Ribosomal RNA sequence suggests microsporidia are extremely ancient eukaryotes

Abstract

The microsporidia are a group of unusual, obligately parasitic protists that infect a great variety of other eukaryotes, including vertebrates, arthropods, molluscs, annelids, nematodes, cnidaria and even various ciliates, myxosporidia and gregarines¹. They possess a number of unusual cytological and molecular characteristics. Their nuclear division is considered to be primitive², they have no mitochondria³, their ribosomes and ribosomal RNAs are reported to be of prokaryotic size^4,5 and their large ribosomal subunit contains no 5.8S rRNA⁶. The uniqueness of the microsporidia may reflect their phylogenetic position, because comparative sequence analysis shows that the small subunit rRNA of the microsporidium Vairimorpha necatrix^7,8 is more unlike those of other eukaryotes than any known eukaryote 18S rRNA sequence. We conclude that the lineage leading to microsporidia branched very early from that leading to other eukaryotes.