Which of the following best explains why triploid bananas do not produce seeds?

Benjamin Burr and Frances Burr, biologists at Brookhaven National Laboratory, offer this explanation:

Fruit development normally begins when one or more egg cells in the ovular compartment of the flower are fertilized by sperm nuclei from pollen. In some plants, however, fruit develops without fertilization, a phenomenon known as parthenocarpy. Parthenocarpic fruit has advantages over seeded fruit: longer shelf life and greater consumer appeal.

Image: VALENCIAN INSTITUTE FOR AGRICULTURAL RESEARCH

SEEDLESS FRUIT such as navel oranges are propagated asexually, usually by grafting.

The most frequent reasons for lack of seed development are pollination failure, or nonfunctional eggs or sperm. In many plants, self-incompatibility genes limit successful fertilization to cross-pollination between genetically different male and female parents. This property is exploited by citrus farmers who grow seedless fruits, such as navel oranges and clementines. Because these cultivars are self-incompatible, they fail to set seed when they are planted in orchards of identical plants (clones). These plants have a high frequency of parthenocarpy, however, so they still produce fruit. Such trees do not require seed for propagation. In fact, propagation by seed would be disadvantageous because the progeny would differ from the parent. Instead nurserymen frequently propagate fruit trees asexually, usually by grafting.

Another frequent reason for lack of successful fertilization is chromosomal imbalance. For example, the common banana is triploid. In other words, it has three sets of chromosomes. Instead of having one set of chromosomes from each parent, it has two sets from one parent and one set from the other parent. Triploids seldom produce eggs or sperm that have a balanced set of chromosomes and so successful seed set is very rare. Bananas, too, are parthenocarpic and produce fruit in the absence of successful fertilization. These bananas are asexually propagated. After the stalk has flowered and borne fruit, it dies. But there are side shoots or suckers at the base of the main stalk, which can be removed and replanted to continue the cultivar. Growers also propagate bananas by tissue culture.

Seedless watermelons are particularly interesting because they must be propagated by seed, and yet growers can still exploit parthenocarpy. One way to make seedless watermelons is to produce triploid seed. As in the case of bananas, triploid watermelons cannot produce functional seed, but they still develop good fruit through parthenocarpy. Plant breeders produce triploid seed by crossing a normal diploid parent with a tetraploid parent, which itself is made by genetically manipulating diploids to double their chromosome number. In the case of watermelons, this manipulation has to be performed each generation, so it is a somewhat expensive proposition but still worthwhile.

Plant biologists have learned that if the plant hormone auxin is produced early in ovule development, parthenocarpic fruit can grow on plants that do not usually exhibit this property. Thus, genetic engineering will most likely give consumers parthenocarpic fruit in many other species in the near future.

Answer originally posted October 2, 2000.

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Short Answer

Banana plants, which are triploid, are seedless and therefore sterile. Propose a possible explanation.

In bananas, the homologous chromosomes cannot pair up during synapsis of meiosis as they are triploid. As a result, meiosis is affected, and viable gametes are not produced.

Thus, a zygote with a triploid number of chromosomes cannot be produced, and therefore, the bananas are sterile.

See the step by step solution

Step by Step Solution

Step 1: Meaning of Ploidy

The number of chromosome sets present in a cell is referred to as ploidy. In other words, it is the number of chromosomes present in the cell nucleus.

The chromosomes usually occur in pairs and are described as called diploids. However, cells with a single set of chromosomes are called haploids, such as gametes cells.

Similarly, cells with three sets of chromosomes are called triploids. Thus, polyploids are cells that have three or more three sets of chromosomes.

Step 2: Meiosis

Meiosis is cell division that occurs in diploid cells. It involves two nuclear divisions that produce haploid daughter cells. These cells possess one set of chromosomes, which means the chromosome number is reduced to half.

During meiosis, the chromosome set is split to produce gametes or germ cells with half the number of chromosomes as the parent cell. As a result, haploid gametes cells are produced.

The gametes fertilize to produce a zygote that gives rise to a diploid organism.

Step 3: Triploid bananas are sterile

In diploid organisms, the homologous chromosomes pair up during synapsis of prophase I. However, bananas have an extra set of chromosomes. This chromosome set with no homologous set fails to pair up during synapsis.

Thus, meiosis is disrupted, and no viable gametes are produced. These gametes cannot form a zygote with triploid sets of chromosomes. As a result, the bananas are seedless and sterile.

Why triploid bananas are sterile and seedless?

Are all bananas triploid?

Why might Triploidy be difficult for cells in meiosis?

Which of the following provides the best explanation for the scientists claim regarding the threat to the Cavendish banana?