4/16/02
Reminder:
Research seminar today (4/16), MO 157, 12:30, mosquito transmitted
viruses
Ch. 18- Changes in chromosome number - Aneuploidy and
Polyploidy
Polyploidy = Nuclei, cells or organisms having 3 or more full sets of
chromosomes
Euploidy = Nuclei, cells or organisms having one or more copies of
the basic chromosome set for that particular organism (i.e., monploid,
diploid, triploid, tetraploid, etc.). For eukaryotes, the normal euploid
condition is diploid (2x) and haploid (1x) for the gametes or
gametophyte.
Aneuploidy = Nuclei, cells or organisms in which single
chromosomes (or portions of a chromosome set) have been gained or
lost usually due to nondisjunction.
Nondisjunction = abnormal chromosome segregation at anaphase of
meiosis or mitosis.
Origin of polyploids
• Results from spontaneous doubling of the chromosome number
during mitosis or meiosis: 1) failure of spindle fibers to fully
separate chromosome to poles such that a single nucleus forms
around all the chromosomes; 2) following first or second meiotic
division (before cytokinesis) two nuclei fuse, producing an
unreduced gamete. Unreduced gametes fusing with normal
gametes can create triploids; 3) two (or more) sperm fertilize an
egg.
• Naturally occurring polypoids found commonly in plant species as much as 50% of all plant species are of polyploid origin. Rare
in animal species with some amphibians, hermaphroditic worms,
fish, among others. Many of these are parthenogenic.
• Induced chromosome doubling: 1) using a chemical mitotic spindle
inhibitor such as colchicine; 2) mechanical or temperature shock
disrupting normal spindle formation
Symbols used to designate polyploids
• n = haploid number or chromosome number in the gamete
• x = base chromosome number of that organism
• normally n and x are the same if the organism in question has a
diploid chromosome number such as humans (2n = 2x = 46;
n=x=23)
• Exceptions are polyploids. For example, cultivated wheat is a
hexaploid (2n = 42) and the chromosome number in the gamete is
triploid (n = 21). The base chromosome number for wheat is x = 7.
The proper designation for cultivated wheat is 2n = 6x = 42
Types of polyploids - autopolyploids and allopolyploids
• Autopolyploid = multiple sets of chromosomes of the same
species; chromosome sets are fully homologous
• Allopolyploid = sets of chromosomes from different species;
chromosome sets are homeologous (partly homologous)
Autotetraploid identification
• Phenotype similar or identical to diploid; usually larger plant, cell,
fruit, and flower
• Karyotype - 2x's the normal diploid number; four of each
chromosome that are indistinguishable
• Meiotic pairing - Quadrivalent expected 2/3 of the time and two
bivalents expected 1/3 of the time.
Origin of allotetraploid
• Classically an allotetraploid arises when two closely related,
diploid species are hybridized. The F1 is diploid, too but often
sterile because the haploid parental genomes are so differentiated
that pairing and meiosis is aberrant in the hybrid. If spontaneous
(or induced) chromosome doubling occurs in the F1, an
allotetraploid is created and fertility is restored since the
chromosomes have homologous chromosomes to pair with and
meiosis is normal. See Raphanus x Brassica example (Fig. 18-9)
Photo of meiotic pairing in an allohexaploid
Helianthus. Note mostly bivalent formation.
Allotetraploid identification
• Phenotype intermediate between, or shares characteristics with two
putative parental species.
• Karyotype - chromosome number consistent with a tetraploid;
displays some divergence among homoeologous chromosomes.
• Meiotic pairing - As indicated in the illustration, nearly exclusive
bivalent formation (i.e., few to no quadrivalents) expected in the
allotetraploid since homoeologues rarely pair when homologues
are available.
Why so many polyploids?
• Bigger
• More robust in stressful environments
• Genetic buffering
• Hybrid speciation - reticulate evolution
• Fixed heterozygosity