Problem 22 - Campbell Biology Concepts & Connections

Question

What you think of as "a banana" is a Cavendish, one variety of the species Musa acuminate. It is a triploid organism \((3 n)\) with three sets of chromosomes in every somatic cell. The Cavendish cannot be naturally bred; it can only be reproduced by cloning. Explain how its triploid state accounts for its inability to form normal gametes. Discuss how the lack of sexual reproduction might make the species particularly vulnerable to a new pest.

Step-by-Step Solution

Answer

Triploid state disrupts meiosis, preventing normal gametes. Cloning leads to lack of genetic diversity, making species vulnerable to new pests.

Step 1: Understand the Triploid State

A triploid organism like the Cavendish banana has three sets of chromosomes \(3n\). This means each somatic cell contains three homologous sets of chromosomes.

Step 2: Meiosis in Triploid Organisms

During meiosis, chromosomes need to pair up to form homologous pairs before they are separated into gametes. In a triploid organism, it is difficult to form these pairs because there are three sets of chromosomes instead of two.

Step 3: Issues in Chromosome Pairing

Because there are three sets of chromosomes, proper pairing and segregation during meiosis are disrupted. This leads to the formation of gametes with an irregular and unbalanced number of chromosomes, which are usually non-viable.

Step 4: Gamete Formation and Cloning

Due to the irregular gametes, Cavendish bananas cannot undergo sexual reproduction. They must be reproduced by cloning, which means they are genetically identical to the parent plant.

Step 5: Vulnerability to Pests

Lack of genetic diversity due to cloning makes the Cavendish banana particularly vulnerable to pests and diseases. If a new pest or disease affects one plant, it can easily affect all plants within the species, leading to widespread damage.

Key Concepts

Chromosome Pairing

In most organisms, chromosomes come in pairs, one from each parent. This pairing is crucial during meiosis, the process that creates gametes like sperm and egg cells. Normally, chromosomes are diploid \((2n)\), meaning they have two sets of chromosomes, one from each parent. In a triploid organism like the Cavendish banana, there are three sets of chromosomes \((3n)\). This means there is an extra set of chromosomes in every somatic cell, making it difficult for these chromosomes to pair up correctly during meiosis.

Given that meiosis depends on the precise pairing and segregation of homologous chromosomes, having an extra set causes confusion. With three sets, it becomes a challenge to form proper pairs. Consequently, this leads to an issue called 'unbalanced gametes.' These gametes are irregular because they carry unequal numbers of chromosomes. This irregularity makes it challenging for these gametes to develop correctly, thereby preventing successful sexual reproduction.

Meiosis Disruption

Meiosis is a special type of cell division that reduces the chromosome number by half, creating four haploid cells from one diploid cell. It consists of two stages: meiosis I and meiosis II. In a typical diploid organism, chromosomes pair up during meiosis I, allowing for a smooth separation. However, in a triploid organism like the Cavendish banana, this process is disrupted.

Since there are three sets of chromosomes, they cannot pair up evenly. This leads to problems in the alignment and segregation of chromosomes. The result is the formation of gametes with an imbalanced number of chromosomes, which are often not viable. This inability to produce viable gametes explains why the Cavendish banana cannot undergo natural sexual reproduction. Cloning thus becomes the only method to reproduce these bananas.

Genetic Diversity

Genetic diversity is essential for the survival and adaptability of species. It provides a pool of genetic variations on which natural selection can act. Typically, sexual reproduction mixes the genetic material of two parents, producing offspring with varied genetic combinations.

For the Cavendish banana, the inability to reproduce sexually means all offspring are clones of the parent. Cloning results in genetically identical organisms. While this can ensure uniformity in crops, it also has a significant downside—lack of genetic diversity.

Without genetic diversity, the entire population of Cavendish bananas becomes highly susceptible to diseases and pests. If a new pest or disease emerges that can attack one plant, all plants are at risk, leading to potential widespread damage. This makes the Cavendish banana particularly vulnerable and highlights the importance of genetic diversity for resilience against environmental challenges.