The informal nomenclature system used to classify banana cultivars was developed by Norman Simmonds and Kenneth Shepherd in 19551 . It forgoes two-part Latinized names (the first part indicating the genus, and the second one the species) and instead classifies edible bananas into genome groups, according to the relative contribution of their ancestral wild species, and into subgroups, sets of closely related cultivars. This system eliminates almost all the difficulties and inconsistencies of a taxonomy based on Musa paradisiaca and Musa sapientum2 .
However, due to difficulties in assigning certain cultivars to a genome group and subgroup, there are inconsistencies in the way the system has been applied. Contributing to the confusion is the continued use of Latin binomials to refer to edible bananas.
In this system, bananas, at least the ones that are related to Musa acuminata and Musa balbisiana, are classified according to the relative contribution of these species designated by the letter A, for acuminata, and B, for balbisiana. A cultivar is assigned to a genome group according to the number of chromosome sets in its genome (its ploidy) and the species that donated them (see Domestication of the banana). Diploid cultivars can belong to the AA or AB genome group, while triploid cultivars fall into three genome groups: AAA, AAB and ABB (see the cultivar diversity portal).
Some taxonomists recognize a BBB genome group, but its existence has not been conclusively demonstrated. Tetraploid cultivars are mostly hybrids produced by breeders.
Genome groups are further divided into subgroups usually defined as a set of cultivars derived from each other through somatic mutations. On the basis of this system, cultivar names are put between inverted commas and preceded by the name of the genus and when known, the name of the group and subgroup. For example: Musa (AAA group Cavendish subgroup) 'Robusta'3 .
The system is based on 15 characters that were chosen because they are different in Musa acuminata and Musa balbisiana4 . Each character is scored on a scale from one (typical Musa acuminata) to five (typical Musa balbisiana). The possible total scores range from a minimum of 15 to a maximum of 75. The expected scores are 15 for AA and AAA, 35 for AAB, 45 for AB, 55 for ABB and 75 for BB.
|Character||Musa acuminata||Musa balbisiana|
|Pseudostem colour||More or less heavily marked with brown or black blotches||Blotches very slight or absent|
|Petiole canal||Margin erect or spreading, with scarious wings below, not clasping pseudostem||Margin inclosed, not winged but clasping pseudostem|
|Peduncle||Usually downy or hairy||Glabrous|
|Ovules||Two regular rows in each loculus||Four irregular rows in each loculus|
|Bract shoulder*||Usually high (ratio<0.28)||Usually low (ratio>0.30)|
|Bract curling||Bracts reflex and roll back after opening||Bracts do not reflex|
|Bract shape||Lanceolate or narrowly ovate, tapering sharply from the shoulder||Broadly ovate, not tapering sharply|
|Bract colour||Red, dull purple or yellow outside; pink, dull purple or yellow inside||Distinctive brownish-purple outside; bright crimson inside|
|Colour fading||Inside bract colour usually fades to yellow towards the base||Inside bract colour usually continuous to base|
|Bract scars||Prominent||Scarcely prominent|
|Free tepal of male flower||Variably corrugated below tip||Rarely corrugated|
|Male flower colour||Creamy white||Variably flushed with pink|
|Stigma colour||Orange or rich yellow||Cream, pale yellow or pale pink|
Efforts to classify cultivars into genome groups and subgroups have not progressed much since Simmonds's extensive treatment of cultivars in Bananas. Recognizing the limitations of the current system, some ProMusa and MusaNet members are discussing aligning Simmonds and Shepherd's genome-based nomenclature system with the International Code of Nomenclature for Cultivated Plants as part of an effort to have it formally recognized5 .