This sh*t is bananas

green bananas

Gwen was right.

Bananas don’t get as much respect and attention as they deserve. Well past when the need became apparent, a banana genome has finally been released this week. Phew! The wait has been surprisingly long for the most popular fruit in the industrialized world (in the US, as many bananas are consumed, by weight, as apples and oranges combined) and a key staple food in many developing countries (up to 50% of calories consumed in many African countries) (Grimm, 2008). Perhaps that’s because they are kinda crazy.

The banana you are probably all familiar with is known as the Cavendish (on the left, below), and its origins are a bit convoluted. It is the love child of at least two different domesticated banana species (Musa acuminata and M. balbisiana) and the product of some confusing chromosomal hijinx. To my knowledge, it went a little something like this. The wild, ancestral species of banana was haploid (has one copy of each of its chromosomes). Occasionally new individuals are produced with an extra copy, a process called polyploidization. These diploids were domesticated in Southeast Asia, as early as 8000 years ago, making banana one of the earliest domesticated plants. At some point these two diploids hybridized, and went through a further round of polyploidization, resulting in the Cavendish, a triploid. In addition to having three copies of its genome, the Cavendish, like most domesticated bananas intended for food, is also sterile and parthenocarpic – that is, it makes no seeds, has no sex, and makes fruit without having to be fertilized. The only way to propagate the Cavendish is vegetatively, through cuttings of ‘suckers’ or rhizomes.

From ScienceShot. Cavendish (left) and DH-Pahang (right)

Because of this, bananas are also the poster child for the problems of monoculture. The Cavendish alone makes up half of the world’s production of bananas – that is half of all the bananas produced each year are clones of a single individual. This stage is set for a commercial scale disaster. Every Cavendish is exactly as susceptible to disease and pathogens as every other Cavendish. Race 4 Panama disease has already hit Asia – once it gets to Latin America, the Cavendish will be wiped out of commercial scale production. It’s happened before. The banana of choice of the 1940s was a different variety all together, the Gros Michel, until it was wiped out by Race 1 Panama disease. But there are few if any viable replacement varieties waiting in the wings, when the Cavendish finally kicks it.

Which is why people have been so keen to get this genome. The genome reported this week is actually from a “doubled-haploid” variety of banana, the DH-Pahang (on the right in the picture above). With two identical copies of its genome, the DH-Pahang sequence would be a lot easier to assemble than the unwieldy, non-homozygous, triploid, Cavendish.

On a side note, the paper reporting the genome sequence contains possibly the sweetest Venn diagram every used in a molecular biology paper. Behold!

From D’Hont et al. Six-way Venn diagram showing the distribution of shared gene families (sequence clusters) among banana, date palm, Arabidopsis, rice, sorghum, and Brachypodium distachyon genomes.

Angélique D’Hont, et al. (2012). The banana (Musa acuminata) genome and the evolution of monocotyledonous plants Nature DOI: 10.1038/nature11241

David Grimm (2008). A Bunch of Trouble Science DOI: 10.1126/science.322.5904.1046