This
article presents key advances in understanding how ferns reproduce sexually,
with a special focus on how archegonia form, how egg cells develop, and how
fertilization takes place. Recent research reveals new details about the origin
of the egg, the formation of the fertilization pore, and the strategies ferns
use to prevent polyspermy—ensuring that only one sperm fertilizes the egg.
Introduction
Ferns
belong to the group of vascular plants known as pteridophytes, which
reproduce through spores rather than seeds. Traditionally, these plants were
split into two groups—ferns (macrophyllous) and fern allies
(microphyllous). Modern studies, however, show a clearer evolutionary divide:
lycophytes form one branch, while all other vascular plants form another major
group called euphyllophytes. Ferns, which fall under euphyllophytes,
include about 9,000 species such as horsetails, whisk ferns, and both
eusporangiate and leptosporangiate ferns.
Ferns
have a free-living gametophyte known as the prothallus, where sexual
reproduction occurs. While spermatogenesis (sperm formation) in ferns is
well-studied, research on egg formation and fertilization has been limited.
This study addresses that gap by highlighting new discoveries in fern oogenesis
and fertilization.
Oogenesis:
Formation of Archegonia and Egg Cells
Origin of Archegonia
Archegonia—the
female reproductive organs—form on the lower surface of the gametophyte, just
behind the apical notch. Studies on several fern species confirm that each
archegonium arises from a specialized initial cell, located beneath the
surface. This cell contains dense cytoplasm and a large central nucleus.
Through
two divisions, the initial cell forms three cells stacked in a tier. The middle
one becomes the primary cell, which later divides unevenly to produce:
- A neck canal
cell
- A ventral
canal cell (VCC)
- A fully
developed egg cell
The
egg eventually becomes isolated from surrounding cells, forming a separation
cavity and a protective egg envelope.
Development of
the Egg Cell
Young Egg Stage
In
the early stage, the egg, the VCC, and the neck canal cell are tightly packed
together. Numerous plasmodesmata—small channels that allow
communication—connect the egg and VCC. The egg’s nucleus is large and rounded,
and the cytoplasm contains active mitochondria, plastids near the nucleus, and
many vesicles.
Formation of the Separation Cavity
One
of the first major steps in egg development is the creation of a separation
cavity. This cavity begins at the upper surface of the egg when the egg
membrane loosens from the surrounding wall. The plasmodesmata in this region
disappear, except for a small central zone that maintains connection with the
VCC. As the cavity expands inward, the egg gradually becomes more independent.
In
some ferns, a temporary cell wall develops between the egg and the VCC—likely
to support proper egg development.
Formation of the Egg Envelope and
Fertilization Pore
The
egg envelope forms outside the mature egg and is thickest at the upper
surface. Its formation varies among fern species:
- In some ferns,
the endoplasmic reticulum helps build the envelope.
- In others,
amorphous material deposits on the egg surface to form it.
Interestingly,
a small region remains uncovered during envelope formation—the fertilization
pore. This pore serves as the only point where the sperm can enter the egg.
Research shows that the VCC plays a role in shaping this pore by keeping the
area free of deposition.
Nuclear Behavior
As
the egg matures, the nucleus becomes irregular and may develop outward
projections called nuclear evaginations. These features are more
developed in advanced ferns and may offer clues about their evolutionary
relationships.
Though
often overlooked, the VCC plays a critical role in oogenesis. It stays
connected to the egg through the pore region, allowing exchange of signals and
materials. This connection helps regulate:
- Egg envelope
formation
- Fertilization
pore development
- Proper
maturation of the egg
In
some species, the VCC may even influence the activity of the endoplasmic
reticulum beneath the pore region.
Fertilization
and Early Zygote Development
Entry of the Sperm
Fertilization
occurs within the archegonium. Observations show that the spermatozoid enters
the egg exclusively through the fertilization pore. The envelope remains
intact, confirming that the pore is the single entry point.
Preventing Polyspermy
Ferns
have an effective system to ensure only one sperm fertilizes the egg. Even
though several sperm may gather above the egg, only one successfully enters.
Two mechanisms help prevent polyspermy:
- A vesicle-rich
sac forms to block the fertilization pore after the first sperm
enters.
- The fertilized egg
shrinks immediately, making its cytoplasm dense and less
accessible.
Features of the Newly Fertilized Egg
Right
after fertilization:
- The egg shrinks
to nearly half its original size.
- The cytoplasm
becomes dense and opaque.
- Most sperm
structures disassemble except starch-rich plastids.
- The egg envelope
remains intact except around the fertilization pore region.
These
changes mark the beginning of zygote development.
Key Takeaways
for Readers
- Ferns use a
highly coordinated system for egg formation and fertilization, showing
surprising complexity despite their simple appearance.
- The
fertilization pore is a crucial adaptation that ensures controlled sperm
entry and prevents polyspermy.
- The close partnership
between the egg and the VCC highlights how even tiny cells communicate
during reproduction.
- Advanced fern
species show more intricate nuclear and envelope structures, offering
insights into evolutionary patterns.
- Understanding
fern reproduction helps deepen our knowledge of plant evolution and
developmental biology.
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