Chapter 17
Large-Scale Chromosomal Changes
Changes in Chromosome Number
Changes in Chromosome Shape
Types of chromosome mutations
all generated by natural mutagens—extreme temps, UV, chemicals, etc.
Euploidy
• euploidy: change of chromosome number
involving 1 or more whole genomes
– autopolyploidy = doubling of genome from
“wild type” (e.g., tetraploid from diploid,
hexaploid from triploid)
– allopolyploid = doubling of genome from
hybrid of two distinct taxa (e.g., varieties,
species, genera)
Examples of ploidy levels
Balanced (normal
meiosis)
• 2n = diploid
• 4n = tetraploid
• 6n = hexaploid
• 8n = octoploid
• and so on
Unbalanced (abnormal
meiosis)
• 1n = monoploid
• 3n = triploid
• 5n = pentaploid
• 7n = heptaploid
[usually hybrids of
ploidy levels on left]
Facts about polyploidy and
allopolyploids
• Uncommon in animals but abundant
(ancient and ancestral?) in plants
• Recent genetic research shows
allopolyploids far more common than
autopolyploids—different from theory
• Many allopolyploids found with multiple
origins—contrary to evolutionary paradigm
of “single origin” for species
Polyploids often produce larger
structures, e.g., guard cells, pollen,...
...and fruits (e.g., tetraploid grapes)
Meiotic pairing in triploids—> unbalanced gametes (sterility)
Mules have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and
number prevents the chromosomes from pairing up properly and creating successful embryos, rendering
mules infertile.
Colchicine used to induce polyploidy
A famous natural allohexaploid:
Bread wheat (Triticum aestivum)
Famous Examples of Allopolyploid
Complexes
• Appalachian Asplenium ferns—several
diploids, triploid hybrids, several tetraploids
• Domesticated coffee (Coffea arabica)-parentage documented through molecular
cytogenetic “chromosome painting”
• Dandelions, roses, blackberries--more
complicated groups that also do
agamospermy (sex without seeds)
Evolutionary consequences of
polyploidy
• polyploids often more physiologically “fit” than
diploids in extreme environments
• polyploids reproductively isolated from original
ploidy levels, may eventually differentiate
• allopolyploids commonly occupy ecological
niches not accessible to parental types
• opportunities for gene silencing or chromosomal
restructuring without disastrous consequences
Monoploid plants grown in tissue culture
Summary
• polyploids common in plants
• autoploids formed by doubling of “wild type” genome,
allopolyploids from doubling of hybrid
• allopolyploids far more common than autopolyploids
• polyploids often more “fit” than parent(s), often in niches
different from parent(s)
• opportunities for evolutionary change through gene
silencing or chromosome restructuring
Facts about aneuploids
• Rare in animals, always associated with
developmental anomalies (if they survive)
• Most well known examples in human
genetic diseases
• Common in plants, sometimes show
phenotypes, sometimes not
Extra chromosome 21
Down Syndrome
Meiotic nondisjunction = aneuploid products
Figure 16-12 step 1
Meiotic nondisjunction = aneuploid products
Figure 16-12 step 2
Meiotic nondisjunction = aneuploid products
Figure 16-12 step 3
Meiotic nondisjunction = aneuploid products
Figure 16-12 step 4
Meiotic nondisjunction = aneuploid products
Figure 16-12 step 5
Meiotic nondisjunction = aneuploid products
Figure 16-12 step 6
Trisomics of Datura (jimsonweed)
Large-Scale Chromosomal Changes
Changes in Chromosome Structure
Types of chromosome mutations
Deletion loops in Drosophila
genes missing
from chromosome
#2
#1
#2
Deletion loops in Drosophila
Deletion origin of “cri du chat” syndrome
see hear: http://www.youtube.com/watch?v=TYQrzFABQHQ
Duplications following polyploidy in
Saccharomyces
Inversions cause diverse changes
breakpoints
between genes
1 breakpoint
between genes,
1 within gene
breakpoints
within 2 genes
Inversion loops at meiosis
Paracentric inversions can lead to deletion
products
Paracentric inversions can lead to deletion
products
Paracentric inversions can lead to deletion
products
Paracentric inversions can lead to deletion
products
Paracentric inversions can lead to deletion
products
Paracentric inversions can lead to deletion
products
Pericentric inversions can lead to duplication-and-deletion
products
Figure 16-29 step 1
Pericentric inversions can lead to duplication-and-deletion
products
Figure 16-29 step 2
Pericentric inversions can lead to duplication-and-deletion
products
Figure 16-29 step 3
Pericentric inversions can lead to duplication-and-deletion
products
Figure 16-29 step 4
Reciprocal translocation revealed
by molecular cytogenetics
Chromosome segregation in reciprocal-translocation heterozygote
Figure 16-30 step 1
Chromosome segregation in reciprocal-translocation heterozygote
Figure 16-30 step 2
Chromosome segregation in reciprocal-translocation heterozygote
Figure 16-30 step 3
Variegation resulting from gene’s
proximity to heterochromatin
Variegation in translocation
heterozygote
Chloroplast rearrangements
• Great evolutionary
significance in
reconstructing
relationships among land
plant lineages
• Can easily be screened for
by PCR amplification of
“universal” chloroplast
gene primer pairs flanking
large regions of
chloroplast
Judd et al. (2002)
Chloroplast rearrangements
• Major inversions found in certain groups of
families of bryophytes, pteridophytes,
gymnosperms and several groups of angiosperms
• Loss of one copy of inverted repeat in a few
families!
• Numerous losses of certain introns across
angiosperms (e.g., rpl2 in Cactaceae)
• Differences in size of large single-copy region by
expansion or contraction of intergenic spacers
Summary
• each different chromosomal change shows
characteristic meiotic pairing as a “signature”
• deletions in diploids often have grave
consequences; in polyploids do not but may lead
to differentiation of new organisms
• duplications (in plants) generally have few or no
consequences, often provide additional genes for
evolutionary processes to act on (silencing, cooption by different functions)