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Explore the world of chromosomal mutations, focusing on changes in number and structure. Learn about polyploidy, aneuploidy, and the evolutionary consequences of these mutations. Discover famous examples and facts about these chromosomal alterations.
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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)
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] Examples of ploidy levels
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,...
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.
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
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
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 Meiotic nondisjunction = aneuploid products
Large-Scale Chromosomal Changes Changes in Chromosome Structure
Deletion loops in Drosophila genes missing from chromosome #2 #1 #2
Deletion origin of “cri du chat” syndromesee hear: http://www.youtube.com/watch?v=TYQrzFABQHQ
Inversions cause diverse changes breakpoints between genes 1 breakpoint between genes, 1 within gene breakpoints within 2 genes
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 Pericentric inversions can lead to duplication-and-deletion products
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 Chromosome segregation in reciprocal-translocation heterozygote
Variegation resulting from gene’s proximity to heterochromatin
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