Horizontal gene transfer involves the transfer of genetic material between organisms without them being offspring. It is common in bacteria and archaea through processes like conjugation, transduction, and transformation. Horizontal gene transfer in eukaryotes is not well understood and occurs less frequently due to sexual reproduction and multicellularity, though it can still happen in some unicellular eukaryotes. About 8% of the human genome originated from viruses through horizontal gene transfer. Horizontal gene transfer complicates evolutionary hypotheses based on vertical descent from a common ancestor by allowing genes to transfer between different species, domains, and kingdoms, fostering new evolutionary changes and representing life as a "web" rather than a tree.

1. Horizontal Gene Transfer

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
translation
Conjugation
Transduction
Transformatio
n

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
 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
anscestor)
 Genes can be transferred between species,
domains, etc. 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