Transcript Chromosomal evolution and speciation

BIOL2007
CHROMOSOMAL EVOLUTION
Genes are found on chromosomes.
Rule:
Gene action usually independent of
chromosomal location.
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”
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 <1 Mb in humans, and
<100 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
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!!
Chromosomal rearrangements:
gross changes in chromosomal
morphology.
Lots of Greek: telomere, centromere,
autosome, chromosome,
heterochromatin...
Chromosomal morphology
Rearrangements
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
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
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?
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.
Evolutionary effect of rearrangements
General rule:
Heterozygous rearrangements often lead to
the production, in meiosis, of:
UNBALANCED GAMETES
(duplications and deletions in progeny)
e.g. Paracentric inversions
Inversion heterozygotes; chromosomes pair in loops
No crossing over in inversion: gametes fine
Crossing over in inversion, problems
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)
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.
Paracentric inversions are commonly polymorphic in Diptera.
There is even evidence for heterozygous ADvantage
 maintains polymorphisms
e.g. Drosophila, & Anopheles, malaria mosquitoes.
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
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
Humans and great apes
9 pericentric
inversions,
and one
reciprocal
translocaton
from Yunis & Prakash (1982)
Science 215, 1525-1530.
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.
"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!
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).
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).
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.
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
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
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.
• 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)
Possible mechanism:
“the shifting
balance”
today, considered
somewhat
controversial,
but must explain
some
chromosomal
evolution?
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.
A translocation polymorphism in humans
One human chromosome involved in translocation
polymorphism is chromosome 21; a heterozygote for this
translocation can produce Down’s syndrome offspring.