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Information: genomes and ploidy

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When scientists refer to an organism's 'genome', they mean the entire genetic material (DNA) in all of the chromosomes in the cell. 'Diploid' organisms, including most larger plants and animals, contain two sets of chromosomes, or two complete genomes: one from the mother, and one from the father. Humans, for instance, have genomes containing N=23 chromosome pairs, making 2N=46 chromosomes in total. During normal cell division, known as 'mitosis', each chromosome divides, so that every cell in the body of a diploid organism has the full set of diploid chromosomes.

In reproductive cells, a special kind of cell division occurs, known as 'meiosis' to produce the reproductive cells, or 'gametes' (eggs and sperm, or, in flowering plants, eggs and pollen). The diploid (2N) cells undergo a 'reduction division' during meiosis to produce daughter cells, so that each gamete contains only a single copy of each chromosome; they are said to be 'haploid', that is they have a total of only N chromosomes. During sexual reproduction, the egg and sperm cells fuse, creating a 'zygote', or diploid (2N) cell that will eventually develop into the offspring.

However, diploidy and haploidy are not the only ploidy levels possible. Some organisms may have even more chromosome sets — these are called 'polyploids'. For example, 'triploids' have three copies of every homologous chromosome (3N), 'tetrapoloids' have four (4N), 'pentaploids' have five (5N) and 'hexaploids' have six (6N). Polyploidy is particularly common in higher plants, in part because plants, unlike most larger animals, are 'hermaphrodites' (contain male and female sexual organs on the same individual), and can fertilize themselves. Polyploidy comes about in two major ways:

In the first route to polyploidy, a single diploid individual may undergo a rare event of chromosome doubling to form a 4N 'autopolyploid'. This individual (often not a whole individual, but a mutant branch on a plant) will at first be unique within the species, because it produces diploid (2N), instead of haploid gametes. If it mates with a normal individual with haploid gametes, the result will be triploid (2N+N) offspring. Triploid (3N) individuals are often healthy, but usually suffer severe problems during sexual reproduction and meiosis because of confusion during the reduction division; instead of whole numbers of genomes being found in each resulting gamete, sometimes there are two chromosomes and sometimes there is only one. When genomes are imbalanced like this, development usually fails, and so the gametes of triploids are usually sterile.

However, because the 4N autopolyploid is self-fertile, it can fertilize itself to produce further tetraploid (2N+2N=4N) offspring. So the autotetraploid, which started off as mere mutant, ends up with a small population of itself and its offspring, which represent, in effect, a new species. Whenever one of these tetraploid individuals selfs, or mates with another tetraploid individual, it produces fertile tetraploid offspring. If it mates with the original diploid species, the triploid offspring are sterile.

There is also a second route to polyploidy, 'allopolyploidy', in which polyploidy arises as a result of hybridisation. It is often the case that two closely related species have the same haploid number (N). If these two species mate together, the diploid (2N) hybrid offspring are often infertile, because homologous chromosomes from the different species do not recognize one another during meiosis, and the 'counting mechanism' of the reduction division fails, leading to duplications and deletions of chromosomes in the gametes, which become sterile due to genomic imbalance, as before.

Rarely, chromosome doubling takes place in these hybrids, and the resulting allopolyploid may become fertile again, because each chromosome can pair with the same type from its own parental species. Fertile meioses are thereby restored, and the new allotetraploid (4N) becomes a new species, interfertile only with itself and other individuals like it, which all produce diploid (2N) gametes.

Many of our most important crops are polyploid; often the result of human intervention or natural interspecific hybridisation. For example, the most popular banana varieties are triploid (3N). They produce abundant fruit, but no seed whatsoever because the seeds are all infertile and aborted — their traces are the small black dots you can see when a section is cut across a banana fruit. Cultivated wheat is hexaploid (6N), consisting of complete diploid genomes from each of three wild progenitors. In this Seeds of Trade website we have included the ploidy level for each plant product to demonstrate the importance of polyploidy for the interaction of plants with human beings.

For more information on ploidy and polyploidy see: Kimball's Biology Pages — an online biology textbook (http://biology-pages.info or http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/)