SYMBIOSIS BETWEEN METHANOGENIC ARCHAEA AND DELTA-PROTEOBACTERIA AS THE ORIGIN OF EUKARYOTES - THE SYNTROPHIC HYPOTHESIS

We present a novel hypothesis for the origin of the eukaryotic cell, o r eukaryogenesis, based on a metabolic symbiosis (syntrophy) between a methanogenic archaeon (methanobacterial-like) and a delta-proteobacte rium (an ancestral sulfate-reducing myxobacterium). This syntrophic sy mbiosis was originally mediated by interspecies H-2 transfer in anaero bic, possibly moderately thermophilic, environments. During eukaryogen esis, progressive cellular and genomic cointegration of both types of prokaryotic partners occurred. Initially, the establishment of permane nt consortia, accompanied by extensive membrane development and close cell-cell interactions, led to a highly evolved symbiotic structure al ready endowed with some primitive eukaryotic features, such as a compl ex membrane system defining a protonuclear space (corresponding to the archaeal cytoplasm), and a protoplasmic region (derived from fusion o f the surrounding bacterial cells). Simultaneously, bacterial-to-archa eal preferential gene transfer and eventual replacement took place. Ba cterial genome extinction was thus accomplished by gradual transfer to the archaeal host, where genes adapted to a new genetic environment. Emerging eukaryotes would have inherited archaeal genome organization and dynamics and, consequently, most DNA-processing information system s. Conversely, primordial genes for social and developmental behavior would have been provided by the ancient myxobacterial symbiont. Metabo lism would have been issued mainly from the versatile bacterial organo trophy, and progressively, methanogenesis was lost.