Polyploidy

Polyploidy, Allopolyploidy, Autopolyploidy, Amphiploidy

Definitions

Polyploidy = when an organism contains 3 or more

times the haploid (1N) number of chromosomes.

E.g. Normal for 2N (homologous pairs), polyploidy = 3N or more)

(Rare in ANIMALS, but common in PLANTS).

Hermaphrodite animals usually are polyploidy, such as flatworms, and earthworms. Polyploid humans usually abort spontaneously.

Human Polyploid Karyotype

Usually they spontaneouslyabort however this one made itto full term!

Polyploid = triploid (3N)

One parent contributes thenormal 22 autosomes +one sex chromosome, theother parent contributesthe full 46 chromosomes.

About 2% of conceptions! BUTcould be more, as most abort.

Definitions cont…Allopolyploidy = type of polyploidy, results

from mating between TWO different SPECIES.

THINK PLANTS!!!

Species A mates with Species B to produce infertile hybrid. Failure during Meiosis (non-disjunction) doubles the number of chromosomes in they hybrid. The hybrid self fertilises to produce a fertile new species.

AA + BB = AB (infertile hybrid)

AB non-disjunction = AABB, gametes AB (pollen), AB (egg)

Self fertilise = AB + AB = AABB (fertile new species)

Definitions cont…Amphiploidy = The name given to the step when there is anerror in meiosis (non disjunction) resulting in double thenumber of chromosomes in the hybrid.

In plant evolution – especially wheat this has happened and with more than two species interacting to form a fertile new species.

• AA mates with BB = AB (infertile hybrid)• Amphiploidy results = AABB (new fertile species)• AABB mates with DD = ABD (infertile hybrid)• Amphiploidy results = AABBDD (new fertile species –

with all the desirable traits)

Allopolyploidy - wheat

Definitions cont…

Autopolyploidy = form of polyploidy, results from

mating between SAME SPECIES.

E.g. Chromosomes fail to separate during meiosis. Species A mates with species A, non-disjunction occurs in both individuals or within one plant resulting in their gametes carrying AA or nothing. When AA + AA fertilise (or self fertilise) = AAAA (fertile hybrid).

If non-disjunction occurs in only one individual then the hybrid will be infertile. AA + A = AAA (infertile)

Autopolyploidy

Summary

Non-disjunction – this is the process that leads to these mutations and how species can go from producing infertile hybrids to fertile new species!

Colchicine = drug used to double chromosome number. Destroys spindle fibers so chromatids can’t separate.

BUT why are hybrids usually infertile?

WHY?All species have their specific number ofchromosomes (plant = 6CH, 3 homologous pairs),during meiosis the pairs have to match up beforethey separate.Scenario:• Non-disjunction occurs in female part (Egg).• One of her eggs containing 6 CH is fertilised by a pollen that has

gone through meiosis without error (pollen = 3 CH).• Seed = 9 CH.• The seed germinates and becomes a plant that is infertile because

when its reproductive structures go through meiosis they can’t pair up as there is not an even number of CH (half of 9 = 4.5). However if you can double the number of CH (9x2 = 18) = amphiploidy, you can get gametes (pollen and eggs) carrying CH that can pair up evenly.

• Results in a new species! – remember this for speciation (evolution)