotaxonomic analyses at the class level. Our comprehensive biomarker study demonstrates that the Hexactinellida and the Demosponges share an �ancestral� design of lipid biosynthesis. That strategy involves the processing of bacterial precursor lipids and results in different biomembrane structures than found in contemporary organisms other than sponges. To the exclusion of the Calcarea, their unique lipid characteristics put the Hexactinellida and the Demospongiae into the same phylogenetic group. Moreover, biomarkers indicate similar patterns of Archaea and specific Eubacteria associated with the Hexactinellida and the Demospongiae.