10. Perspectives on Phylogenetic tree handouts

Learning Objectives

  • Understand the concept of horizontal gene transfer (HGT).

  • Illustrate mechanisms of gene transfer in prokaryotes and eukaryotes.

  • Identify and compare the wen and ring models of phylogenetic relationships.

  • Describe differences between ring and web models and the original phylogenetic tree concept.

Horizontal Gene Transfer (HGT)

  • Definition: Introduction of genetic material from one species to another via mechanisms that do not involve parental transmission (vertical transmission).

  • Commonality: More prevalent in prokaryotes (bacteria and archaea) but can also occur in eukaryotes.

  • Significance: Provides insights into the origins of organelles like mitochondria and chloroplasts.

Mechanisms of Horizontal Gene Transfer

In Prokaryotes

  • Transformation: Bacteria uptake naked DNA from the environment.

  • Transduction: Viruses (bacteriophages) transfer genetic material from one bacterium to another.

  • Conjugation: A hollow pilus forms to transfer genes between organisms directly.

In Eukaryotes

  • Gene Uptake: Involves some eukaryotic organisms obtaining genes from other species, often during interactions such as feeding (e.g., aphids feeding on fungi).

Genome Fusion

  • Overview: Occurs when two symbiotic prokaryotic organisms form a stable internal relationship.

  • One organism is absorbed into the cytoplasm of another, leading to endosymbiosis.

  • DNA Organization: This process results in separate nuclear DNA and mitochondrial DNA within eukaryotic cells.

Hypotheses of Eukaryotic Evolution

  • Nucleus-first Hypothesis: The nucleus developed in a prokaryote and later fused with bacteria to create mitochondria.

  • Mitochondria-first Hypothesis: Mitochondria evolved within a prokaryotic host which then acquired a nucleus.

  • Eukaryote-first Hypothesis: Prokaryotes arose from eukaryotes that lost essential genes and complexity.

New Perspectives in Phylogenetics

Web Model

  • Integrates the concept of horizontal gene transfer while maintaining a rooted structure of common ancestry.

Ring Model

  • Suggests that all three major domains of life evolved from a shared pool of primitive prokaryotes, highlighting a more interconnected evolutionary history.