Two main themes underlying the observations on chromosomal changes
Karyotypes generally remain constant within a species
most genetic imbalances result in a selective disadvantage
Related species usually have different karyotypes
closely-related species differ by only a few rearrangements on their karyotypes
distantly-related species differ by many rearrangements on their karyotypes
Correlation between karyotypic rearrangements and speciation
Four types of chromosomal rearrangement: deletion, duplication, inversion, and translocation
Fluorescent in situ hybridization can detect large chromosomal rearrangements using spectral karyotyping with probes specific for two different chromosomes showing chromosomal translocation
Phenotypic and genetic effects of deletions
homozygosity for deletions is often lethal or harmful depending on size of deletions and affected genes
deletion in heterozygotes can have a mutant phenotype due to gene dosage effects (haploinsufficiency)
deletion in heterozygotes increases risk of altered phenotype due to mutation in the other copy of the gene, may uncover recessive mutant alleles
Transposable elements (TEs) are movable genetic elements
TEs are any segment of DNA that has the ability to move from place to place within a genome
Marcus Rhoades (1930s) and Barbara McClintock (1950s) inferred existence of TEs from genetic studies of corn
TEs have now been found in all organisms with some functions beneficial to the host, can be present in hundreds of thousands of copies per genome
Instability of TE assiciated alleles
a nonautonomous TE (Ds) inserted into gene C and disrupted its function. In kernals that have autonomous TE (AC), Ds can hop out, restoring gene function
Structure of DNA transposons
Most DNA transposons contain inverted repeats (IRs) of 10-200 bp long at each end, gene encoding trasnposase recocgnizes IRs and cuts at border between the IR and genomic DNA
TEs can disrupt genes and alter genomes
TE insertion can result in altered phenotype. TE can insert within coding region of a gene or insert near a gene and affect its expression. TE-associated alleles can be unstable
TEs can trigger spontaneous chromosomal rearragnements due to unequal crossing over between TEs
Gene relocation due to transposition, formation of composite TE
Aneuploidy is loss or gain of one or more chromosomes
Aneuploids: individuals whose chromosome number is not an exact multiple of the diploid number for that species
Euploidy: 2n
Nullisomy: 2n-2
Monosomy: 2n-1
Trisomy: 2n+1
Aneuploidy usually results in multiple abnormalities or lethality
autosomal monosomy is usually lethal
most trisomies are lethal; a few result in characteristic syndromes
sex chromosome aneuploidy is tolerated due to X chromosome inactivation
Trisomic 21: down syndrome
Trisomic 13: Patau syndrome
Trisomic 18: Edwards syndrome
XO, monosomic: turner syndrome
XYY trisomic, normal
XXY: trisomic Klinefelter syndrome
XXYY, XXXY: tetrasomic Klinefelter syndrome
XXXXY: pentasomic Klinefelter syndrome
XXXXXY: hexasomic Klinefelter syndrome
Aneuploidy is caused by nondisjunction
nondisjunction: failure of chromosomes to segregate normally, can occur during meiosis I or meiosis II
Some euploid species are not diploid
basic chromosome number (number of chromosomes in a single, complete set = x
monoploidy and polyploidy are rarely observed in animals, except ants, bees, hermaphroditic worms, and some fish
Formation of a triploid (3x) organism
diploid gametes may arise from a 4x parent or from a diploid with defects in meiosis (defect in spindle or at cytokinesis)
Meiosis in a triploid organism
regardless of how chromosomes pair, there is no way to ensure that gametes contain a complete, balanced set of chromosomes. all polyploids with odd numbers of chromosome sets are sterile because they cannot produce balanced gametes.
Generation of tetraploid (4x) cells
tetraploid cells occur during mitosis in a diploid when chromosomes fail to separate into two daughter cells
if tetraploidy occurs in gamete precursors, diploid gametes are produced
union of two diploid gates produces a tetraploid organism
autopolyploid: all chromosome sets are derived from the same species
In tetraploid, pairing of chromosomes as bivalents generates balanced gametes
four copies of each group of homologs pair two by two to form two bivalents
successful tetraploids produce balanced 2X gametes and are fertile
Polyploids in agriculture
1/3 of all known flowering plant species are polyploid
polyploidy often results in increased size and vigor in plants
many polyploid plants have been selected for agricultural cultivation
tetraploids: alfalfa, coffee, peanuts, Macintosh apples, Bartlett pear
octaploids: strawberries
Euploid: a complete set of chromosomes (usually diploid)
Polyploid: a euploid species that carries 3+ complete sets of chromosomes
Autopolyploid: a kind of polyploid that derives all its chromosome sets from the same species
Allopolyploid: hybrids in which chromosome sets come from distict, but related, species. Usually infertile ebcause different chromosome sets cannot easily pair and segregate properly
Amphidiploid: two diploid genomes, each from a different parental species
Raphanobrassica: sterile F1 from crossing cabbages and radishes, has 18 chromosomes (9 from each parent)
Creation of the allopolyploid Triticale
F1 hybrid of wheat and rye, sterile because there are no pairing partners for the rye chromosomes
fertile Triticale can be created from infertile F1 hybrid Triticale
some triticale hybrids combine high yield of wheat with ability of rye to grow in unfavorable environments
some triticale hybrids combine high level of protein from wheat and high level of lysine from rye
Colchicine treatment prevents spindle formation and results in doubling of chromosome numbers
Duplicate genes may acquire new functions
gene families: sets of related genes with slightly different functions
genome in common ancestor of all cereal grasses was duplicated. The two copies are related, but have diverged in sequence and function
Rapid chromosomal evolution in house mice on the island of Madeira
one population of mice introduced to island in 1400s; two populations evolved different sets of Robertsonian translocations, hybrid offspring are sterile
