Mixing the Banana Genome

If you are human, your egg cell or sperm cell carries a full set of chromosomes that includes a single copy of each chromosome. Haploid is the term used when a cell has only one set of chromosomes, which is very useful because males and females can then produce offspring that has the characteristics of both parents. The number of haploid cells in sperm and eggs is 23, which results in a diploid number of chromosomes of 46 (twice 23) in complete humans.

So far, so good.

Having more than two sets of chromosomes is called polyploidy and it is most commonly found in plants. Triploids have three sets of chromosomes, tetraploids have four, etc.

Most of the cultivated sweet bananas and plantains are triploid varieties that evolved from the two wild species, Musa acuminata and Musa balbisiana. That proved easy to designate because the former got the genome designation ‘AA’ (from acuminata) and the latter got the genome designation ‘BB’ (from balbisiana)[1].

The formation of homogenomic triploids hybrids with the AAA genotype occurred within Musa acuminata, leading to cultivars that mostly comprise the sweet bananas. Crosses of the diploid and triploid types of Musa acuminata with Musa balbisiana resulted in the formation of heterogenomic triploid hybrids that are mostly plantains (AAB genotype) and other cooking bananas (ABB genotype). Tetraploid bananas do also exist.

[Image: www.frontiersin.org]

Both the ‘Gros Michel’ and the ‘Cavendish’ belong to the genotype AAA. The modern day edible bananas are therefore a maddening mix of wild and cultivated species, varieties and hybrids, all associated with Musa acuminata and Musa balbisiana.

The breeding of bananas needs triploids or tetraploids, rather than diploids, because only those result in virtually seedless bananas. The ‘Cavendish’ does so very rarely set seed that it is considered sterile. But it isn’t.

[1] Simmonds et al: The taxonomy and origins of the cultivated bananas in Journal of Linnean Society of London - 1955