Chapter 17 Large-Scale Chromosomal Changes Changes in Chromosome NumberChanges in Chromosome Shape.

Chapter 17Large-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 21Down Syndrome

Figure 16-12 step 1

Meiotic nondisjunction = aneuploid products

Figure 16-12 step 2


Figure 16-12 step 3


Figure 16-12 step 4


Figure 16-12 step 5


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 missingfrom chromosome#2

#1

#2

Deletion loops in Drosophila

Deletion origin of “cri du chat” syndrome

see hear: http://www.youtube.com/watch?v=TYQrzFABQHQ

http://www.youtube.com/watch?v=TYQrzFABQHQ

http://www.youtube.com/watch?v=TYQrzFABQHQ

Duplications following polyploidy in Saccharomyces

Inversions cause diverse changes

breakpointsbetween genes

1 breakpointbetween genes,1 within gene

breakpointswithin 2 genes

Inversion loops at meiosis

Paracentric inversions can lead to deletion products

Figure 16-29 step 1

Pericentric inversions can lead to duplication-and-deletion products

Figure 16-29 step 2


Figure 16-29 step 3


Figure 16-29 step 4


Reciprocal translocation revealed by molecular

cytogenetics

Figure 16-30 step 1

Chromosome segregation in reciprocal-translocation heterozygote

Figure 16-30 step 2


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, co-option by different functions)

Chapter 17 Large-Scale Chromosomal Changes Changes in Chromosome NumberChanges in Chromosome Shape.

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