The return of Cytogenetics

Meadowfoam meiosis

Metaphase I Anaphase I

n

n

2n

Reduction division

Meadowfoam meiosis

Metaphase I Anaphase I

a

A a

A

Mendel’s first law – law of segregation

Meadowfoam meiosis

Metaphase I Anaphase I

a

B

a

B

A

bA

b

Meadowfoam meiosis

Metaphase I Anaphase I

a

B

a

b

A

BA

b

Mendel’s second law – law of independent assortment

But what if the genes are on the same chromosome?

Metaphase I Anaphase I

a

B

a

b A

BAb

But what if the genes are on the same chromosome?

Metaphase I Anaphase I

a

B

a

b A

BAb

No independent assortmentDeviation from Mendel’s second law

Creating gene maps

Creating gene maps

Many genes were mapped to specific chromosomes and specific chromosome arms in wheat and barley before the advent of molecular genetics

How was this possible?

Chromosome mapping

Many genes were mapped to specific chromosomes in wheat and barley before the advent of molecular genetics

How was this possible?

Deviation from Mendel’s laws

Chromosome mapping

- Monosomics

- Trisomics

- Deletions

Metaphase I

non-disjunction

Anaphase I

n - 1

n + 1

Wheat Monosomic

R.J. Singh 2002

Barley trisomic 6

Barley telotrisomic 6S

If there is a chromosome or part of a chromosome missing the inheritance of any gene on the missing part will deviate from Mendel’s laws

If there is an extra chromosome or chromosome arm the inheritance of any gene on the extra chromosome will also deviate from Mendel’s laws

Combining cytogenetics and molecular genetics for gene mapping

Wheat deletion series – mapping wheat gene locations

Wheat is a hexaploid with 3 genomes A, B and D.The genes are arranged on the same chromosomes in the different genomes, making the chromosomes homeologous (1A, 1B and 1D carry similar genes)

Set of deletion lines

A A A a a a gene expression

Set of deletion lines

location of gene

A A A a a a gene expression

How do you know which chromosome to test?

- Screen plants that carry 4 copies of one chromosome and no copies of a homeologous chromosome (nulli-tetrasomic)

Example: nulli-tetrasomic for chromosome 1D 0 copies of chromosome 1D, 4 copies of either chromosome 1A or chromosome

1B

Dis

omic

Nul

li 1A

Nul

li 1B

Nul

li 1D

Southern blot results with marker used as a probe – what is the chromosome location of the probe?

Dis

omic

Nul

li 1A

Nul

li 1B

Nul

li 1D

Southern blot results with marker used as a probe – probe is on chromosome 1D

Dis

omic

Nul

li 1A

Nul

li 1B

Nul

li 1D

Telo

1D

S

Telo

1D

LSouthern blot results with marker used as a probe – what is the chromosome arm location of the probe?

Dis

omic

Nul

li 1A

Nul

li 1B

Nul

li 1D

Telo

1D

S

Telo

1D

LSouthern blot results with marker used as a probe – Chromosome arm location – 1DS

Dis

omic

Nul

li 1A

Nul

li 1B

Nul

li 1D

Telo

1D

S

Telo

1D

L

Del

1D

S-1

Del

1D

S-2

Del

1D

S-3

Southern blot results with marker used as a probe – what section of chromosome IDS is the location of the probe?

Dis

omic

Nul

li 1A

Nul

li 1B

Nul

li 1D

Telo

1D

S

Telo

1D

L

Del

1D

S-1

Del

1D

S-2

Del

1D

S-3

Gene is on the short arm of chromosome1D in the region covered by deletion 1DS-3

Genome sequencing

- Barley

How can you separate out a specific chromosome for sequencing?

Genome sequencing

- Barley

How can you separate out a specific chromosome for sequencing?

Use differences in chromosome size provided by the presence of telochromosomes in telo-trisomics

Barley telotrisomic 6S

Transposable elements

Barbara McClintock- cytogeneticist- worked on corn- in the 1940’s worked on a trait that appeared to change locations on a chromosome (inversions) and/or move between chromosomes (translocations)- problem, no observable chromosome changes- so she proposed the idea of transposable elements

21st century Cytogenetics- molecular cytogenetics

In situ hybridization

Canola / Rapeseed

Visualization of C genome chromosomes

BC1 between wheat and jointed goatgrass - which species is the recurrent parent?

Count number of yellow chromosomes, greater than 7 – recurrent parent jointed goatgrass

BC1 between wheat and jointed goatgrass - which species is the recurrent parent?

Maize chromosomes