BIOL 265 - Chapter 8 (8.4 - polyploidy)

Polyploidy

Presence of more than two sets (>2n) of chromosomes.
(Ex. triploid: 3n, tetraploid: 4n, etc.)

• More common in plants; less common in animals

• two types

Autopolyploidy

chromosome sets are all from one species.

How does an autopolyploidy arise?

Arises by nondisjunction of all types of chromosomes or failure of cytokinesis in mitosis or meiosis.

Ex. nondisjunction of (2n) chromosomes in mitosis (no cell division = 4n cell

Ex. nondisjunction of (2n) chromosomes in meiosis I creates 2n gametes, which fuse to normal 1n gametes = 3n zygote

Ex. nondisjunction of (2n) chromosomes in meiosis I creates 2n gametes in BOTH parents, which fuse = 4n zygote

For an autopolyploidy, explain how chromosomes interact during meiosis and the result

All chromosome sets are homologous and may attempt to pair during prophase I. This usually results in sterility

Explain what occurs in meiosis for an autotriploid

In meiosis in an autotriploid, homologous chromosomes can either pair two of three, pair all three, or not pair at all. The resulting gametes will be unbalanced, as they vary in chromosome number. (don’t produce viable offspring).

Explain what occurs in meiosis for an autopolyploid with even numbers

Autopolyploids with even numbers of chromosome sets usually also produce unbalanced gametes due to failure of equal segregation of chromosomes during meiosis. (sterile)

Allopolyploidy

chromosome sets are from two or more species.

How does an allopolyploidy arise?

-       Arises from hybridization between two different species.

-       However, the resulting hybrid’s nonhomologous chromosome are all different, so they don’t pair or segregate properly (unbalanced, nonviable gametes)

-       To fix this, nondisjunction occurs in mitosis, doubling all chromosomes (so they have a pair), creating an amphidiploid.

-       may also arise when nondisjunction during meiosis leads to a 2n gametes, which fuses with a 1n or 2n gametes from a different species to produce a polyploid individual.

Amphidiploid

A type of allopolyploid with combined diploid genomes of two different species

Amphidiploid formula

2n + 2n (add the two totals together)

Consider that species A has 2n = 8 chromosomes and species B has 2n = 14 chromosomes.

What would be the chromosome number for an amphidiploid formed from A and B?

22 only (14+8)

Ploidy	Genome formula	General name	Specific name
		Haploid
		Diploid
		Triploid	Autotriploid
		Triploid	Allotriploid
		Tetraploid	Autotetraploid
		Tetraploid	Allotetraploid
		Hexaploid	Autohexaploid
		hexaploid	allohexaploid

Ploidy	Genome formula	General name	Specific name
N	A	Haploid
2n	AA	Diploid
3n	AAA	Triploid	Autotriploid
3n	AAB	Triploid	Allotriploid
4n	AAAA	Tetraploid	Autotetraploid
4n	AABB	Tetraploid	Allotetraploid
6n	AAAAAA	Hexaploid	Autohexaploid
6n	AABBDD	hexaploid	allohexaploid