Spring 2008: Section 3 – lecture 6

Reading – Chapter 9

Spring 2009: Section 3 – lecture 6

Reading – Chapters 8

Additional explanation on the problems during meiosis in autopolyploids that can reduce gamete viability

Disadvantages of an autopolyploid:

- problems in meiosis with chromosome pairing and even distribution of chromosomes.

- low or no seed set

When the multiple homologous chromosomes in an autopolyploid attempt to pair during meiosis you will get multi-valents such as tetravalents and trivalents produced in addition to the normal bivalents. Univalents will also be produced that may be lost during completion of the reduction and equational divisions of meiosis

Besides losing univalents, dividing the homologous chromosomes during anaphase I can lead to an unequal distribution of the chromosomes leading to gametes having extra chromosomes or lacking chromosomes. These imbalances will reduce the viability of the gametes

If only bivalents form during meiosis, it is possible for an autopolyploid to be fertile. An example would be alfalfa, an autotetraploid.

If the chromosome sets originated from more than one related species then the organism is said to be an allopolyploid.

The chromosomes that are from related species but carry the same genes in relatively the same linear order but do not pair during meiosis are said to be homeologous chromosomes.

Advantages of allopolyploids

- plants and seeds are larger

- more genetically diverse so able to adapt to more environments

- fertile because only bivalents are formed in meiosis because the homeologous chromosomes will not pair.

Example: wheat

An allohexaploid with chromosome sets donated by 3 related species. All the donor species (AA, BB, DD) were diploids with a 2N number of 14.

First two diploid species need to cross to produce an allotetraploid

AA X BB

AB double the number of

chromosomes through endomitosis

AABB now have an allotetraploid (4X)

Then the allotetraploid crosses to another related diploid species

AABB X DD

ABD double the number of

chromosomes by endomitosis

AABBDD allohexaploid (6X)2N = 42 N = 21

Chromosomes 1A, 1B, and 1D would be called homeologous chromosomes because they carry the genes for the same traits.

Example of allotetraploid – rapeseed

Brassica napus (4X) - 38 chromosomes

(AACC)

three possible donor species

Brassica oleracea (2X) – 18 chromosomes

(CC)

Brassica nigra (2X) – 16 chromosomes

(BB)

Brassica campestris (2X) – 20 chrom.

(AA)

Triangle of U

Why are mules sterile?

Example of an allodiploid

Horse (HH) X Donkey (DD)

Mule (HD)

In meiosis would you have proper chromosome pairing and distribution?

Changes in the number of chromosomes in a chromosome set will result in aneuploidy.

With aneuploidy the addition or deletion of a chromosome may have a major or minor effect.

Effect depends on the genes that are on the missing or additional chromosome and the tolerance of the species to aneuploidy.

Mammals have a low tolerance while plants have a high tolerance, especially polyploids.

Aneuploidy occurs due to non-disjunction of either the homologous chromosomes or the sister chromatids during meiosis.

Non-disjunction during the first or second division of meiosis

first division - non-disjunction

n+1

n+1 n+1

2N

n-1 n-1

n-1

second division – non-disjunction

n+1

n n-1

2N

n n

n

Meadowfoam meiosis

Metaphase I Anaphase I

n

n

Metaphase I

non-disjunction

Anaphase I

n - 1

n + 1

Example of the addition of a chromosomehumans - 2N = 46

- If one chromosome is added the condition is called trisomic with

2N+1= 47 - If a pair of homologous chromosomes

are added the condition is called tetrasomic with 2N+2 = 48

- If two non-homologous chromosomes are added the condition is called double trisomic with 2N+1+1 = 48

Examples in humans

- trisomic for chromosome 21 - Down’s syndrome

- trisomic for a sex chromosome - Klinefelter’s syndrome (XXY)

examples of trisomy in plants include:

barleycorn

The presence of an extra chromosome can be useful in determining the chromosome location of a gene because the presence of the extra chromosome disrupts normal Mendelian segregation.

The presence of an extra chromosome (trisomy) is observed more in diploid species than the deletion of a chromosome (monosomy) because diploids do not have the necessary gene redundancy to tolerate the loss of any genetic information.

Example of the deletion of a chromosomewheat - 2N = 42

- If one chromosome is missing the plant is said to be monosomic with

2N-1 = 41. - If a homologous pair of chromosomes

is missing the plant is said to be nullisomic with 2N-2 = 40. - If two non-homologous chromosomes

are missing the plant is double monosomic with 2N-1-1 = 40.

Example in humans:

Monosomic for a sex chromosome - Turner’s syndrome (X instead of XX)

This form of aneuploidy in polyploid plants is useful for genetic analysis to determine the chromosome location of a gene.

Changes within a chromosome

a) deletions

b) additions

c) inversions

d) translocations

Deletions

Loss of a section of a chromosome. This loss could be from an end or from within (intercalary) the chromosome.

Deletions can be caused by unequal crossing-over, by radiation or by movement of transposable elements or recombination between elements.

Terminal deletion by radiation

Intercalary deletion by radiation

Can observe deletions in meiosis during prophase I if the individual is heterozygous for the deletion.