1. Section 26.5: Horizontal Gene Transfer By Monica Macaro

2. Learning Objectives  Explain why horizontal gene transfer can complicate evolutionary hypothesis  Explain why the "Tree of Life" is being replaced by the "Web of Life"

3. Horizontal Gene Transfer  Process in which an organism incorporates genetic material from another organism without being its offspring  Common in bacteria and archaea, rare in eukaryotic cells  Genetic information transferred via conjugation, transduction, and transformation Conjugation Transduction Transformation

4. Horizontal Gene Transfer in Eukaryotes  Not well understood  Hard to detect  Frequency and importance in multicellular eukaryotes hard to evaluate  Rare due to sexual reproduction and multicellularity  More common in certain unicellular eukaryotes (e.g protists)  HGT genes only transferred to offspring in multicellular eukaryotes if recipient was a gamete/germ cell

5.  Eukaryote may ingest bacterial cell via endocytosis and incorporate a bacterial gene  Bacterial gene may escape degradation  Gene incorporated into genome via nonhomologous recombination  Genes may be transferred via viruses  Viral DNA incorporated into recipient's DNA  About 8% of human genome originates from viruses

6. Horizontal Gene Transfer in Eukaryote

7. Horizontal Gene Transfer and Evolution  Evolutionary hypothesis focuses on vertical evolution (changes in species due to descent from common ancestor)  Genes can be transferred between species via HGT  New genes may benefit/hinder organism’s chance for survival  May foster evolutionary change

8.  HGT may have been prevalent during life’s early stages  Common ancestor may have been a community of cell lineage evolving as a whole rather than a single cell lineage  Muddles concept of monophyletic groups  “Web of Life” instead of “Tree of Life”  HGT indicated by horizontal connections

9. The Web of Life