A MITOCHONDRIAL HSP70 ORTHOLOGUE IN VAIRIMORPHA NECATRIX - MOLECULAR EVIDENCE THAT MICROSPORIDIA ONCE CONTAINED MITOCHONDRIA

Microsporidia are small (1-20 mu m) obligate intracellular parasites o f a variety of eukaryotes, and they are serious opportunistic pathogen s of immunocompromised patients [1]. Microsporidia are often assigned to the first branch in gene trees of eukaryotes [2,3], and are reporte d to lack mitochondria [2,4]. Like diplomonads and trichomonads, micro sporidia are hypothesised to have diverged from the main eukaryotic st ock prior to the event that led to the mitochondrion endosymbiosis [2, 4], They have thus assumed importance as putative relies of pre-mitoch ondrion eukaryote evolution, Recent data have now revealed that diplom onads and trichomonads contain genes that probably originated from the mitochondrion endosymbiont [5-9], leaving microsporidia as chief cand idates for an extant primitively amitochondriate eukaryote group. We h ave now identified a gene in the microsporidium Vairimorpha necatrix t hat appears to be orthologous to the eukaryotic (symbiont-derived) Hsp 70 gene, the protein product of which normally functions in mitochondr ia, The simplest interpretation of our data is that microporidia have lost mitochondria while retaining genetic evidence of their past prese nce. This strongly suggests that microsporidia are not primitively ami tochondriate and makes feasible an evolutionary scenario whereby all e xtant eukaryotes share a common ancestor which contained mitochondria.