Problem 78 - Objective Biology for NEET

Question

The sporophyte of bryophytes develops after (a) Reduction division of zygote (b) Multiplication in zygote (c) Before the formation of zygote (d) None of these

Step-by-Step Solution

Answer

The correct answer is option (b) Multiplication in zygote.

Step 1: Understanding the Life Cycle of Bryophytes

In the lifecycle of bryophytes, the sporophyte is the diploid phase that follows the fertilization of the gametes. It grows from the gametophyte, which is the haploid phase of the cycle.

Step 2: Identifying Zygote Formation

The zygote is formed immediately after fertilization, which is the union of the male and female gametes. This zygote, which is diploid, will eventually develop into the sporophyte.

Step 3: Applying Knowledge to the Options

With the understanding of when the sporophyte develops in the life cycle, option (a) Reduction division of the zygote might be misleading as reduction division or meiosis does not occur in zygote, instead, it undergoes mitotic division to form the sporophyte. It develops after the multiplication in zygote i.e., the zygote divides mitotically to form the sporophyte and not before or after meiosis, thus option (b) is the correct answer.

Key Concepts

Sporophyte Development

In the unique life cycle of bryophytes, understanding sporophyte development is crucial. After fertilization takes place, the sporophyte stage begins. This stage is diploid, meaning it contains two sets of chromosomes. Unlike angiosperms, where the sporophyte is the dominant phase, in bryophytes, the sporophyte is relatively small and often dependent on the gametophyte for nutrition. Here’s how the process unfolds:

• The zygote, a single cell formed from the union of male and female gametes, develops inside the archegonium of the gametophyte.
• It undergoes repeated mitotic division, forming the multicellular sporophyte.
• This new structure ultimately extends out from the gametophyte, producing a stalk and a spore capsule.

The sporophyte is responsible for spore production, which ensures the continuation of the bryophyte life cycle.

Zygote Formation

Zygote formation in bryophytes is the initial step that leads to sporophyte development. It occurs through the fusion of gametes during fertilization. Here's what happens:

• The male reproductive organ called antheridium releases sperm.
• The female reproductive organ called archegonium contains the egg.
• When sperm reaches the egg, fertilization occurs, leading to the formation of the zygote.

This process is critical because it marks the transition from the haploid gametophyte phase to the diploid sporophyte phase. By being genetically diverse, the zygote lays the foundation for the future sporophyte and potentially new genetic traits within the species.

Reduction Division

While reduction division, or meiosis, is a significant step in the life cycles of many organisms, in bryophytes, it doesn't occur immediately following zygote formation. Instead, it happens later in the lifecycle:

• Following the development of the sporophyte, spores are produced inside the spore capsule.
• Within the capsule, reduction division takes place, converting diploid spore mother cells into haploid spores.
• This reduction in chromosome number ensures the spores can germinate into haploid gametophytes, restarting the life cycle.

Reduction division efficiently bridges the generational transition from sporophyte to gametophyte, maintaining the cycle's balance between diploid and haploid phases.

Gametophyte Phase

The gametophyte phase is the dominant phase in bryophyte life cycles, characterized by its haploid nature. Unlike the sporophyte, the gametophyte is more pronounced and independent. Here are some important aspects:

• This phase begins when spores, produced by the sporophyte, land in a suitable environment and germinate.
• They grow into mature gametophytes, developing into structures bearing gametangia, responsible for producing sperm and eggs.
• The gametophyte supports the subsequent sporophyte by providing it with nutrients.

By dominating the life cycle, gametophytes of bryophytes underline a unique evolutionary direction compared to other plant groups, where the sporophyte tends to dominate.