Summary of large-scale chromosomal changes Chp 17

1. Deletion of small or larger segments 2. Missing 1 or a few chromosomes 3. Additional chromosomes 4. Duplication of small to larger segments 5. Inversion of segments 6. Translocation of segment from one to anthor See fig 17-2

Significance of chromosomal mutations or changes

1) They often characterize species differences and are often responsible for reproductive isolation between species.

2) A number of crop plants have undergone such changes and chromosome manipulation may be important in agriculture (breeding).

3) A number of such changes are responsible for human genetic diseases. 4) They may disrupt gene function directly if a break occurs in a gene.

5) Can lead to genetic redundancy, and the subsequent evolution of gene function, differential expression, or gene loss

Polyploidy

Possession of more than two haploid sets of chromosomes.

Diploids have two homologs of every chromosome.

Polyploids might have 3, or 4, or 5 etc of every homolog. So their chromosome number will usually be a multiple of the haploid chromosome number, n.

Some genera may have different species that form a polyploid series such as:

2x (diploid); 3x (triploid), 4x tetraploid, 6x hexaploid, 8x octoploid, where x is the basic haploid number of chromosomes.

1st metaphase of meiosis in diploid Turnera subulata, 2n=10

1st metaphase of meiosis in autotetraploid Turnera subulata, 4x=20.

1st metaphase of meiosis in autotriploid Turnera subulata, 3x=15

1st metaphase of meiosis in allohexaploid Turnera velutina, 6x=30

Taraxacum officionale (Dandelion) is a triploid apomict 2n = 3x = 24 It flowers, and sets seeds, but circumvents meiosis producing seeds through mitosis not meiosis and fertilization. All progeny and the parental plants should be geneticially identical to on another.

Triploid parthenogenetic salamanders and lizards are known to occur.

Musa acuminata x M. balbisiana 2n = 3x = 33. Genome are AAA, AAB, or ABB Triploid parthenocarps (fruits but no seeds).

Autopolyploids versus Allopolyploids Autopolyploids: chromosome derived from a single species. - Exhibit tetrasomic inheritance - Quadrivalents at MI Allopolyploids: chromosomes derived from more than one species - Genome-wide gene duplication - Bivalent formation at MI

Tetrasomic Inheritance in an autotetraploid For two alleles, A and a, 5 possible genotypes occur AAAA, AAAa, AAaa, Aaaa, aaaa To deduce results of crosses determine gametic output of each genotype: AAAA yields diploid gametes all of which are AA AAAa - ½ AA : ½ Aa AAaa – 1/6AA : 4/6 Aa : 1/6 aa Aaaa – ½ Aa : ½ aa aaaa – all aa So the cross AAaa x aaaa give 5 A- : 1 aaaa ratio What does this produce AAaa x AAaa?

Tetrasomic inheritance of the aconitase-2 locus in autotetraploid Turnera subulata 2n = 4x = 20.

From Shore JS. 1991. Heredity 66: 305—312

Origins of tetraploid Brassica species See fig 17-8

Brassica oleraceae 2n=18 cabbage, cauliflower, broccoli, kale kohlrabi, brussels sprouts

B. nigra, 2n = 16 Black mustard

B. campestris, 2n = 20 Chinese cabbage, turnip, turnip rape

B. juncea, 2n = 36 Leaf mustard

B. carinata, 2n = 34 Abyssinian mustard

B. napus, 2n = 38 Canola, Rutabaga

Origin of allopolyploid Wheats, Triticum See fig 17-9

T. urartu, 2n = 14 AA genome donor Wild diploid wheat

Aegilops speltoides, 2n = 14 BB genome donor Diploid goatgrass or extinct relative of it

T. turgidum, 2n = 28 AABB genomes Emmer wheat ~10,000 year ago

Aegilops tauschii, 2n = 14 DD genome donor Another wild grass species

T. aestivum, 2n = 42 AABBDD genomes Common bread wheat

A genome - yellow D genome - red B genome - brown Green - chromosome fragments of Thinopyrum intermedium

Genomic in situ hybridization (GISH) of allohexaploid wheat, Triticum aestivum 2n = 42 Kato et al. 2005

GISH in a Gossypium hirsutum x G. sturtianum hybrid 2n = 3x = 39 (ADC triploid) G. hirsutum (cotton) is an allotetraploid AADD

G. hirsutum A genome pink G. hirsutum D genome dark blue G. sturtianum C genome light blue

Guan et al. 2007

Triploid Salamander, Ambystoma jeffersonianum-laterale 2n = 3x = 42 Genomes are JJL. Photo by Dr. L.E. Licht

CHANGES IN CHROMOSOME NUMBER AND STRUCTURE 1. ANEUPLOIDY – MONOSOMY, TRISOMY

2. GENE “BALANCE”, DOSAGE, AND INVIABILITY

3. DELETIONS AND DUPLICATIONS

4. INVERSIONS

5. TRANSLOCATIONS

ANEUPLOIDY Changes in chromosome number not involving complete haploid sets Humans: XXY, XYY, XXX, X0 Causes: Nondisjunction Monosomy 2n-1 (lethal in humans with exception of X0) X0 – Turner syndrome – phenotypic effects including some level of congitive impairment Trisomy 2n+1 XXY – Klinefelter syndrome, Males, lower IQ, sterile XYY – once thought to have enhanced violence. Not clear XXX – females normal Trisomy 21 – Down syndrome 13- Patau syndrome – 130 day life expectancy 18 – Edwards syndrome – a few weeks

Gene balance may provide an explanation for detrimental effects of trisomy and monosomy.

CHROMOSOME REARRANGEMENTS Possible routes to chromosome rearrangements See fig 17-19 Two break points are typically required. If these occur on a single chromosome this can result in a deletion or and inversion. If two nonhomologous chromosome each has a break, a Reciprocal translocation can occur. If two homologous chromosomes each have break point this can produce both duplications and deletions

Breakage an reunion leading to rearrangements

a b c d e f g h i

See fig 17-19

Breakage an reunion leading to rearrangements

a b c d e f g h i a b c d e f g h i

See fig 17-19

Chromosome deletions and inversions may be visible using microscopy especially in the giant/polytene chromosomes of Drosophila or a pachytene

–see fig 17-20

A B C D E J K L M

A B C D E J K L M

H G F I

Possible routes to chromosome rearrangements Via repetitve elements See fig 17-19 It is possible that crossing over between repetitive elements (e.g. transposable elements) generates the various rearrangements Crossing over between elements on a single chromosome can result in a deletion or inversion. If two nonhomologous chromosome each have the same repeated Element, crossing over between them can yield a reciprocal translocation. If two homologous chromosomes each have the same repeated element, crossing over can produce both duplications and deletions

Chromosome 5 showing deletion responsible for Cri du chat syndrome See fig 17-22.

Deletion

Note that deletions can be recognized and are often very useful in identifying thelocations of genes of interest.

a b c d e f g h

A E F G H

Inversions See Fig 17-27

A B C F E D G H I a b c d e f g h i

Reciprocal translocations See fig on page 589 for photograph of chromosomes Fig 17-30 for pairing and results of segregation of reciprocal Translocations Fig 17-33 occurrence of Down syndrome when chromosome 21 is involved in a reciprocal translocation

Reciprocal translocation

Breaks and rejoining

Alternate disjuntion - Viable gametes

Adjacent disjunction -often inviable due to deletions and duplications

Robertsonian fusion – asymmetrical reciprocal translocation involving centromere loss see fig 17-33

Chr 14 chr 21

Normal Robertsonian Lost

Familial form of Down Syndrome involving Robertsonian fused chromosome 21

Carrier – Normal Down syndrome

Fate of human zygotes resulting from various chromosome abnormalities see fig 17-37 Spontaneously aborted embryos (15% of conceptions) Of 150,000 such embryos 75,000 due to chromosome abnormalities 39,000 of these are trisomics for various chromosomes 13,500 are XO 12,750 triploids 4,500 tetraploids 5,250 other chromosome abnormalities. For live births (i.e. the remaining 85% of conceptions

For live births (i.e. the remaining 85% of conceptions) About 0.6% of those result from chromosome abnormalities Including various numbers of Sex chromosome aneuploids Trisomics for chromosomes 13, 18, 21 Robertsonian translocations and other reciprocal translocations Inversions Others, small deletions etc.