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 spontaneously
abort however this one made it
to full term!
Polyploid = triploid (3N)
One parent contributes the
normal 22 autosomes +
one sex chromosome, the
other parent contributes
the full 46 chromosomes.
About 2% of conceptions! BUT
could 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 an
error in meiosis (non disjunction) resulting in double the
number 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 of
chromosomes (plant = 6CH, 3 homologous pairs),
during meiosis the pairs have to match up before
they 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)