Lecture 27: Sea
Urchin development
Campbell,
5th, pp.937- mid 945
I.
Overview of development
A. Fertilization
1. Union of sperm and egg
(gametes)
2. Mechanisms to bring
together
3. Species-specificity
4. Prevention of
polyspermy
(fertilization by multiple
sperm)
Fertilizationin
animals is the association and fusion of two
gametesto produce a
new individual. This, the
union of sperm and egg, presents several challenges for sea
urchins.
Fertilization
is external.They live and
spawn in tide pools and reefs in the ocean, where there is a
tremendous amount of water rushing about. To prevent the
sperm and eggs from being washed away and diluted, sea
urchins have evolved mechanisms to bring the gametes
together, including synchronizing spawning and
chemotaxisof the sperm
towards the egg.
At the
same time, many animals, even many echinoderms coexist in
the same habitats, so that there need to be safeguards to
ensure
species-specificityand
prevent association wih
different species. Both the sperm and the egg have
specific receptors for the
otherthat must bind and
transduce a signal sequentially for fertilization to
occur.
In
addition, all those attractive forces can work too well and
bring many sperm to each egg. Two is good, but more is not
better; fusion with multiple sperm will bring in multiple
genomes and multiple centrioles and result in the death of
the embryo. Therefore, there have to
bemechanisms to prevent
polyspermy,
fertilization with more than one sperm.
B. Gametes
1. Sperm
a. Nucleus - haploid set of
condensed chromosomes
b. Flagella
c. Basal body
d. Mitochondria
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