POLYPLOIDY

=Polyploidy= [image:http://i.imgur.com/ha4gsaB.png?1] Polyploidy occurs when an individual inherits additional chromosome sets (3n or greater). Many plants are polyploid and a few examples are listed (''you don't need to learn these examples''). Many polyploids are infertile, depending on the number of chromosome sets they have inherited. If the individual has inherited an odd number of chromosome sets (3n, 5n, etc), they are usually infertile. This is because the chromosomes cannot pair up correctly during meiosis and thus no functional gametes are produced. If the individual has an even number of chromosome sets, they are usually fertile. This is because the chromosomes can still pair up during meiosis and produce functional gametes. This explains why most naturally occurring polyploids have an even number of chromosome sets. It also explains why most organisms with an odd number of chromosomes are plants. This is because plants can undergo vegetative growth, resulting in numerous individuals without producing gametes or the need for meiosis. =Autopolyploidy:= [image:https://i.imgur.com/8WXjPJmh.png] '''Autopolyploidy:''' Autopolyploidy results from a failure of the chromosomes to separate during meiosis. However, unlike aneuploidy, not just a single pair of chromosomes failure to separate, all of them fail to separate. This total non-disjunction of all chromosomes results in gametes carrying two sets of chromosomes (2n). When one of the resulting gametes (2n) combines with a regular haploid (n) gamete, the resulting offspring are triploid (3n). Offspring produced in this way are normally infertile because they have an uneven number of chromosomes that won't pair correctly during meiosis. When two of these gametes (2n) combine, the resulting offspring are tetraploid (4n). This is common in plants as they produce both male and female gametes and are often capable of self-fertilisation. The resulting offspring a generally fertile as they have an even number of chromosomes. If the new tetraploid offspring cannot reproduce with the parental type (diploid) plants, but can reproduce with each other, a new species has been formed. =Allopolyploidy= [image:https://i.imgur.com/iVNs2dch.png] '''Allopolyploidy:''' Allopolyploidy occurs when two closely related species mate and produce a hybrid containing chromosome sets from both parent species. The resulting hybrid is usually sterile because the chromosomes from each species cannot pair correctly during meiosis. The two different species involved may also contribute different numbers of chromosomes which again prevents chromosome pairing during meiosis, rendering the hybrid sterile. '''Amphipolyploidy:''' Doubling the chromosome number in a sterile hybrid can often produce a fertile hybrid. This process is known as Amphipolyploidy. Allopolyploidy generally produces infertile hybrids because the chromosomes from each of the parent species cannot pair correctly. In this example, you can almost think of the resulting hybrid as being haploid (n) with 5 individual chromosomes rather than any chromosome pairs. However, Autopolyploidy can double the chromosome number, producing a fertile hybrid with two of each chromosome. If these new fertile hybrids cannot reproduce with the parental types, but can reproduce with each other, a new species has been formed. =Evolutionary Significance= As discussed above, polyploidy can cause speciation (the formation of new species). It has played an important role in the evolution of many species, especially plant crops, that we see today. [image:http://i.imgur.com/hnf2Vtd.png?1] '''The Evolution of Wheat''' Common wheat has developed as a result of several polyploid events after the formation of hybrids between different grass species. The diagram (left) outlines the key polyploid events. Can you spot instances of Autopolyploidy, Allopolyploidy and Amphipolyploidy? ''Note: the letters (A, B and D) each represent entire chromosome sets. Infertile hybrids have been shown in blue.''