How did sex pop up? complexity of egg-sperm union (Introduction)

by David Turell , Sunday, February 11, 2024, 16:19 (77 days ago) @ David Turell

Quite a complicated set of events:

https://www.sciencealert.com/simulations-reveal-what-happens-when-a-sperm-kisses-an-egg...

"The moment when a slithering sperm propels itself head-first into a gelatinous egg is one of sudden change. Within seconds to minutes, chemical changes in the egg's membrane and outer coat are enacted to block any more sperm from attaching to and entering the oocyte.

"A series of reactions also takes place as the sperm and egg recognize each other, chemically speaking, and then begin to merge their membranes together. But despite the significance of these delicate molecular events, their details haven't been fully resolved.

***

"'It was assumed that the combination of the two proteins [JUNO and IZUMO1] into a complex initiates the recognition and adhesion process between the germ cells, thereby enabling their fusion," explains Paulina Pacak, a bioinformatician at ETH Zurich and first author of the study.

"This interaction of JUNO – which is located on the outer membrane of the female egg cell – and IZUMO1, found on the male sperm cell surface, is the first known physical link between two newly fusing sex cells.

"However, efforts to develop small molecular inhibitors of the JUNO-IZUMO1 union, as a potential contraception, haven't amounted to much so researchers suspect there might be more to their molecular interactions than we know.

***

"Each simulation spanned just 200 nanoseconds each, but they showed that the JUNO-IZUMO1 complex is initially stabilized by a host of short-lived and weak non-covalent interactions between the protein molecules.

"These contacts lasted less than 50 nanoseconds each, and understanding what happens when they are interrupted, either by other molecules or mutations, could provide insights into contraceptives and infertility, the researchers suggest.

***

"Minutes after a sperm and egg unite, the fertilized egg releases a flood of charged zinc atoms which are thought to prevent other sperm from entering the egg by hardening its outer coat.

"The simulations showed that the presence of zinc ions bent IZUMO1 into a boomerang shape, so it could no longer firmly bind to JUNO. This suggests the egg's zinc release could also hinder the binding of approaching sperm."

Comment: each reaction by sperm and egg must be precisely correct for the union to occur. This is a good example of irreducibly complex. Another reason to realize the evolutionary step to sexual reproduction had to be designed.