Phylogenetic position and inter-relationships of the osmotrophic euglenids based on SSU rDNA data, with emphasis on the Rhabdomonadales (Euglenozoa).

Abstract

In order to reconstruct the evolution of euglenid flagellates, euglenozoan SSU rDNA data have been used to investigate phylogenetic relationships with a focus on osmotrophic taxa and especially on the Rhabdomonadales. The dataset consisting of the SSU rDNAs of osmotrophic, phagotrophic and phototrophic taxa was used in parsimony, maximum-likelihood and distance analyses. Five genera make up the Rhabdomonadales, all of them osmotrophic : Gyropaigne, Menoidium, Parmidium, Rhabdomonas and Rhabdospira. According to our analyses they form a strongly supported monophyletic assemblage which is characterized by a low sequence divergence compared to the euglenids in general. Closest relatives are the members of the osmotrophic genus Distigma. All primary osmotrophic species constitute a larger monophyletic group with the phototrophic euglenids and the phagotroph Peranema trichophorum. The combination of three rhabdomonadalian species Rhabdomonas gibba, Rhabdomonas spiralis and Rhabdospira spiralis with nearly identical SSU rDNA sequences is strongly recommended. The phagotroph Petalomonas cantuscygni branches at the bottom of the euglenid subtree with significantly weaker support. The inter-relationship of the three distinct euglenozoan taxa (euglenids, kinetoplastids and diplonemids) could not be convincingly resolved by this study.

Character evolution in heterotrophic euglenids

“…However, this union of character states begs the question: Is P. cantuscygni more closely related to euglenids or kinetoplastids? Phylogenetic analyses of small subunit rDNA using non-euglenozoan outgroups incontrovertibly demonstrate that P. cantuscygni forms the sister taxon to the other euglenids in the ingroup (Müllner et al 2001;Preisfeld et al 2000Preisfeld et al , 2001. We can infer from these data that the Euglenida was derived from an ancestor possessing kinetoplasts and, consequently, the Kinetoplastida is paraphyletic.…”

“…Arrows indicate homologous heel-like structures marking the articulation zones between fused strips. sistent with the finding of Müllner et al (2001) and Preisfeld et al (2001) using small subunit rDNA sequences. Clade I was rather surprisingly derived from within the eukaryotrophs (Fig.…”

“…Two bodonids, Bodo saltans and B. caudatus, were designated as the outgroup based on previous studies on the molecular phylogeny and comparative morphology of euglenozoans (Willey et al 1988;Farmer 1991a, 1991b;Montegut-Felkner and Triemer 1997;Linton et al 2000;Preisfeld et al 2000Preisfeld et al , 2001. The support of the data for each node on the most parsimonious tree(s) was estimated using decay indices using Autodecay 4.0.2 (Eriksson 1998) and nonparametric bootstrap percentages (Felsenstein 1985) from 500 replications using PAUP* 4.0.…”

“…Figure 2 illustrates the progression of thought regarding the phylogenetic relationships of taxa possessing the four primary modes of nutrition. When the distinction was not made in the literature, "phagotroph" refers to bacteriotrophs and eukaryotrophs collectively; we refer only to distigmids and rhabdomonads sensu lato (Leedale and Hibberd 1974;Cann 1986;Preisfeld et al 2001) (Fig. 1d-e) as "osmotrophs" and not to those taxa that are clearly derived from phototrophs.…”

“…motrophs like distigmids and rhabdomonads (Fig. 1d-e) is less obvious; however, small subunit (18S) rDNA sequences indicate that they diverged prior to the eukaryotrophic Peranema and the phototrophs (Preisfeld et al 2000(Preisfeld et al , 2001Leander and Farmer 2001b;Müllner et al 2001). We are interested in identifying the key morphological innovations in euglenid evolution associated with the transitions between bacteriotrophy, eukaryotrophy, phototrophy, and osmotrophy, but it is first necessary to establish a robust phylogenetic hypothesis for euglenids as a whole.…”