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
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
Viral DNA incorporated into recipient's DNA
About 8% of human genome originates from viruses
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
