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Section 26.5: Horizontal Gene
Transfer
By Monica Macaro
Learning Objectives
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Explain why horizontal gene transfer
can complicate evolutionary hypothesis
Explain why the "Tree of Life" is being replaced by the
"Web of Life"
Horizontal Gene Transfer
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Process in which an
organism incorporates
genetic material from
another organism without
being its offspring
Common in bacteria and
archaea, rare in eukaryotic
cells
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Genetic information
transferred via conjugation,
transduction, and
transformation
Conjugation
Transduction
Transformation
Horizontal Gene Transfer in Eukaryotes
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Not well understood
Hard to detect
Frequency and importance in multicellular eukaryotes
hard to evaluate
Rare due to sexual reproduction and multicellularity
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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
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Eukaryote may ingest bacterial cell via endocytosis and
incorporate a bacterial gene
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Bacterial gene may escape degradation
Gene incorporated into genome via nonhomologous
recombination
Genes may be transferred via viruses
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Viral DNA incorporated into recipient's DNA
About 8% of human genome originates from viruses
Horizontal Gene Transfer in Eukaryote
Horizontal Gene Transfer and Evolution
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Evolutionary hypothesis focuses on vertical evolution
(changes in species due to descent from common
ancestor)
Genes can be transferred between species via HGT
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New genes may benefit/hinder organism’s chance for survival
May foster evolutionary change
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HGT may have been prevalent during life’s early stages
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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”
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HGT indicated by horizontal connections
The Web of Life