Primary Structure and Phylogenetic Relationships of Glyceraldehyde‐3‐Phosphate Dehydrogenase Genes of Free‐Living and Parasitic Diplomonad Flagellates

Abstract

Complete nucleotide sequences have been established for two genes (gap1 and gap2) coding for glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12) homologs in the diplomonad Giardia lamblia. In addition, almost complete sequences of the GAPDH open reading frames were obtained from PCR products for two free-living diplomonad species, Trepomonas agillis and Hexamita inflata, and a parasite of Atlantic salmon, an as yet unnamed species with morphological affinities to Spironucleus. Giardia lamblia gap1 and the genes from the three other diplomonad species show high similarity to each other and to other glycolytic GAPDH genes. All amino-acyl residues known to be highly conserved in this enzyme are also conserved in these sequences. Giardia lamblia gap2 gene is more divergent and its putative translation reveals the presence of a cysteine and serine-rich insertion resembling a metal binding finger. This motif has not yet been noted in other GAPDH molecules. All sequences contain an S-loop signature with characteristics close to those of eukaryotes. In phylogenetic reconstructions based on the derived amino acid sequences with neighbor-joining, parsimony and maximum-likelihood methods the four typical GAPDH sequences of diplomonads cluster into a single clade. Within this clade, G. lambia gap1 shares a common ancestor with the rest of the genes. The latter are more closely related to each other, indicating an early separation of the lineage leading to the genus Giardia from the lineage encompassing the morphologically less differentiated genera, Trepomonas, Hexamita and that of the unnamed species. This result is discordant with the orthogonal evolution of diplomonads suggested on the basis of comparative morphology. In neighbor-joining reconstructions G. lamblia gap2 occupies a variable position, due to its great divergence. In parsimony and maximum likelihood analysis however, it shares a most recent common ancestor with the typical G. lamblia gap1 gene, suggesting that it diverged after the separation of the Giardia lineage. The position of the diplomonad clade in broader phylogenetic reconstructions is firmly within the typical cytosolic glycolytic representatives of GAPDH of eukaryotes.

A possible mitochondrial gene in the early-branching amitochondriate protist Trichomonas vaginalis

“…In addition, it has been suggested that typical eukaryotic cytosolic glyceraldehyde-3-phosphate dehydrogenase genes may derive from the mitochondrial endosymbiont (26). If this is correct, then the existence of eukaryotic cytosolic glyceraldehyde-3-phosphate dehydrogenase genes in G. lamblia (26), other diplomonads (34) and Microsporidia (A.J.R., unpublished data) may also betray the secondary loss of mitochondria in these groups. However, further evidence is needed and a concentrated search for endosymbiotically-derived genes in these deeply-branching amitochondrial groups may help to decide which groups, if any, truly never had mitochondria.…”

Origin and Evolution of the Mitochondrial Proteome

“…There is, however, some reason to believe that a number of nominally eukaryotic proteins, including those of the amitochondrial lineages, may be the descendants of diverse bacterial ancestors. We postpone a resolution of this issue until we consider other data suggesting that the nominal archaeazoans are not taxa from deeply diverging branches of the eukaryote tree (50,53,55,86,96,154,156).…”

Biology ofGiardia lamblia

“…The classification of H. inflata with G. lamblia is also supported by a comparison of the glyceraldehyde-3-phosphate dehydrogenase sequences of G. lamblia, Trepomonas agilis, H. inflata, and Spironucleus sp. (287). A comparison of the SS rRNA sequences of Giardia, Hexamita, Trepomonas, and Spironucleus, all diplomonads, showed that all were phylogenetically related and that the last three comprised one clade while Giardia occupied another clade (46).…”