From the point of view of botany, the fruit is a structure that the plant forms around the seeds, protecting them and, frequently, helping them to disperse . It comes from the transformation of the ovary, sometimes involving other plant tissues, after pollination and fertilization of the ovules. When it does not occur, the usual thing is that the growth of the ovary stops early and the fruit does not develop.
However, and exceptionally, there are situations in which the fruits grow without pollination, and even cases in which the ovary does not contain ovules —and, therefore, could not fertilize anything—. In these cases, a seedless fruit develops, which in botany is called ” parthenocarpic “. And that is exactly what happens with the banana we eat.
Are there bananas with seeds?
Like all the fruits that we find in the market, the banana is a product of human ingenuity. Ever since we started growing plants 10,000 years ago, we have been altering their appearance and composition, ultimately genetically modifying them . Initially, involuntarily, selecting those that provided the best products, based on our needs and tastes, and more recently, deliberately, either through artificial selection, variety crossings or various genetic engineering techniques.
And like almost all the fruits that we find in the market, the banana has a wild variety. Among the different cultivars of plantain we find diploid, triploid and tetraploid forms of two species, Musa acuminata and M. balbisiana , and hybrid forms of both. A variety is diploid when its chromosomes are arranged in pairs and it is triploid when it has them arranged in groups of three, a condition that causes reproductive sterility in plants. Most commercial bananas are triploid and thus seedless .
However, wild forms of M. acuminata and M. balbisiana do have seeds, and continue to reproduce sexually. When we open a wild banana we find multiple seeds, small and black , that are arranged throughout the fruit.
How do banana plants reproduce?
A commercial banana, without seeds, cannot reproduce sexually. Therefore, a different method is needed. After all, the banana, in terms of production, is the fourth most important food product in the world after rice, wheat and milk.
The way in which commercial bananas reproduce is exclusively vegetative, and the most common is through suckers . A sucker is a shoot that emerges from the underground rhizome of a plant, giving rise to a new stem. Actually, this sucker is part of the same individual as the main stem; when it reaches a certain size, it separates from the adult, extracting it with part of the roots and cutting the rhizome that links it to the original plant. In this way a clone is obtained that can be transplanted elsewhere. The new plant will grow, bearing its own fruit—seedless—and, when it grows large enough, producing its own suckers.
This process of reproduction has some advantages over sexual reproduction. On the one hand, the fruit does not have seeds, which makes it more attractive to the consumer. Also, being a genetic clone , unless it accumulates some unlikely mutation, it will be exactly the same as the original; If a variety of bananas is highly appreciated, its production can be increased quickly, cheaply, and with certain guarantees that the product will not change.
However, it has drawbacks. As there is no genetic variability, the populations will be very vulnerable to diseases . And that can be very expensive.
The Cavendish banana: a story with an ending
Most of the currently cultivated commercial banana varieties belong to the so-called Cavendish group , which includes cultivars such as the Grande Naine , the Poyo , the Valery , the Robusta , or the Enano —which includes the Canarian banana—. In all cases, they are triploid forms of M. acuminata .
The first Cavendish banana was obtained in China in 1826, and from that first specimen, it began to be distributed throughout the world. The different cultivars arose as a result of mutations during the breeding process, but essentially all bananas are clones of that first Cavendish .
Until the 30s of the last century, the most cultivated variety of banana was the Gros Michel . But a disease caused by the fungus Fusarius oxysporum brought about a drastic change in the world banana scheme. The fungus was first detected in Australia at the end of the 19th century, but the banana fusarium epidemic did not start until the middle of the 20th century, from Panama. In the 1950s, the Gros Michel variety had almost completely disappeared, a victim of the so-called “ Panama disease ”.
The Cavendish variety, however, was found to be resistant to the fungus. That was what allowed, little by little, banana cultivars to replace one variety with another, making the Cavendish the majority crop that it is today. However, it won’t last forever.
It has been known for more than a decade that there is a tropical strain of Fusarius oxysporum —called race 4— capable of infecting the Cavendish variety . The disappearance of such an appreciated banana variety may only be a matter of time. Perhaps a different variety, resistant to this race 4 of Panama disease, will replace the Cavendish bananas in our greengrocers, until a new strain of the fungus appears.
Or maybe the answer is transgenics . Since 2017 we have been working on the development of Cavendish bananas resistant to this race 4. After all, and as explained at the beginning, since we began to grow plants 10,000 years ago, we have genetically modified them; now we do it deliberately, and we have the tools to carry out the modifications with much more precision and speed. Perhaps the answer lies in biotechnology to prevent an epidemic of fusarium from happening again.
References:
Dale, J. et al. 2017. Transgenic Cavendish bananas with resistance to Fusarium wilt tropical race 4. Nature Communications, 8(1), 1496. DOI: 10.1038/s41467-017-01670-6
Frison, É. A. et al. 1996. New frontiers in resistance breeding for nematode,fusarium and sigatoka: proceedings of the workshop held in Kuala Lumpur,Malaysia 2-5 october 1995. INIBAP IPGRI.
Pérez Morales, C. 1999. Morphology of spermatophytes . Ed. Celarayn.
Ploetz, R. C. 2005. Panama Disease: An Old Nemesis Rears Its Ugly Head: Part 1. The Beginnings of the Banana Export Trades. Plant Health Progress, 6(1), 18. DOI: 10.1094/PHP-2005-1221-01-RV
Price, N. S. 1995. The origin and development of banana and plantain cultivation. En S. Gowen (Ed.), Bananas and Plantains (pp. 1-13). Springer Netherlands. DOI: 10.1007/978-94-011-0737-2_1
