"What are you doing?"
"Eating a banana."
"Did you know that banana trees are seedless? They only reproduce asexually and hence are all genetically identical."
(Me, chewing) "Hmm-mmm."
"If a parasite were to kill one, it would kill all of them because there's no genetic variation among the plants."
See, this is what you get from growing up with a biologist father. Over a meal, you can learn infinitely many new things, like the fact that shellfish is an unfortunate name for something that really isn't a fish. The story behind bananas, though, is fascinating. Between 8,000 and 7,000 years ago humans started selecting and hybridizing a number of banana tree species, which eventually lead to the creation of the domesticated banana tree we know today. For the most part, they derived from two species, Musa acuminata and Musa balbisiana. About half of the global banana production comes from this two species. A recent study published in Nature by D'Hont et al. examined the whole genome of the Musa acuminata and reconstructed the history of its domestication through phylogenetic analysis .
These plants are mostly triploid, meaning they have three copies of each chromosome. Most sexually reproducing organisms have two copies, and are hence called diploid. A whole genome duplication happens when an organism inherits an additional copy of the entire genome. Triploidism is mostly observed in plants, and often artificially sought to create seedless fruits because triploid organisms are usually sterile. In fact, banana trees are propagated by replanting cuttings. This of course cuts many opportunities for genetic variation. The species ends up being genetically homogeneous, which means that any potential threat to one organism, will be a threat to the whole species. There isn't enough variation to grant a fitness advantage of a subgroup over the other individuals.
As D'Hont et al. conclude in their Nature Letter,
"The reference Musa genome sequence represents a major advance in the quest to unravel the complex genetics of this vital crop, whose breeding is particularly challenging. Having access to the entire Musa gene repertoire is a key to identifying genes responsible for important agronomic characters, such as fruit quality and pest resistance."
Angélique D’Hont,, France Denoeud,, Jean-Marc Aury,, Franc-Christophe Baurens,, Françoise Carreel,, Olivier Garsmeur,, Benjamin Noel,, Stéphanie Bocs,, Gaëtan Droc,, Mathieu Rouard,, Corinne Da Silva,, & et al. (2012). The banana (Musa acuminata) genome and the evolution of monocotyledonous plants Nature DOI: 10.1038/nature11241