Evolutionary diversity of eukaryotic small-subunit rRNA genes.

Abstract

The small-subunit rRNA gene sequences of the flagellated protists Euglena gracilis and Trypanosoma brucei were determined and compared to those of other eukaryotes. A phylogenetic tree was constructed in which the earliest branching among the eukaryotes is represented by E. gracilis. The E. gracils divergence far antedates a period of massive evolutionary radiation that gave rise to the plants, animals, fungi, and certain groups of protists such as ciliates and the acanthamoebae. The genetic diversity in this collection of eukaryotes is seen to exceed that displayed within either the eubacterial or the archaebacterial lines of descent.The eukaryotes can be represented as four major divisions: the Protista, the Fungi, the Plantae, and the Animalia (1). The order of succession for these major eukaryotic lineages is an unsettled question, but it is generally accepted that the first eukaryotes were most similar to certain members of the present-day Protista. Efforts to reconstruct the evolutionary history of protists, and hence the phylogenetic origins of the major eukaryotic divisions, have been frustrated by the enormous physiological, cytological, and biochemical diversity exhibited by these "simple" microorganisms. Traditional phylogenetic analyses-i.e., examination of the fossil record and comparative studies of phenotypes-are suitable for defining relationships between multicellular organisms, but they are of limited value in determining relationships among the protists. These soft-body organisms are not well represented in the fossil record, and there is little agreement regarding which phenotypic characters are most useful for inferring protist relationships. Furthermore, comparative studies of phenotypes do not provide quantitative measures of genetic similarity that can be used to deduce the order of branching for the major eukaryotic lines of descent.As an alternative, comparisons of the nucleotide or amino acid sequences of functionally equivalent macromolecules can be used to infer quantitative phylogenetic relationships between diverse organisms (2). Amino acid sequence homologies among cytochrome c molecules (3, 4), ferredoxins (5-7), and superoxide dismutases (8-10) have been useful for refining classical eukaryotic phylogenies, but they are of limited value in resolving the deepest phylogenetic branching patterns. The use of cytochrome c and other genes that are defined by plastid and mitochondrial genomes as "molecular chronometers" for measuring eukaryotic relationships is further complicated by their non-nuclear origin. It is now apparent that plastids and mitochondria arose as bacterial endosymbionts within some ancestral eukaryotes (11-13). Therefore, genes that are contained within the plastid or mitochondrial genomes or genes that were present in the original bacterial endosymbionts and then transferred to the nucleus-e.g., cytochrome c genes-should be regarded as markers of prokaryotic rather than eukaryotic evolution.By comparison, the rRNAs are better suited for defining evolutiona...

Complete nucleotide sequence and deduced polypeptide sequence of a nonmuscle myosin heavy chain gene from Acanthamoeba: evidence of a hinge in the rodlike tail.

“…Second, the amino acid sequences that comprise the globular head regions of myosin II and muscle myosins show a high degree of similarity. Acanthamoeba castellanii is estimated to have diverged from the main line of eukaryotic descent sometime between the divergence of yeast (,'ol.2 x 109 yr ago) and the divergence of plants and animals (,,ol x 109 yr ago) (15,38). Therefore, we speculate that as far back as 1 × 109 yr ago the ancestral myosin gene contained introns in at least the three positions that interrupt the present-day amoeba gene.…”

The Evolution of Trypanosomes Infecting Humans and Primates

“…It is conserved throughout the eukaryotes, while the range of conserved and variable regions allow diverse rates of genetic evolution to be studied, making it ideal for elucidating both higher evolutionary relationships and those between closely related species (Sogin et al 1986). Its high copy number also facilitates ease of PCR amplification.…”

Function of N‐terminal import signals in trypanosome microbodies

“…As trypanosomatids are evolutionarily the earliest-branching eukaryotes to contain microbodies [17], it is of considerable interest to determine to what extent the import apparatus is conserved. Here we provide evidence that the PTS-2 pathway is present in trypanosomes, indicating that it has been conserved throughout evolution.…”