yes, we have some bananas

Prominent creationist Ray Comfort once (in)famously commented that the ‘design elements’ that make up a banana, including its so-convenient shape, are evidence for the existence of a Designer. A comment that has been pretty resoundingly debunked – unsurprisingly, since the banana-as-we-know-it is due in large part to the hand of man, selecting for those features of bananas that make them desirable as a food – lack of seeds (wild-type, uncultivated bananas have almost more seeds than flesh) & that wonderfully unzippable peel. Something that last year’s Schol Bio examination asked students to think about.  

Image: inside a wild-type banana (from Wikipedia)

Bananas belong to the genus Musa. If you think back to the last banana you ate, you’ll rememember that it’s seedless, unlike the ‘wild’ banana shown above. All commercially-grown banana plants are produced asexually, from suckers or sprouts. The varieties we import also tend to be rather large, but the fruit can be much smaller (apparently the name ‘banana’ derives from an Arabic word for finger, banan). They’re imported as unripe fruit, & ripened before sale by being exposed to ethylene. Ripe fruit (not just bananas) release this gas, but for commercial ripening the fruit are placed in a room that’s then flooded with it. 

Incidentally that lack of any sort of sex life places banana crops at some risk: because they’re clonal, if a pathogen comes along to which they have no resistance, much of the crop may fail. For example, the ‘Black Sigatoka fungus’ can lower production by up to 50% in infected crops. This problem is magnified by the loss of genetic diversity in wild bananas as rainforests are felled. Another pathogen, bacterial banana Xanthomonas wilt (BXW) disease, poses a significant threat in East Africa. While we may view the loss of bananas as something of a misfortune, for others it would be much worse: after rice, maize & wheat, bananas are the world’s 4th most important food crop.

Now, back to the question. The examiner tells us that "Many different species of banana exist today, all of which are descended from one or other of the ‘wild’ Asian species: Musa acuminata (AA:2n = 22) and Musa balbisiana (BB: 2n = 22). Students needed to pay careful attention to the AA & BB. They represent the genome – the full chromosome set – rather than alleles at a particular locus. This is important, because the question goes on to give you information about the genomes of several of the major cultivars in modern bananas:

So, the examiner asked scholarship candidates to "[d]iscuss the sequence of events and processes that have resulted in the three different species of banana with the followign genomes, arising from the original ‘wild’ species of banana: AAA, AB, and ABB," and advised using annotated flow diagrams in doing this. The discussion also needed to include "the genetic processes that could have occurred to produce the different cultivars of Gros Michel, Grande Naine and Cavendish within the one species of the AAA genome." (The question went on to ask candidates to design an experiment to investigate the action of ethylene on ripening rate in bananas, but I think we have enough to look at without going into that right now!)

O-kay. I am not one of those clever people who can do all sorts of flowcharts on their webpages, but if I was, I’d be drawing a flow diagram or two showing things like polyploidy & hybridisation. This is because that first AAA genome must be the result of a cross between 2 M.acuminata individuals where one produced normal (i.e. n = 11) haploid gametes, A, while the other produced gametes (at least some!) which were diploid (AA) due to complete non-disjuncti on in meiosis. The 3n (AAA) individuals thus produced are autopolyploids (both parents from the same species. Because as triploid organisms you’d predict they’d have difficulty producing gametes of their own, these AAA plants are sterile & can reproduce only asexually.

The second (AB) species is the result of a hybridisation event involving one M.acuminata parent and one from M.balbisiana, both producing haploid gametes. And tthe third, ABB, species is another polyploid organism. This time it’s an allopolyploid (2 different parents) with the M.acuminata parent contributing a haploid gamete (A: n = 11) while the M.balbisiana parent must have produced some BB (2n = 22) gametes, again through complete non-disjunction during meiosis. (You could probably get away with not drawing flow diagrams, but in that case you’d need to give a comprehensive & logical description of what was happening.)

What about those different cultivars with the one genotype (AAA)? Two possibilities here. One is mutations after the AAA triploid was formed. In some cases mutations – in different plants – could result in different phenotypes. And if those phenotypes were viewed as ‘desirable’ by farmers, they’d be propagated by cuttings, ending up as the different cultivars.

The other possibility is that the original parent plants (those AA individuals) had differences in their genotypes. This is really only to be expected, given the fact that independent assortment, crossing-over & recombination routinely shuffle alleles between homologous chromosomes, and homologues assort independently when gametes are formed. Again, when the AAA polyploids were produced (I think it’s safe to assume this could happen more than once) then they’d receive these variations from their parents. And again, any ‘desirable’ phenotypes would be selected for.

There! Isn’t that a rather more satisfying explanation than Mr Comfort’s?