Question

If you tried to fertilize sea urchin embryos in sodium-poor seawater, how would this affect the fast block to polyspermy? How would it affect the slow block to polyspermy? What might you observe if you watched this under the microscope?

Answer 1

Polyspermy describes an egg that has been fertilized by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome—one from the egg and one each from multiple sperm. Usually, the result is an non-viable zygote.

The prevention of polyspermy in sea urchins depends on a change in the electrical charge across the surface of the egg, which is caused by the fusion of the first sperm with the egg. Unfertilized sea urchin eggs have a negative charge inside, but the charge becomes positive upon fertilization. When sea urchin sperm encounter an egg with a positive charge, sperm-egg fusion is blocked. Thus, after the first sperm contacts the egg and causes the change, subsequent sperms are prevented from fusing. This "electrical polyspermy block" is thought to result because a positively charged molecule in the sperm surface membrane is repelled by the positive charge at the egg surface. This mechanism is called Fast block to polyspermy. This is a transient phase before the slow block to polyspermy has to be activated.

Slow block to polyspermy is achieved through cortical granule reaction in Sea Urchins. Through this reaction a fertilisation envelope/egg membrane is made which prevents polyspermy.

Effect of sodium poor water on fast block to polyspermy

Sodium does indeed play an essential role in the fast block mechanisms in sea urchins.Lower-than-standard levels of sodium lead to successful fertilization, though not normal development,The fast block mechanism is mediated by sodium, which floods into the egg via Na+ channels once the sperm and egg interact. The influx of external sodium causes an electrical depolarization of the egg’s plasma membrane, changing the membrane potential from its resting, negative potential of -70mV to a positive potential . This rapid change of membrane potential prohibits other sperm from interacting and fusing with the egg, thus giving the egg additional time to complete the cortical reaction. Lowering of sodium levels interrupts this process and can induce polyspermy

Effect of sodium poor water on slow block to polyspermy

Since slow block mechanism is not as reliant as the fast block on sodium levels, its effect is not so direct with the lowering of sodium levels. Experiments shows that once the egg membrane is formed, the effect of lowering sodium level does not cause any effect in fertilization. Normal fertilisation happens in this condition.

So under this circumstance, you could find embryos with fertilization membrane (which denotes successful fertilization) under a microscope.