How did sex pop up? How did sperm get in? (Introduction)

by David Turell , Friday, July 19, 2019, 02:12 (1953 days ago) @ dhw

The big problem is the female immune system and being allowed to advance:

https://www.sciencedaily.com/releases/2019/07/190718145352.htm

"Why does it take 200 million sperm to fertilize a single egg?

One reason is that sperm, when they arrive in the uterus, face a bombardment by the immune system. Perhaps, says molecular anthropologist Pascal Gagneux, many are needed so that some will survive. On the other hand, there may be a benefit to culling so many sperm.

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"Gagneux's lab at the University of California, San Diego, has discovered the makings of a "secret handshake" between sperm and the cells lining the uterus. Uterine cells, they report in the Journal of Biological Chemistry, express a receptor that recognizes a glycan molecule on the surface of sperm cells. It's possible that this interaction may adjust the female's immune response and help sperm make it through the leukocytic reaction

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"The leukocytic reaction is not well understood. What we do know, Gagneux explained, is that "after crossing the cervix, millions of sperm -- a U.S. population worth of sperm -- that arrive in the uterus are faced by a barrage of macrophages and neutrophils."

"This attack by the innate immune system kills a majority of the sperm cells in semen, winnowing hundreds of millions of sperm down to just a few hundred that enter the fallopian tubes. The defensive response may be beneficial in preventing polyspermy, when an egg is fertilized by more than one sperm and cannot develop.

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"..the team observed sialic acid binding receptors called siglecs on endometrial cells. In solution, these endometrial receptors can bind to whole sperm. According to Gagneux, the binding interaction may help the sperm run this gantlet -- for example, by dampening the immune response. Alternatively, it may be a way for uterine cells to weed out faulty sperm.

"In the immune system, this receptor class helps cells to recognize sialic acid molecules as "self," and in that context they can either turn up or down inflammation.

"'It's somewhat embarrassing how little we can say about what this [interaction] means," he said. The first step in understanding its physiological significance will be to look for direct interaction between sperm and intact uterine tissue -- this paper looked at only sperm interacting with purified proteins.

"In some ways, Gagneux added, it's humbling to be working in such a poorly understood area. Reproduction, he said, "is a very, very delicate tug-of-war at many levels. The fact that there is (also) this immune game going on is completely fascinating.'"

Comment: This study demonstrates the difficulty in imagining anything else but evolution by design. Animals must have a strong immune system to survive infections. That same immune system has to allow sperm an entry to the uterus. The sperm is like a foreign invader. This could not be developed in a stepwise way. It had to be developed in one step for sexual reproduction to work and sexual reproduction continue throughout the animals that subsequently evolved.