Why Sexually Reproduce?
After diatoms within the genus Pseudo-nitzschia (as well
as most other genera of diatoms) have vegetatively multiplied for
an extended period of time, the cells ultimately face the problem
of size reduction. Sexual reproduction is a diatom's response to
this problem. Size can be restored through the formation of an auxospore
following meiosis and sexual reproduction. A specialized zygotic
cell called an auxospore swells in sexually mature cells and act
to restore size. The physiological basis for how a diatom is able
to switch from vegetative growth to sexual reproduction is not entirely
Centric vs. Pennate Diatoms
Those species of pennate diatoms within the genus Pseudo-nitzschia differ
in their method of sexual reproduction when compared to centric diatoms.
Sexual reproduction in centric diatoms involves oogamy whereupon
the large non moving female gamete is fertilized by small, flagellate
sperm (thought that chemotaxis plays a part in recognition). Sexual
reproduction in Pseudo-nitzschia, on the other hand, involves
morphologically indistinguishable gametes and thus is called isogamous.
- Differentiation of parent cells into gametangia
- Pairing of parent cells = gametangiogamy, lie valve to valve in
a parallel fashion
- Next state is called gametogenesis: each cell divides meiotically
forming spherical gametes.
- Each gametangium yields two morphologically isogamous nonflagellated
gametes with anisogamous behavior.
- One gametangium produces two active gametes while the other
produces two passive gametes.
- The frustules completely open, allowing both active gametes to
move via amoeboid action towards the passive gametes in the other
- Each gamete fuses with the nearest one in the other gametangium,
forming a spherical zygote in 1-2 minutes.
- Gametes were unable to fuse if they had lost contact with
their parent frustule: substrate probably needed for amoeboid
- Two auxospores are formed from the two sets of gametes.
- They remain attached to only one parent frustule (or may
not be associated to any frustule).
- Gamete formation to appearance of initial cells (auxospores) takes
2-4 days (entire process of sexual reproduction).
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copyright Jennifer Shin 1999.