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Home Q & A Board Biology-Related Homework Help General Biology
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SoTired67 SoTired67
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3 years ago Edited: 3 years ago, SoTired67
Tl;DR ... What biological processes/mechanisms enable the fusion of sperm and egg, but prevent the fusion of two sperm or two eggs? Are androgenesis, gynogenesis and parthenogenesis sexual or asexual "reproduction"? And what makes a polar body different from an egg?

To ask my 3 questions in more detail:

Q1) How is it that in all species, humans themselves included, only sperm and egg can fuse or create a zygote, and two sperm or two eggs can not, as in it is impossible for two sperm or two eggs to fuse, or create a zygote?

What (biological) processes/mechanisms are there that make it so two sperm or two eggs can not fuse or create a zygote, and only a sperm and an egg can? And what are these processes/mechanisms called?

I've heard there are certain proteins that make it impossible for two sperm or two eggs to fuse or create a zygote. What proteins are they?

Q2) Are androgenesis, gynogenesis and parthenogenesis sexual or asexual "reproduction"?

Androgenesis is a process in which a sperm uses an egg, but later deactivates the egg and becomes the only source of "genetic material".

Gynogenesis is similar to androgenesis, except it's an egg that uses a sperm, but later deactivates the sperm and becomes the only source of "genetic material".

Does this make androgenesis and gynogenesis asexual or sexual "reproduction"? What are the precise definitions of sexual and asexual "reproduction"?

I've seen people claim zygotes "created" from androgenesis "have no mother", and from gynogenesis "have no father".

Don't see how that is the case given in androgenesis egg is still used, and in gynogenesis sperm is still used meaning the zygotes do have a "mother" and a "father" respectively.

It's just that the egg and sperm become deactivated, respectively. Doesn't mean they are not necessary in the "reproduction" and play no role, right? It's not as if a male "creates" a zygote on its own in androgenesis, without a female, and it's not as if a female "creates" a zygote on its own in gynogenesis, without a male, right?

It's false to claim the female plays no role in the "creation" of zygote in androgenesis, and the male plays no role in the "creation" of zygote in gynogenesis, right?

Which means the zygotes do have a "mother" in androgenesis, and a "father" in gynogenesis, right?

What about parthenogenesis? In parthenogenesis, an egg either "creates" a zygote on its own, or combines with a polar body and "creates" a zygote.

Is that sexual or asexual "reproduction"?

Q3) As mentioned, in parthenogenesis, an egg either "creates" a zygote on its own, or combines with a polar body and "creates" a zygote.

Is polar body different from an egg? What makes a polar body not an egg, and different from an egg?
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wrote...
#1
Educator
3 years ago Edited: 3 years ago, bio_man
Hi SoTired67

Quote from: SoTired67 (3 years ago)
Q1) How is it that in all species, humans themselves included, only sperm and egg can fuse or create a zygote, and two sperm or two eggs can not, as in it is impossible for two sperm or two eggs to fuse, or create a zygote?

To prevent polyspermy, the zona pellucida, a structure that surrounds mammalian eggs, becomes impermeable upon fertilization, preventing the entry of further sperm. The zona pellucida undergoes a structural change immediately after fertilization where cortical granules are exocytosized. Cortical granules are membrane bound organelles located in the cortex of unfertilized oocytes. [1]

Please watch this support video:


Source: [2]

Quote
Q2) Are androgenesis, gynogenesis and parthenogenesis sexual or asexual "reproduction"?

Androgenesis is asexual. There was an experiment conducted by Dr. A. R. Whiting of the University of Pennsylvania that investigated this. Dr. Whiting treated female wasps with ultraviolet radiation and destroyed the chromosomes of the unfertilized eggs. Mating then proceeded and the offspring grew up without any trace of maternal characters and with only paternal character being displayed. This type of insect production has been termed androgenesis, meaning from the Greek, 'birth from a male.' Given that the female genome did not participate in the formation of the offspring, then it only involves one sex, hence asexual.

The same can be said about parthenogenesis - it too is considered asexual. It is common in the animal kingdom up through the class Insecta, but it occurs much more rarely thereafter. Keep in mind that these insects can also reproduce sexually depending on the conditions. Take, for example, aphids. Inside the ovaries of an adult female aphid are dozens of tiny aphid embryos, patiently waiting to be released. Inside these embryos, in an arrangement reminiscent of those wooden nesting dolls from Russia, are even tinier embryos—also on tap, as it were. During the summer, when the weather is warm and food is abundant, female aphids release these stockpiled generations into the world. In the cooler, less nurturing months of spring and fall, the same female aphids mate with any available males, then give birth to a few generations of nonparthenogenetic young.

Quote
And what makes a polar body different from an egg?

Polar bodies are byproducts of meiosis. They form because the egg cell (oocyte) does not divide evenly; the one with more cytoplasm is used, while the polar body gets degenerated. The polar body has equal amount of DNA as the secondary oocyte, however.


Source: [3]

Hope this helped you understand!
wrote...
#2
3 years ago
Although many sperm can bind to an egg, normally only one fuses with the egg plasma membrane and injects its nucleus and other organelles into the egg cytoplasm. If more than one sperm fuses - a condition called polyspermy - multipolar or extra mitotic spindles are formed, resulting in faulty segregation of chromosomes during cell division; nondiploid cells are produced, and development usually stops.

Two mechanisms can operate to ensure that only one sperm fertilizes the egg. In many cases, a rapid depolarization of the egg plasma membrane, which is caused by the fusion of the first sperm, prevents further sperm from fusing and thereby acts as a fast primary block to polyspermy. But the membrane potential returns to normal soon after fertilization, so that a second mechanism is required to ensure a longer-term, secondary block to polyspermy. This is provided by the egg cortical reaction.

When the sperm fuses with the egg plasma membrane, it causes a local increase in cytosolic Ca2+, which spreads through the cell in a wave. In some mammalian eggs, the initial increase in Ca2+ is followed by prolonged Ca2+ oscillations. There is evidence that the Ca2+ wave or oscillations are induced by a protein that is introduced into the egg by the sperm, but the nature of the protein is unknown.

The Ca2+ wave or oscillations activate the egg to begin development, and they initiate the cortical reaction, in which the cortical granules release their contents by exocytosis. If the cytosolic concentration of Ca2+ is increased artificially - either directly by an injection of Ca2+ or indirectly by the use of Ca2+-carrying ionophores - the eggs of all animals so far tested, including mammals, are activated. Conversely, preventing the increase in Ca2+ by injecting the Ca2+ chelator inhibits activation of the egg in response to fertilization. The contents of the cortical granules include various enzymes that are released by the cortical reaction and change the structure of the zona pellucida. The altered zona becomes “hardened,” so that sperm no longer bind to it, and it therefore provides a block to polyspermy. Among the changes that occur in the zona is the proteolytic cleavage of ZP2 and the hydrolysis of sugar groups on ZP3.
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