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Horizontal
gene transfer

The transfer of genetic information
from one genome to another.
1
Conjugation = transfer of
DNA from one organism to
another by means of a
plasmid.
2
Transformation = uptake of
free DNA from the
environment.
3
Transduction = transfer of
DNA from one organism to
another by a bacteriophage.
4
5
An organism into which genetic information from
a different organism has been incorporated as a
stable part of its genome is a transgenic
organism.
Aeqorea victoria
green fluorescent
protein (GFP)
ANDi
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Orthology
Paralogy
Xenology
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Horizontal gene transfer is suspected when there is a
discrepancy between gene phylogeny and species
phylogeny, in particular when the tree reflects
geographical proximity rather than phylogenetic affinity.
8
Patchy phylogenetic distribution
Aedes aegypti
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nucleus
cytoplasm
(CuZn-SOD)
SOD = superoxide dismutase
cyanobacteria
+ chloroplast
(Fe-SOD)
bacteria +
mitochondria
(Mn-SOD)10
Leiognathus daura
Photobacterium leiognathi
(CuZnSOD)
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12
Crown Gall Disease
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Agrobacterium tumefaciens
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Plant Gene
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P
M
P elements
&
hybrid dysgenesis
Margaret
Kidwell
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P elements were not found in any D. melanogaster strains
collected before 1950, and collections made subsequently
showed increasing frequencies of P with decreasing age.
North and South America
Europe, Africa, Middle East
Australia, Far East
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Two hypotheses:
1. Most D. melanogaster strains in
nature carry P elements, but they
tend to lose them in the laboratory.
2. P elements were recently
introduced into D. melanogaster
populations in nature.
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The recent acquisition hypothesis is
supported by:
1. P strains that have been monitored in
the lab for ~15 y were never observed to
lose P.
2. There is a geographical cline in the
temporal appearance of P in nature.
3. There is evidence for horizontal gene
transfer.
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Species tree
P-element tree
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Drosophila melanogaster
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Drosophila willistoni
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Proctolaelaps
regalis
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Conditions for horizontal Pelement transfer:
(1) two Drosophila females from the
donor and the recipient species
must lay eggs in proximity to one
another
(2) the recipient egg must be less than
3 hours old (< 512 cells)
(3) the germline of the recipient
embryo must incorporate a
complete copy of P
Marilyn A. Houck
(4) the receiving embryo must survive
the biting injuries
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type-C virogene
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Informational genes:
Genes involved in
replication, transcription,
reverse transcription, and
translation.
Operational genes:
All others.
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The complexity hypothesis
informational
operational
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Genes move within the
genome and between
genomes.
Genetic “Mutatis mutandis.”
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Promiscuous
DNA
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Einat Hazkani-Covo et al.
Mitochondrial-sequence
invasions into the nuclear
genome
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Numts* (nuclear mitochondrial
DNA sequences) are a type of
promiscuous DNA, i.e.,
nuclear sequences of
mitochondrial origin.
*pronounced “new mights”
35
The transfer of functional genes
from the mitochondria to the nucleus
is thought to have has stopped in
evolution after the emergence of
animals (~1,000 MYA).
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The reason is thought to be the
differences between the nuclear
and mitochondrial genetic codes.
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The transfer of nonfunctional
pieces of mitochondrial genetic
information continues to this day.
Numts have been found so far
in 83 eukaryote species.
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Most species whose genomes
have been completely
sequenced contain very few
numts.
Saccharomyces cerevisiae
Caenorhabditis elegans
Drosophila melanogaster
Plasmodium falciparum
17 numts
3 numts
3 numts
3 numts
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In the human genome we find
~1,000 numts
total length = 831 Kb
~0.02% of the nuclear genome
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Numts: Evolution’s
misplaced witnesses
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Junk DNA
Domestic
Imported
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