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Intelligent Design, the best explanation of Origins » Molecular biology of the cell » Development biology » Sperm activation

Sperm activation

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1Sperm activation Empty Sperm activation on Tue Jan 08, 2019 1:40 pm

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Sperm activation

Species-specific sperm attraction has been documented in numerous species, including cnidarians, mollusks, echinoderms, amphibians, and urochordates. In many species, sperm are attracted toward eggs of their species by chemotaxis—that is, by following a gradient of a chemical secreted by the egg. These oocytes control not only the type of sperm they attract, but also the time at which they attract them, releasing the chemotactic factor only after they reach maturation. The mechanisms of chemotaxis differ among species, and chemotactic molecules are different even in closely related species. In sea urchins, sperm motility is acquired only after the sperm are spawned. As long as sperm cells are in the testes, they cannot move because their internal pH is kept low (about pH 7.2) by the high concentrations of CO2 in the gonad a . However, once sperm are spawned into seawater, their pH is elevated to about 7.6, resulting in the activation of the dynein ATPase. The splitting of ATP provides the energy for the flagella to wave, and the sperm begin swimming vigorously. But the ability to move does not provide the sperm with a direction. 

In echinoderms, direction is provided by small chemotactic peptides called sperm-activating peptides (SAPs). One such SAP is resact, a 14-amino acid peptide that has been isolated from the egg jelly of the sea urchin Arbacia punctulata. Resact diffuses readily from the egg jelly into seawater and has a profound effect at very low concentrations when added to a suspension of Arbacia sperm. When a drop of seawater containing Arbacia sperm is placed on a microscope slide, the sperm generally swim in circles about 50 μm in diameter. Within seconds after a small amount of resact is injected, sperm migrate into the region of the injection and congregate there.  As resact diffuses from the area of injection, more sperm are recruited into the growing cluster. Resact is specific for A. punctulata and does not attract sperm of other urchin species. (An analogous compound, speract, has been isolated from the purple sea urchin, Strongylocentrotus purpuratus.) A. punctulata sperm have receptors in their cell membranes that bind resact. When the extracellular side of the receptor binds resact, it activates latent guanylyl cyclase in the cytoplasmic side of the receptor 

Sperm activation OwSruAW
Model for chemotactic peptides in sea urchin sperm.
Resact from Arbacia egg jelly binds to its receptor on the sperm. This activates the receptor’s guanylyl cyclase (RGC) activity, forming intracellular cGMP in the sperm. The cGMP opens calcium channels in the sperm cell membrane, allowing Ca2+ to enter the sperm. The influx of Ca2+ activates sperm motility, and the sperm swims up the resact gradient toward the egg.

Active guanylyl cyclase causes the sperm cell to produce more cyclic GMP (cGMP), a compound that activates a calcium channel in the cell membrane of the sperm tail, allowing the influx of calcium ions (Ca2+) from the seawater into the tail. These sperm-specific calcium channels are encoded by CatSper genes—the same genes that control the direction of sperm migration in mice and humans. The increases in cGMP and Ca2+ activate both the mitochondrial ATP-generating apparatus and the dynein ATPase that stimulates flagellar movement in the sperm . In addition, the sperm sense the SAP gradient by curving their tails, interspersing straight swimming with a “turn” to sense the environment. The binding of a single resact molecule may be enough to provide direction for the sperm, which swim up a concentration gradient of this compound until they reach the egg . Thus, resact functions as a spermattracting peptide as well as a sperm-activating peptide. (In some organisms, the functions of sperm attraction and sperm activation are performed by different compounds.)

a A gonad, sex gland, or reproductive gland[1] is a mixed gland that produces the gametes (sex cells) and sex hormones of an organism.

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2Sperm activation Empty Re: Sperm activation on Tue Jan 08, 2019 2:01 pm

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Sperm activation

In order to fertilize the maternal egg, the male sperm must be attracted to its destination. This happens through chemotaxis. That is, the sperm has a flagellum which permits it to swim. The attraction is promoted by a gradient of a chemical secreted by the egg. This chemical is only secreted, when the oocyte it reaches maturation ( a female gametocyte or germ cell involved in reproduction. A gamete is a haploid cell that fuses with another haploid cell during fertilization (conception) in organisms that sexually reproduce. Haploid is the term used when a cell has half the usual number of chromosomes.  )

As long as sperm cells are in the testes, they cannot move because their internal pH is kept low (about pH 7.2) by the high concentrations of CO2 in the gonad . A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes (sex cells) and sex hormones of an organism.

pHi regulation is of the utmost importance for sperm. Intracellular pH (pHi) regulation is essential for cell function. Notably, several unique sperm ion transporters and enzymes whose elimination causes infertility are either pHi dependent or somehow related to pHi regulation. 1  It is thus clear that pHi regulation is of the utmost importance for sperm physiology.

Once sperm are spawned into seawater for example by sea urchins, their pH is elevated to about 7.6, resulting in the activation of the dynein ATPase. This enzyme is a multisubunit protein complex associated with microtubules.
The splitting of ATP provides the energy for the flagella to wave, and the sperm begin swimming vigorously. But the ability to move does not provide the sperm with a direction. 

In echinoderms, direction is provided by small chemotactic peptides called sperm-activating peptides (SAPs). One such SAP is resact, a 14-amino acid peptide that has been isolated from the egg jelly of the sea urchin Arbacia punctulata. Resact diffuses readily from the egg jelly into seawater and has a profound effect at very low concentrations when added to a suspension of Arbacia sperm. When a drop of seawater containing Arbacia sperm is placed on a microscope slide, the sperm generally swim in circles about 50 μm in diameter. Within seconds after a small amount of resact is injected, sperm migrate into the region of the injection and congregate there.  As resact diffuses from the area of injection, more sperm are recruited into the growing cluster. Resact is specific for A. punctulata and does not attract sperm of other urchin species.  A. punctulata sperm have receptors in their cell membranes that bind resact. When the extracellular side of the receptor binds resact, it activates latent guanylyl cyclase in the cytoplasmic side of the receptor 

Active guanylyl cyclase causes the sperm cell to produce more cyclic GMP (cGMP), a compound that activates a calcium channel in the cell membrane of the sperm tail, allowing the influx of calcium ions (Ca2+) from the seawater into the tail. These sperm-specific calcium channels are encoded by CatSper genes—the same genes that control the direction of sperm migration in mice and humans. The increases in cGMP and Ca2+ activate both the mitochondrial ATP-generating apparatus and the dynein ATPase that stimulates flagellar movement in the sperm . In addition, the sperm sense the SAP gradient by curving their tails, interspersing straight swimming with a “turn” to sense the environment. The binding of a single resact molecule may be enough to provide direction for the sperm, which swim up a concentration gradient of this compound until they reach the egg . Thus, resact functions as a sperm-attracting peptide as well as a sperm-activating peptide. (In some organisms, the functions of sperm attraction and sperm activation are performed by different compounds.)

In order to have this complex system functioning, following parts are essential: 

1. flagellum which permits it to swim
2. pHi regulation
3. dynein ATPase multisubunit protein complex
4. sperm-activating peptides (SAPs) like resact in seaurchins from the egg
5. Receptors on the sperm cell membranes that bind sperm-activating peptides (SAPs) like resact of sea urchins
6. guanylyl cyclase (RGC)
7. intracellular cGMP's in the sperm cell

making sperm cell activation and its ability to find the egg an irreducible. Both, the sperm, and the egg, must be fully functional, specified, and each contributing for fertilization to occur:

This is clear evidence that a gradual formation of both specialized germ cells is impossible. Furthermore, if the mechanisms to keep the ph level correct is also essental at all stages. The whole system must be fully functional from day one, demonstrating why the evolutionary hypothesis of sexual reproduction is not supported by the evidence.

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3Sperm activation Empty Re: Sperm activation on Tue Jan 08, 2019 5:20 pm

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Co-option of alternate sperm activation programs in the evolution of self-fertile nematodes

The ability to co-opt specific genetic programs might underlie the origin of many complex traits and could explain some examples of parallel evolution. Our data strongly support this model, as we show that nematodes have co-opted male sperm activation programs for use in newly evolving hermaphrodites. More importantly, we found that two different sperm activation programs can be co-opted for this purpose. Usually, developmental biases or constraints are thought to prevent certain types of variation from occurring, which prevents some evolutionary transitions. Our results imply that some patterns of developmental regulation favour certain types of change, as the existence of two sperm activation pathways in nematodes helps explain why self-fertility has originated so frequently in this group. Thus, this system provides a concrete example of the concept of evolvability in evolutionary developmental biology.

might underlie
could explain parallel evolution
strongly support
can be co-opted
are thought
our results imply
concept of evolvability in evolutionary developmental biology

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