Problem 11 - Campbell Biology

Question

The Cavendish banana, the world's most popular fruit, is currently threatened by extinction due to a fungus. This banana variety is "triploid" (3n, with three sets of chromosomes) and can only reproduce through cloning by cultivators. Given what you know about meiosis, explain how the banana's trip- loid number accounts for its inability to form normal gametes. Considering geneticdiversity, discuss how the absence of sexual reproduction might make this domesticated species vulnerable to infectious agents.

Step-by-Step Solution

Answer

The triploid nature prevents meiosis, resulting in no viable gametes, leading to cloning. Lack of genetic diversity makes it vulnerable to disease.

Step 1: Understand the Triploid Nature

Triploid organisms have three sets of chromosomes (3n). In the case of the Cavendish banana, this results in an uneven number of chromosome sets which complicates the process of meiosis because chromosomes usually pair up in meiosis.

Step 2: Explain Meiosis Issue

During meiosis, homologous chromosomes need to pair and segregate evenly into gametes. A triploid banana, having an odd number of chromosome sets (3n), means there is no way to evenly divide the chromosomes, resulting in imbalanced gametes which are not viable.

Step 3: Inability to Form Normal Gametes

Since the Cavendish banana cannot form viable gametes through meiosis due to the imbalance caused by the odd number of chromosome sets, it can only reproduce asexually through cloning.

Step 4: Lack of Genetic Diversity

Asexual reproduction results in clones, which means all offspring are genetically identical to the parent. Without the genetic variation that sexual reproduction provides, these clones have uniform genetic makeup.

Step 5: Vulnerability to Infectious Agents

The lack of genetic diversity means that if one banana plant is susceptible to a disease, all clone plants will likely be susceptible. This uniformity makes it easier for infectious agents to spread and wipe out entire populations.

Step 6: Conclusion

Thus, the inability of the Cavendish banana to undergo sexual reproduction and the resulting lack of genetic diversity make it more vulnerable to extinction events such as the current fungal threat.

Key Concepts

Triploid Organisms

Triploid organisms have three sets of chromosomes (3n). This is different from the diploid (2n) organisms, which have two sets of chromosomes, one from each parent. In triploid organisms, the uneven number of chromosome sets complicates the process of meiosis, which is typically designed to work with pairs of chromosomes. For example, Cavendish bananas have three sets of chromosomes. This odd number makes it difficult for the chromosomes to pair up correctly during meiosis. As a result, triploid organisms often cannot produce viable gametes and hence are usually sterile.

Meiosis and Chromosome Segregation

Meiosis is a type of cell division that reduces the chromosome number by half, resulting in four offspring cells, each with half the original number of chromosomes. It's essential for sexual reproduction. During meiosis, homologous chromosomes must pair up and segregate evenly into the daughter cells. With triploid organisms like the Cavendish banana, which have an odd number of chromosome sets (3n), the chromosomes cannot pair evenly. This leads to imbalanced gametes that are not viable. Therefore, triploid bananas cannot undergo normal sexual reproduction.

Asexual Reproduction and Cloning

Since triploid bananas cannot form viable gametes through meiosis, they reproduce asexually through cloning.

• Asexual reproduction results in genetically identical offspring, called clones.
• This method bypasses the need for gametes and fertilization.

In the case of the Cavendish banana, cultivators propagate new plants using techniques like tissue culture. While asexual reproduction ensures that desirable traits are preserved, it also means that all the offspring are exact genetic replicas of the parent plant.

Genetic Diversity and Vulnerability

The lack of genetic diversity in cloned populations is a double-edged sword. On the one hand, it ensures consistency in traits like fruit size and taste. But it also means that all individuals share the same genetic weaknesses. This uniformity makes these plants highly susceptible to infectious agents.

• If one plant is vulnerable to a disease, all its clones will likely be vulnerable too.
• This can lead to the rapid spread of diseases and pests across the entire population.

In the case of the Cavendish banana, this lack of genetic diversity has made it more vulnerable to a fungus that is threatening its extinction. Without genetic variation for natural resistance, a single successful infection can potentially wipe out the entire population.