On the Origins of Eukaryotic Life

1. On the Origins of Eukaryotic Life • Christophe Anslinger 31 March 2010

2. The Earliest Life on Earth • Earliest precursors to life thought to be protobionts, self-replicating macromolecule aggregates which posses characteristics akin to true life forms. • Fossils indicate first true living organisms, prokaryotic cells, appear on earth as early as 3.5 billion years ago. • Eukaryotic cells first appear as early 2.7 billion years ago, given fossilized evidence of biomarkers.

3. Prokaryotes and Eukaryotes • Defining characteristics of prokaryotes: • Lack nucleus • Chromosomes are found in localized area of cell known as nucleoid • Reproduce via cellular fission • Generally unicellular • Lack membrane-bound organelles • Some species also posses plasmids • Supercoiled DNA molecules

4. Prokaryotes and Eukaryotes, cont. • Eukaryotes: • Posses a nucleus • Posses multiple membrane-bound organelles • Mitochondria, chloroplasts, etc. • Large in size, relative to prokaryotes • Reproduce through mitosis and meiosis • Contain a cytoskeleton • Rigid system of protein fibers

5. A Visual Comparison

6. Resolving the Rise of Eukaryotes • Appearance of eukaryotes is logically explained by the endosymbiotic theory of evolution. • First advanced by Lynn Margulis in 1981, the theory postulates that membrane bound organelles in eukaryotes were once free-living prokaryotes that were engulfed by a larger cell. • Ingested prokaryote gained protection and a supply of carbon, while the host cell benefited from increased ATP production and the ability to utilize oxygen in cellular respiration.

7. Evidence for Endosymbiosis • Mitochondria posses their own genome and ribosomes. • They produce their own proteins independently of the host cell. • Mitochondria are protected by a double membrane. • Mitochondrion size is comparable to that of an average bacterium. • Duplication of mitochondria occurs via fission . • This process is also independent of the host cell division.

8. Further Evidence • Most compelling correlation that provided credence to endosymbiosis was provided by comparison of mitochondrial DNA to that of other cellular organisms. • Mitochondrial gene sequences were more closely related to that of α-proteobacteria than to nuclear DNA from eukaryotes.

9. Further Issues to be Resolved • The endosymbiosis theory provides a valid, well-supported explanation for the presence of mitochondria and chloroplasts in eukaryotes, but it does not fully account for characteristic eukaryotic traits such as the cytoskeleton and nucleus. • This is a point of contention between scientists, with multiple explanations proposed for its resolution.

10. Margulis and Purves • Two contending theories for the development of the nucleus and cytoskeleton are seen in the work of Margulis, Dolan, et al. and Purves, et al.

11. Invagination • Purves proposes that the nucleus and the cytoskeleton arose from the invagination of the cell membrane following the loss of a cell wall. • Loss of structure that the cell wall provided enabled the organism to fold in upon itself.

12. Evidence of Infolding • Infoldings of the plasma membrane are evidenced in certain modern bacteria. • The Endoplasmic reticulum and nuclear envelope are continuous in modern eukaryotes.

13. Evolutionary Basis • The evolution of the nuclear envelope allowed early eukaryotes to separate transcription and translation. • This allowed for alternative splicing and other forms of RNA processing, enabling gene expression.

14. The Chimeric Eukaryote • Margulis, Dolan, et al. postulate that eukaryotes instead arose from a merger between an archaebacterium and a eubacterium. • The combination was between cells resembling a spirochaete, a gram negative bacteria, and something resembling Thermoplasma, a thermoacidohil that lacks a cell wall. • Ancestral Eukaryotic cells were formed through of fusion and integration of the two organisms’ genomes.

15. Evolutionary Basis of Chimeric Cells • The formation of a chimera is favored by complementary chemical processes that the two symbiotic organisms perform. • The eubacteria generated carbon-rich products through the oxidation of sulfide to elemental sulfur. • The archaebacteria utilized carbon products and tended to form tight associations with globules of sulfur by means of its early cytoskeletal system.

16. The Karyomastigont- Evidence of Cellular Fusion • An ancestral feature of eukaryotes that is present in early branching protists, the karyomastigont is an organellar system coined by Janicki, C. to refer to a eukaryotic flagellum attached via a rhizoplast to the nucleus. • Preceded unattached nuclei. • Provides kinetosomes that may have in-folded to produced microtubule based cytoskeleton.

17. Amitchondriate Protists • Evidence for the chimera theory of endosymbiogenesis, as put forth by Margulis, et al., can be seen in amitochondriate protists, unicellular organisms lacking mitchonchondria. • Most primitive eukaryote to display characteristic nucleus. • Thought to be representative of earliest examples of cells containing karyomastigonts.

18. Further Support for the Chimeric Eukaryote • Gupta, et al., in study of protein phylogenies, concurs with assessment of chimeric origin of eukaryotes, but posits instead a relationship between an archaebacterium and a gram positive bacterium as the source of the chimera. • Information transfer processes are derived from archaebacteria • Critical proteins, such as Hsp70, display evidence of origin independent of later eubacteria endosymbiosis, while being distinctly foreign from the eukaryote. • Unpublished work by Hua on eukaryotic genome finds evidence of eukaryotic genes homologous to genes fond in both Archaebacteria and Eubacteria.

19. A Visual Representation

20. Subsequent Dispute of Phylogenetic Tree of Life • Margulis, et al. argues that no third domain for archaebacteria is required. • Though some prokaryotes lie as intermediates between acraheabacteria and eubacteria, there exists no such intermediate between prokaryotes and eukaryotes. • rRNA similarities and genetic relations are less important than morphological observations, and are irrelevant when comparing prokaryotes to symbiotically derived eukaryotes. • rRNA has been shown to be susceptible to horizontal gene transfer, making it less stable than previously assumed in relation to evolutionary pressures.

21. Argument for Three Domain Taxonomy • In a landmark 1990 paper, Woese, et al., successfully proposed the establishment of a three domain phylogenetic tree as the basis biological classification. • Replaced five kingdom system • Basis lay in study of subunits in ribosomal RNA • Claimed that combining eubacteria and archaebacteria into single kingdom was inefficient given that the largest shared similarity is dissimilarity to eukaryotes.

22. Current Phylogenetic Division Three domains, as established by Woese, et al.

23. Personal Response and Conclusion • Based on the work of Margulis, et al., and others, I find the concept of a chimera organism to be a reasonable explanation for the origin of eukaryotes. • Provides plausible genetic basis for function and structure of eukaryotes. • Provides intermediates in the form amitochondriates. • Relates the importance and function of karyomastigont. • I agree with the classification of life into three domains, based on genetic evidence and inherent physical differences visible between the different phylogenetic branches.