Problem 105 - Objective Biology for NEET

Question

Zygote of bryophyte (a) Multiply and forms gametophyte (b) Multiply and forms sporophyte (c) Undergoes reduction division just after formation (d) All are correct

Step-by-Step Solution

Answer

None of the provided options are correct, so the answer is neither (a), (b), (c) nor (d). In bryophytes, a zygote grows into a sporophyte.

Step 1: Comprehending Terminology

Before solving the exercise, it's necessary to understand terms such as zygote, gametophyte, sporophyte, and reduction division. A zygote forms when a male and female gamete combine. The zygote then develops into an organism. In terms of plants, the zygote goes through mitotic cell division to form a multicellular plant body called sporophyte. The sporophytes form spores through a process called reduction division (also referred to as meiosis) and these spores eventually grow into gametophyte.

Step 2: Understanding Bryophyte Lifecycle

Bryophytes exhibit a haplodiplontic life cycle, where they cyclically alternate between the gametophytes (haploid) and sporophytes (diploid). The dominant stage in Bryophytes is of the gametophyte. The sporophyte stage is totally dependent on the gametophyte for its nutrition.

Step 3: Choosing the Correct Option

(a) and (b) are incorrect as in bryophytes the zygote (2n) produced after fertilisation divides by mitosis to form a multicellular, nutritionally dependent sporophyte. (c) is incorrect too as the zygote does not immediately undergo reduction division but instead grows into a sporophyte. So, none of the choices are accurate, hence the right answer is not among the options provided.

Key Concepts

Zygote

In the lifecycle of bryophytes, the zygote is a crucial starting point of development. A zygote is formed when the male and female gametes unite through the process of fertilization. This fusion results in a single cell with a diploid set of chromosomes, denoted as 2n.
This diploid zygote does not directly develop into a gametophyte. Instead, it marks the beginning of the sporophyte generation.
The transition from zygote to sporophyte distinguishes the generation phases in plants.

• The zygote undergoes mitotic divisions, which means it divides and re-divides to produce a multicellular body.
• This process does not involve reduction or meiosis; hence, the sporophyte remains diploid.
• The zygote's role is central in bridging the gap between the generations.

It is important to note that in bryophytes, the zygote is the starting point of the dependent sporophyte, highlighting its non-independent nature.

Gametophyte

The gametophyte represents the haploid phase of the bryophyte lifecycle. It is the dominant and more visible stage compared to the sporophyte.
This stage arises from spores, which are produced by the sporophyte after undergoing meiosis or reduction division.
In bryophytes, the gametophyte stage is crucial for reproduction and survival.

• The gametophyte produces gametes (sperm and eggs) through mitosis, which are haploid in nature.
• The male and female gametes unite to form the diploid zygote, restarting the cycle.
• This phase is independent and performs vital functions like photosynthesis and nutrient absorption.

In essence, the gametophyte is self-sufficient and was once believed to be the primary phase of early terrestrial plants.

Sporophyte

The sporophyte phase in bryophytes is dependent on the gametophyte. It emerges from the zygote after mitotic division.
Unlike in vascular plants, the sporophyte in bryophytes is usually smaller and relies on the gametophyte for nutrition.
Several key points define the sporophyte's role:

• The sporophyte generation is diploid (2n).
• It consists mainly of a stalk and a capsule where spores are produced.
• Spores are formed in the capsule through meiosis, also known as reduction division, reducing the chromosome number by half.

This reduced form of sporophyte reflects the evolutionary step where early land plants transitioned into more complex forms, leading to vascular plants.

Reduction Division

Reduction division is scientifically termed meiosis. It is a process that occurs in the sporophyte generation of the bryophyte lifecycle.
During this process, diploid cells undergo division to form haploid spores.
This division is essential because it reintroduces genetic variability and ensures that gametes remain haploid.

• Meiosis reduces the chromosome number from diploid (2n) to haploid (n).
• Through this division, genetic variation is introduced, which is beneficial for adaptation and survival.
• The haploid spores produced can grow into new gametophytes.

Without reduction division, bryophytes wouldn't be able to maintain their cyclical life cycle between distinct haploid and diploid phases.

Haplodiplontic Life Cycle

Bryophytes exhibit a haplodiplontic life cycle, characterized by alternating haploid and diploid generations.
This cycle entails a rhythmic transition between the gametophyte and sporophyte phases.
The term "haplodiplontic" itself indicates the presence of both haploid (gametophyte) and diploid (sporophyte) stages.

• The cycle begins with the haploid spores germinating to form a new gametophyte.
• This gametophyte produces gametes that fuse to create the diploid zygote, giving rise to the sporophyte.
• The sporophyte culminates in producing spores via reduction division, closing the cycle.

This type of lifecycle balances genetic stability and variation, allowing bryophytes to thrive in various environments.