Double Fertilization

Question Asked in Class - During double fertilization, which event take place first - syngamy or triple fusion ?

The Direct Answer

In many investigated angiosperms (flowering plants), Triple Fusion (formation of the endosperm) often takes place slightly before Syngamy (formation of the zygote).

While the events are tightly coupled and often described as simultaneous "double fertilization," detailed cytological studies (such as in Lilium and Arabidopsis) have shown that the sperm nucleus often fuses with the polar nuclei (central cell) before the other sperm nucleus completes fusion with the egg cell.

Why "Short Distance" is Incorrect

Your hypothesis that "syngamy happens first due to short distance" is logical but biologically incorrect for two key reasons:

  • The "Dwell" Phase: Upon release from the pollen tube into the synergid cell, the sperm cells do not immediately fuse. In Arabidopsis, for example, there is a "dwell time" (approx. 7 minutes) where the sperm cells sit between the egg and central cell before migrating to their targets.
  • Active Transport: The sperm cells are non-motile and are actively transported via actin filaments to their targets. This transport is a controlled cellular process, not a simple race based on proximity.

Why "Prerequisite" is the Best Explanation

Your hypothesis that the order is "irrelevant because endosperm formation is a prerequisite" is actually the most biologically accurate way to think about it.

  • Metabolic Prerequisite: The primary endosperm nucleus (formed by triple fusion) divides rapidly to form the nutritive tissue (endosperm) needed to support the embryo.
  • Zygotic Rest: The zygote (formed by syngamy) typically enters a resting period (dormancy) immediately after fertilization. It waits for the endosperm to develop enough cellular mass to provide nutrition. Therefore, triple fusion occurring first or simultaneously ensures the "food" is ready before the "baby" starts growing.
Feature Explanation
Order of Events Triple Fusion → Syngamy (or simultaneous).
Why not Syngamy first? Sperm release involves a "pause" and active signaling; it is not just about which cell is closer.
Biological Logic Endosperm is the "prerequisite" fuel; the zygote rests until the endosperm is established.
Variability While the order of fusion is generally triple fusion first or simultaneous, the time gap varies. In some plants (like Lilium), the triple fusion is clearly observed earlier; in others, it is millisecond-close.

Conclusion

You should lean towards Triple Fusion occurring first, driven by the biological necessity of establishing the endosperm (prerequisite) rather than the physical distance of the gametes.