lecture 19: external fertilization Flashcards
how to efficiently broadcast spawn?
increase number to increase success
examples of organisms that do this
echinoderms
- urchins
- sea stars
why is broadcast spawning good?
inexpensive
- not a lot of energy used to make internal or external anatomy
- lots of gametes
- no reproductive organs in males
risks of external fertilization
high risk of wasted gametes
gamete dilution
non-synchrony
***experiment showed high success of gametes when released right over urchin
current speeds and distance impacted success of fertilization
strategies to minimize gamete waste
1) reproductive aggregations
2) synchronized release of gametes
3) sperm attractants
spawning cues for Great Barrier Reef corals
1) increased seawater temp or increasing photoperiod
2) lunar cue
- wait till calmer water so less water movement
3) light-off cue –> spawn only after sunset
sperm attractants
VERY specific
even between closely related species
acrosome reaction
acrosome reaction happens when sperm finds egg
1) sperm makes contact w jellycoat of an egg
2) exocytosis of acrosomal vesicle
- the acrosomal membrane breaks down
3) acrosomal enzymes digest the jellycoat
4) actin polymerization
- subacrosomal granule
is induced to polymerize
- turns acrosomal vesicle
inside out
- the dotted part now outside
– receptors on the vitelline envelope bind bindin
***if bindin doesnt bind, no fertilization
***bindin only binds to conspecific vitelline envelope of species specifics —> conspecifics
bindin
bindin only binds to conspecific vitelline envelope of species specifics —> conspecifics
- gamete recognition protein of sea urchins
- exposed on acrosomal process by acrosome reaction
- bindin receptors on vitelline envelope of the egg
- facilitate conspecific gamete binding
what is polyspermy and how to stop it
- simultaneous fusion of two or more sperm
- Both sperms introduce a centriole and nucleus to egg
- Results in cleavage abnormalities – fatal for egg
HOW IS IT PREVENTED:
- fast block
- slow block
How is polyspermy prevented?
- Fast block – egg membrane depolarization
– as soon as bindin binds to vitelline envelope - a wave of depolarization goes thru - any sperm coming up behind, sperm cannot bind in state of depolarization - this wave doesnt last v long - like miliseconds —> so slow block then necessary
- Slow block – cortical granule exocytosis
- ‘harden’ vitelline envelope preventing entry of additional sperm
– vitelline envelope undergoes exocytosis reaction —> where lots of crosslinks happen w glyoprotein —> gets larger and hard —> then next sperm that start to swim up reach this hard barrier
For broadcast spawning organisms, reproductive failure has different causes at different male densities:
Low density males –> excessive sperm dilution
High density males –>
polyspermy
Different alleles of bindin gene can have subtle effects:
- enhance fertilization success at LOW sperm density
- enhance fertilization success at HIGH sperm density by minimizing polyspermy
change in selection pressure on urchins bc of otters
- Shift in allele frequencies of bindin over last 200+ years
- Animals conceived 200 years ago have higher LD frequency (low population density at time of fertilization)
- Animals conceived recently have higher HD frequency (high population density at time of fertilization)
spermcast mating
- males broadcast spawn sperm
- females sequester the sperm to fertilize retained eggs
– common in sessile marine inverts - suspension feeders will filter food and collect sperm this way
