Gametogenesis is induced by nitrogen starvation.
The gametes demonstrate several differences from the dividing vegetative cells they are derived from. The flagella become agglutinable (able to recognize and adhere to the flagella of gametes of the opposite
mating type). Gametes also develop mating structures on their plasma membrane.
When gametes of opposite mating types are mixed, their flagella immediately agglutinate. This initial recognition event (Stage 1 in the diagram below), which is both gamete
and mating type specific, is followed by one or more signals. These signal(s)
trigger a series of events (Stage 2):
(a) the cells release autolysin, an enzyme which
digests the cell walls,
(b) the flagella, which initially show a loose
interaction along their entire length, now demonstrate tipping, a tighter interaction that occurs only at the flagellar tips,
and
(c) the mating structures, located on the cell
(plasma) membrane, in close proximity to the flagellar origin, become "activated".
Activation of cells of mating type + (mt+)
results in production of a long, membrane-enclosed mating tubule (the activated mating structure) covered with a glycoprotein
"fringe", and containing polymerized actin filaments. This activated mating structure
is analogous to the acrosome found on sperm (the first sperm component to react with the egg).
The mt- cells respond by moving membrane proteins (particles) to the center
of the mating structure, a region of the plasma membrane that had been cleared of particles during gametogenesis, and producing
a short-lived tubule with no microfilaments. When the two activated mating structures
come into contact, they appear to be held together by an adhesive interaction (Stage 3), finally leading to fusion between
opposite mating type cells (Stage 4)
