Tuesday, 27 November 2007

Polyploidy in Canna

Each cell of a canna plant naturally has 18 chromosomes, or 9 pairs. These plants with their paired chromosomes are termed diploid, from the Greek word for "double".

The pollen and ovules, are formed through a process called meiosis, which results in the production of cells that have only half the parent organism's chromosomal material; these cells are termed haploid, from the Greek word for "half". The haploid pollen and ovule join in fertilisation to form the new diploid cell that eventually becomes the offspring organism, and jointly provide the 18 chromosomes.
Canna 'President', a triploid cultivar

Sometimes, however, the process of meiosis fails, and pollen or ovules are produced that have the full complement of parental chromosomes; this type of cell is called a non-reduced gamete. When such a reproductive cell participates in fertilisation with a haploid cell, an event that does not occur as easily as normal fertilisation, the resulting offspring has three sets of chromosomes instead of the normal two and is termed triploid. When both pollen cell and ovule cell have the diploid chromosome number, the offspring has four sets and is termed tetraploid. All organisms with more than the normal number of chromosomes are collectively called polyploid.

Polyploid cannas tend to be larger, stronger, more substantial, and more persistent in every respect. This has obvious advantages in any growing context.

The offspring of a triploid and a diploid parent is a tetraploid. Such a cross usually produces few or no normal seeds. With their 27 chromosomes, diploid cannas are difficult to cross with other cannas.

Although in other plants it has been possible to create cultivars with a higher chromosome level, the tetraploid seems to be the limit of the Canna, even after extensive laboratory experiments. Tetraploids can occur naturally as the resulting failure of meiosis, as described for triploids, if there are irregularities in the formation of both parent's reproductive cells. They can also be produced vegetatively under laboratory conditions.


References

Origin and Evolution of Cultivated Cannas, T.N. Khoshoo and I. Guha (Neé Mukherjee)

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