Phylogenetic positions of several amitochondriate protozoa——Evidence from phylogenetic analysis of DNA topoisomerase II

Abstract

Several groups of parasitic protozoa, as represented by Giardia, Trichomonas, Entamoeba and Microsporida, were once widely considered to be the most primitive extant eukaryotic group--Archezoa. The main evidence for this is their 'lacking mitochondria' and possessing some other primitive features between prokaryotes and eukaryotes, and being basal to all eukaryotes with mitochondria in phylogenies inferred from many molecules. Some authors even proposed that these organisms diverged before the endosymbiotic origin of mitochondria within eukaryotes. This view was once considered to be very significant to the study of origin and evolution of eukaryotic cells (eukaryotes). However, in recent years this has been challenged by accumulating evidence from new studies. Here the sequences of DNA topoisomerase II in G lamblia, T. vaginalis and E. histolytica were identified first by PCR and sequencing, then combining with the sequence data of the microsporidia Encephalitozoon cunicul and other eukaryotic groups of different evolutionary positions from GenBank, phylogenetic trees were constructed by various methods to investigate the evolutionary positions of these amitochondriate protozoa. Our results showed that since the characteristics of DNA topoisomerase II make it avoid the defect of 'long-branch attraction' appearing in the previous phylogenetic analyses, our trees can not only reflect effectively the relationship of different major eukaryotic groups, which is widely accepted, but also reveal phylogenetic positions for these amitochondriate protozoa, which is different from the previous phylogenetic trees. They are not the earliest-branching eukaryotes, but diverged after some mitochondriate organisms such as kinetoplastids and mycetozoan; they are not a united group but occupy different phylogenetic positions. Combining with the recent cytological findings of mitochondria-like organelles in them, we think that though some of them (e.g. diplomonads, as represented by Giardia) may occupy a very low evolutionary position, generally these organisms are not as extremely primitive as was thought before; they should be polyphyletic groups diverging after the endosymbiotic origin of mitochondrion to adapt themselves to anaerobic parasitic life.

Isolation of DNA topoisomerase II gene from Pleurotus ostreatus and its application in phylogenetic analysis

“…1995; Wang 1996). The associated genes have been identified in eukaryotes, including protozoa such as Trichomonas vaginalis (He et al. 2005a), Giardia lamblia (He et al.…”

“…Homologous DNA topo II genes have proven to be reliable markers for distinguishing the pathogenic Candida species (Kato et al. 2001), and the amitochondriate protozoa (He et al. 2005a,b).…”

Identification and evolutionary implication of four novel box H/ACA snoRNAs from Giardia lamblia

“…Therefore, Giardia is previously regarded as a missing link between prokaryotes and eukaryotes [25] . Although the evolutionary position of G. lamblia has been challenged by recent researches and some biologists suggest that it is possibly not the most primitive eukaryote [26][27][28][29] , G. lamblia undoubtedly represents an important primitive eukaryotic model for the study of biological evolutions. Previously, we have primarily investigated the snoRNAs in G. lamblia and found that the G. lamblia snoRNAs have unique features different from the counterparts previously identified from Archaea and other unicellular protozoan in gene structures, functions, genomic organizations and expression models [30] .…”