BIOL2007

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BIOL2007 CHROMOSOMAL EVOLUTION Genes are found
on chromosomes. Rule Gene action usually
independent of chromosomal location.
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Exception 1 position effects. e.g. Hox gene
clusters order reflects order of segments in the
body that they control Suggests cis-acting,
functional reasons   Nonetheless, success in much
genetic engineering implies that position often
unimportant. Most genes trans-acting
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Exception 2 Tight linkage and epistasis may also
influence evolution. Or linkage disequilibrium.
With epistasis or linkage disequilibrium, genes
do not act independently. Epistasis usually not
strong except Papilio, HLA. Linkage
disequilibria up to lt1 Mb in humans, and lt100 Kb
in Drosophila
Reich, D. et al. 2003. Nature 411, 199 - 204
Humans 3,400,000,000 bp per genome ½ of 1
mllnth 160 kb
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However, the fact that genes are on chromosomes
influences evolution far beyond the minor effects
of position effects and linkage disequilibria.
  Because the genes are arranged on long
strings, and because chromosomes themselves act
as genetic elements-
Holistic selective effects act on 100s to 1000s
of genes at a time.
WOW!!
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Chromosomal rearrangements gross changes in
chromosomal morphology.   Lots of Greek
telomere, centromere, autosome, chromosome,
heterochromatin...   Chromosomal morphology
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Rearrangements
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Chromosomal genotypes called karyotypes.
Karyotype often means number of chromosomes,
for instance, "the human karyotype is 2n 46".
Polyploidy. Common chromosomal mutation
involving a doubling of numbers of copies
autopolyploidy (doubling of endogenous
chromosomes) allopolyploidy (hybridisation ?
doubling)   Abnormal numbers aneuploidy
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• Autopolyploidy and allopolyploidy popular in
  flowering plants
• 30 of species are of polyploid origin
• 2-3 of plant speciation assoc. with new
  polyploidy
• Probably because monoecious and hermaphrodite
  plants can self ? polyploid offspring with fully
  balanced gametes.
• If tetraploid mates with diploid, the F1 is
  triploid causes aneuploidy in the offspring,
  offspring almost invariably sterile.
  Duplications of some but not all genetic material
• So polyploidy can lead to speciation

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How do rearrangements occur? Chromosome
breakage. Can occur via radiation, mutagens etc.
Repeated sequences, especially transposable
elements, in the DNA may frequently be involved,
i.e. non-homologous recombination e.g. P-
elements in Drosophila. Alu elements probably
do in mammals perhaps in us?
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Breakage leads to "sticky ends" (? something to
do with the function of a telomere?). Telomere
repeated motifs.   Telomere's function to "cap"
sticky ends, prevents chromosomal mutation.
Grows to replace losses in DNA synthesis
telomerase.
Telomeric inversions are rare   ? Most
(successful) rearrangements are reciprocal
paracentric or pericentric inversions
reciprocal translocations also preserve the
telomere, and are common too.
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Evolutionary effect of rearrangements General
rule Heterozygous rearrangements often lead to
the production, in meiosis, of UNBALANCED
GAMETES (duplications and deletions in progeny)
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e.g. Paracentric inversions Inversion
heterozygotes chromosomes pair in loops
No crossing over in inversion gametes fine
Crossing over in inversion, problems
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dicentric bridge (breaks at cell division)
acentric fragment (lacks centromere, becomes
lost) duplications and deletions of chromosomal
material ? heterozygote disadvantage ? fixation
(except in many flies, where paracentric
inversions act crossover suppressors)
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Evolution of paracentric inversions Paracentric
inversions in Diptera No crossing over in male,
so no damage to sperm. In female,
dicentrics/acentrics go to polar bodies. So
little or no damage to egg chromosomes. Paracentr
ic inversions are commonly polymorphic in
Diptera. There is even evidence for
heterozygous ADvantage ? maintains
polymorphisms e.g. Drosophila, Anopheles,
malaria mosquitoes.
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Pericentric inversions Like paracentric
inversions, only worse. Reciprocal
translocations Approximately 50 (or more)
unbalanced gametes due to non-disjunction, or
non- separation of homologous parts of
chromosomes duplications and deletions result
Because pericentric inversion and translocation
heterozygotes produces such unbalanced gametes,
the rearrangements cause heterozygote
disadvantage. ? usually fixed within
populations may differ between populations
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Phylogeny from rearrangements Banding patterns
can identify chromosomes and chromosome
parts. In humans/apes, chromosome banding
patterns first showed that chimps are more
closely related to humans than gorillas Humans
differ from closest relatives by 9 pericentric
inversions and 1 centric fusion
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Humans and great apes
9 pericentric inversions, and one reciprocal
translocaton
from Yunis Prakash (1982) Science 215,
1525-1530.
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• Evolutionary oddities about chromosomes
• Poorly understood.
• Chromosome number is variable.
• In Drosophila melanogaster, 4 pairs of
  chromosomes (n 4, 2n 8). Of these, only 3
  very active, X, 2 and 3.
• In humans, 23 pairs (n 23, 2n 46). Mammals in
  general are highly variable in chromosome number.
  
• Across the whole Lepidoptera, some variability
  (10-100s!), but strong modal number of n 31.

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"Karyotypic orthoselection"
Similar repeated change in many chromosomes at
once. Not fully explained. For example, the
primitive chromosome number of chromosomes in Mus
musculus domesticus, the house mouse, is 2n 40,
all acrocentrics. However, by a series of
Robertsonian fusions, there are multiple
chromosomal races with less, some of which have
as few as 2n 22. Nobody knows why!
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• What explains these patterns?
• Not entirely clear.
•  
• Approx. 1 chiasma (causing a crossover) per
  chromosome arm
• ? perhaps chromosome number is an adaptation
  (like sex) which affects overall recombination in
  the genome.
•  
• Many chromosomes
• lots of of recombination (50 recombination
  between chromosomes, plus a lot of chiasmata).

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Evolutionary significance Heterozygous
disadvantage may prevent evolution of new
chromosome rearrangements Most populations
should be fixed. In general, true. But
polymorphisms occur. e.g. Diptera. Often,
non-disjunction rates low e.g Mus. However,
mostly some heterozygous disadvantage, leading to
fixation Can cause a partial barrier between
populations fixed for different rearrangements
(e.g. species).
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Chromosomal evolution and speciation Species
absence of hybrids, hybrid inviability, or
sterility of hybrids Barriers between chromosome
races therefore similar to barriers between
species ? chromosomes important in
speciation? Controversial (MJD White, Guy Bush
1970s, "stasipatric speciation").   Chromosomal
rearrangements certainly contribute to isolation
species often differ chromosomally. But
generally doubted that drift important.
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For example, humans 2n 46, chimps 2n 48 9
pericentric inversions 1 centric
fusion human-chimp hybrids would almost
certainly be very infertile, due to chromosomal
problems alone
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Did chromosome change cause speciation? Or did
it occur since separation? Most now think
genic differences more important than
chromosomes in speciation (except
polyploidy). Recent suggestions Rearrangements
trap groups of genes effecting ecological
differences? Due to suppression of recombination
by rearrangement. e.g. in Drosophila
pseudoobscura vs. D. persimilis
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• TAKE HOME POINTS
• "Position effects" known, but often unimportant
• But karyotypes have strong holistic, selective
  effects
• Chromosomal rearrangement heterozygotes
• reduced fertility, heterozygote disadvantage
• Rearrangement polymorphisms usually rare, found
  in
• hybrid zones between species or
• chromosome races only
• But, in some groups, chromosomal polymorphisms
• common within species.
•  

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• Species often differ in karyotype
• Rearrangements contribute to hybrid
  sterility/inviability. But not much?
• Rearrangements may prevent recombination
• allowing distinct populations to arise,
• maybe in initial stages of speciation
• FURTHER READING
• FUTUYMA, DJ 2005. Evolution Ch 8 pp. 181-185.
• YUNIS PRAKASH 1982. Science 215, 1525-1530.
• (Human, chimpanzee, gorilla, orang-utan
  chromosomes)

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Possible mechanism the shifting
balance today, considered somewhat
controversial, but must explain some chromosomal
evolution?
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Translocations and Robertsonian rearrangements
common in mammals. Usually, populations are
fixed for a translocation. Populations fixed
for alternative rearrangements often called
chromosomal races. Common in species such as the
european house mouse Mus musculus
domesticus. Non-disjunction rates often low, 0
- 15, not approx. 50 as one might expect.
Mammals, like Drosophila, appear to have
mechanisms which reduce production of unbalanced
gametes.
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A translocation polymorphism in humans One human
chromosome involved in translocation polymorphism
is chromosome 21 a heterozygote for this
translocation can produce Downs syndrome
offspring.