The banana’s we eat don’t have seeds, they are a result of a naturally occurring mutation that was subsequently exploited by human selection for breeding. Banana’s are a cultivar; they are a domesticated crop, reproduced in manner that selects for and maintains certain desired characteristic, such as, being seedless. Banana’s reproduce vegetatively through an asexually process known as parthenocarpy, in which no fertilization is required for their propagation. Parthenocarpy is a genetic trait or characteristic imparted by a gene, but this process has nothing to do with genetic modification, and banana’s are not genetically modified organisms (GMO) — at least not yet.
Although many imagine them to grow on trees, bananas are the fruit of a herbaceous plant. They originated in the islands of Southeast Asia and Melanesia, and are related to cassava and taro. Wild bananas, the predecessors of todays bananas, were inedible fruits with little flesh and many stony seeds. Edible variants arose randomly as a result of a naturally occurring mutation that produced seedless fruit. 7000 years ago, human began exploiting this natural occurrence and domesticated the banana by selecting bananas with the desirable trait of being seedless, for breeding. Seedless bananas were able to reproduce asexually due to parthenocarpy, which did not require pollination (fertilization). The resulting fruit was not only seedless, but also fleshier and sweeter. Today, bananas are cultivated in over 130 countries and there are over a thousand banana cultivars. Wild bananas are also still in existence, and there are about 60 wild-type bananas known. Wild bananas are used in breeding processes to improve the genetic variation amongst cultivars, occasionally.
Banana’s (and plantains) belong to the family Musaceae, which includes 25 species, which are divided into four sections: Australimusa, Callimusa, Rhodochlamys, and Eumusa, the latter being the most common. The Eumusa Section includes the two major species, Musa Acuminata and Musa Balbisiana. These two species were cross-bred to produce the commercial banana cultivars that we typical eat today. From the Acuminata came the trait for parthenocarpy and female sterility, while the Balbisiana contributed hardiness. Seedlessness is the result of parthenocarpy plus female sterility. While all edible bananas are parthenocarpic, they are not all completely sterile. Edible bananas display various levels of female fertility and may produce a few seeds, due to polyploidization.
Banana’s are a Polyploid Crop
Wild bananas, the ancestors of todays cultivars, were fertile diploids. Most bananas today are polyploids, they may be triploid or tetraploid, however most are triploid. This means that they have more than two sets of the same chromosomes. The initial domestication of bananas involved the inter and intra-specific hybridization of several wild musa, including the Acuminata and the Balbisiana, with the Musa Acuminata designated as genome A and Musa Balbisiana as genome B. The resulting hybrid bananas were also diploid, but were either AA or AB. Continued cross-breeding led to triploid banana’s, which are either AAA, AAB, or ABB. Triploids are the result of unbalance meiosis, and it is this irregular meiosis that results in sterility. Meiosis is a cell division process involved in the sexual reproductive process that occurs in order to reduce and maintain the ploidy (the number of sets of chromosomes in a cell, or in the cells of an organism) of the species after fertilization. The Cavendish banana is an example of a cultivar that is triploid (approximately 45% of commercial bananas are ‘Cavendish’). Another triploid cultivar, the ‘Gros Michel’ has low female fertility, but produces no seeds because various rates of fertility decrease the likelihood of sexual reproduction, thereby allowing for parthenocarpy to take precedence. Tetraploid bananas were more recently introduced, by cross-breeding triploid bananas with diploid banana’s, which resulted in a more balanced ploidy, thereby increasing the likelihood of fertility. Tetraploid banana’s are used to increase the genetic diversity of banana cultivars, by providing breeders with seeds to start new distinct banana plants that are not clones (wild bananas are also introduced into the breeding process for this reason).
Seedless Fruit & Parthenocarpy
Seedless plants either produce fruits bearing no seeds, produce fruit with traces of aborted seeds, or produce fruit with far fewer seeds than usual. There are several ways by which fruits come to be seedless, parthenocarpy is only one cause of seedlessness. In Parthenocarpic fruits the ovary is able to develop without fertilization of the ovule.
Parthenocarpy naturally occurs in a number of species of fruit, such as, summer squash, cucumber, sweet peppers, eggplants, and tomatoes. Parthenocarpy is the ability of a plant to develop fruit without the need for pollination or subsequent seed formation; it gives rise to seedless fruit. There are two types of parthenocarpy: facultative and vegetative. Tomatoes for example, are facultative parthenocarpic fruit; only producing seedless fruit by stimulation under conditions in which fertilization does not normally occur, like at low temperatures or in a greenhouse. Banana cultivars, are on the other hand, a vegetative parthenocarpic fruit; they do not require any external stimulation to trigger asexual reproduction.
Advantages to Parthenocarpy and seedlessness
Parthenocarpy maintains the seedless nature of bananas, and also allows for the crop to be cultivated year round. Seedless fruit are more fleshy, having more edible tissue and firmness. Seedless fruit also have a longer self-life, as seeds produce hormones that trigger senescence, causing the fruit to deteriorate faster, becoming overripe and mealy in texture. Fruits that lack their seeds are sweeter, less tart, and less fibrous than their seeded counterparts.
Breeding vs Genetic Modification
Edible banana’s are essentially clones. Initially bananas were propagated by growing cuttings from suckers from the parent plant. Then as natural mutants arose within a particular species or variety of banana, the genetically divergent variants were selected and bred amongst each other (intra-specific hybridization). In addition, crossbreeding amongst bananas of different species was also performed (inter-specific hybridization), to expand the genetic diversity of bananas. These and other methods such as in vitro cell culture are ordinarily used to breed bananas today. However, because edible bananas do not produce seeds or produce very few seeds, bananas are difficult to breed and to improve as a crop. Also, since bananas are clones, reproducing asexually by parthenocarpy and sterile, they lack genetic diversity. Low genetic diversity makes bananas susceptible to disease and environmental stress. Although bananas are currently not GMO (genetically modified organisms), research is being done to employ biotechnology to enhance genetic diversity to improve breeding, crop survival, and nutrient content.
Genetic modification is the introduction of a gene or genes from other organisms or noncrossable species into another, producing what is known as a transgenic crop. Transgenic crops are genetically altered to resist herbicides and insecticides, pest or pathogens, or to increase micronutrient (vitamins) content. Otherwise, they are comparable to conventional crops and pose no increased risk to health.
There are a host of pathogenic disease that threaten banana cultivars worldwide. Highly contagious fungi and bacterium infect banana plants, adversely affecting their growth and development. Fusarium Wilt, better known as Panama Disease destroyed many cultivars, such as the Gros Michel in the last century, and spread fear that bananas would become extinct. A new form of the virulent fungi has resurfaced in recent times, and there is no known chemical cure for it. In order to protect against diseases that threaten to wipe-out entire banana species, genes from other organisms could be used to produce banana plants that are resistant to a host of fungi and bacterium. One example of this is the placing of a gene known as Pflp (plant ferredoxin-like protein) from red peppers into banana cells during the embryogenic stage, to produce banana plants that resist Xanthomonas Wilt—Xanthomonas Wilt is a very contagious bacterial infection that destroys the banana plant. (this would be done in a laboratory)
Bananas tolerant to other environmental stressors besides pathogens and pests could also be produced. Banana plants can be engineered to grow under harsh conditions such as drought or flood, which is especially important, considering climate change.
Furthermore, genetic modification can also be used to increase the nutritional value of fruit to combat hunger and malnutrition. Regions of the world where vitamin A deficiency is high, like Africa and South East Asia, could benefit from transgenic banana’s designed to express high levels of carotenoid provitamins, the precursors of vitamin A, as banana and plantain are staples in these regions. There are some bananas that already express high levels of beta-carotene, like the ‘Fe’i’ bananas found in Micronesia and Papua New Guinea. However, most dessert bananas such as the ‘Cavendish’ and ‘East African Highland Banana’ produce much lower, and varying levels of the vitamin A precursor. As such, banana’s could be altered with genes from Fe’i bananas that would increase their expression of micronutrients. Biofortification, a process which increases the nutritional quality of food crops, can be accomplished by conventional breeding practices, but because most banana cultivars are sterile (lacking both male and female fertility), transferring the necessary genes via breeding the ‘Fe’i’ with the ‘Cavendish’ is almost impossible. Researchers have successfully conducted trials in which transgenic bananas were produced that expressed higher levels of carotene than normal. They found the trait to be stable, meaning it did not weaken or become “lost” overtime. In fact with consecutive breeding, each successive vegetative generation of bananas had greater levels of carotene than the previous ones. These bananas are also golden or orange in color, like carrots, which are also high in beta-carotene.
Bananas are seedless because of a naturally occurring mutation that took place centuries ago, and not because of genetic modification. This mutation was fortuitous, as it provided humans with a good source of food, that was previously unavailable, as ancient wild bananas were inedible. The mutation made it possible not only for the fruit to be eaten, but also for it to be domestically reproduced, as the trait conferred by the mutated gene resulted in parthenocarpy; the ability for a fruit to reproduce like a vegetable, without the need for seeds or pollen.
Although cultivated bananas are not yet genetically modified, this may change, owing to the many environmental stressors that may threaten the sustainability of bananas as a commercial and even local crop. As great as parthenocarpy was for making bananas fleshier and sweeter, it also made them harder to breed, which limited their genetic diversity and made them susceptible to disease. Disease and climate change, may destroy banana plants, and reduce the quality and yield of bananas. However, genetic modification can be used to introduce genetic material from other bananas, plants, or entirely different species, like bacteria, into the banana genome to impart tolerance to pathogens, pests, drought, and flood. Genetic modification may even be used to improve the nutritional value of the fruit, and combat hunger and malnutrition globally.