Sperm are very different from all other cells
bbc.com58 points by viewtransform 12 hours ago
58 points by viewtransform 12 hours ago
Seems very Star Trek Enterprise when they polarize the hull plating.
"To prevent this from happening, once a sperm cell has made contact with it, the egg quickly employs two mechanisms. First, its plasma membrane rapidly depolarises – meaning it creates an electrical barrier that further sperm cannot cross."
Whatever relates to DNA in an organism is under strict control and huge protective mechanisms. For example DNA replication has an error rate of 1/10^9. Obviously, selecting 1 out of 10-150 million "persons" is some fierce form of survival of the fittest, that happens early and is very cheap. The next layer of selection is embryonic development, next the labor and in the end early life diseases. At every level the selection becomes weaker but the price larger.
We are some pretty special and die-hard persons.
We need to get the male’s genetic material into the female’s body. How many redundant copies should we send. 100? 1000? A voice in back of the room: 50 million.
Survival of the fittest. You don’t guarantee there’s nothing majorly wrong with the sperm that gets through, but by rejecting 99.99% as failing to be the fastest or survive the longest in a harsh environment we can drastically cut down on the issues with the next generation before investing significant resources into trying to form an embryo.
Is this really the case? Do the fittest sperm become the best people? Feels like a streach
It is just a filter for certain cellular traits. It doesn't filter for many other important traits. A good flagellum doesn't indicate good kidneys. A sperm's existence indicates the father made faithful copies of cells, however the new sperm DNA could be terrible at making faithful copies (ability not proven until after fertilisation).
It also depends on how much of the sperm cell comes from the fathers genes, and how much is generated from the new DNA. I didn't find a clear answer to this but the following indicates that the new sperm cell is at least somewhat generated by the sperm DNA:
Because nucleotide recombinations can occur during meiosis I, the genetic code of chromosomes of gametes can differ from that of somatic parent cells (ie., progeny cells might express cell-surface antigens that are recognized by the host [the father] as foreign and thus be eliminated by humoral or cellular immune mechanisms). Occluding junctions that interconnect adjacent Sertoli cells shield secondary spermatocytes, spermatids, and spermatozoa from autoimmune recognition.
And I'm completely ignoring the mitochondria (Dad's copy are not passed on so should be mostly irrelevant to sperm selection pressures). I'm pretty ignorant of this whole topic - high school biology only.
100% of sperm dna comes from father.
but each chromosome could be "grandfathers" or "grandmothers", and usually those chromosomes have one or two crossover events, so, the chromosome goes FFFFFFFFMM for example. (where F = grandfather and M = grandmother)
Does that mean gendering a baby is more determined by factors on the father's side?
Whats interesting to me is that along with the autosomes (chr1-22) the sperm contributes either an X or Y sex chromosome. The X chromosome is large and carries a lot of important genes. The Y chromosome on the other-hand is much smaller and carries very few functional genes (primarily just SRY).
So If you think about it girls get ~5% more genetic material from their father than boys.
2.5% (but not really, also remember the chromosomes have different sizes).
this is probably why distribution of traits in men has fatter tails than in women.
I couldn't be assed to do the math, not just chromosome size but also number of genes etc, get's tricky to quantify.
Fascinating to think about the variability introduced by having only a single copy of the X chromosome. Lot's of interesting genes in there, MAOA/MAOB (primary neurotransmitter breakdown pathways), AR (androgen receptor), OPN1LW/OPN1MW (red green color blindness), G6PD, etc.
I never explored this scientifically but I have this hunch that a man with more sisters than brothers will tend to have more daughters than sons
It's a common idea a with some decently strong evidence it's false, https://www.cnn.com/2020/02/18/health/boys-girls-run-in-fami...
Yes. Sperm cells are haploid. If the sperm that fertilizes the egg has an X chromosome, the baby will be female. If it has a Y chromosome, the baby will be male.
There are rare exceptions for genetic disorders relating to sex development, but generally speaking the above is true.
I suppose it’s a question of perspective.
The best sperm will likely result in the next generation of sperm also being good.
We look at the human as the organism, the sperm as the gamete - but perhaps our logic is anthropocentric - perhaps the sperm is the organism, and we are just the ridiculously elaborate reproductive mechanism.
That was the common theory for awhile, but I’ve been seeing a few articles that talk about the egg doing things to “select” amongst the sperm
I don't think the fittest sperm become the fittest people no. But I do think that seriously fucked up genetic errors will make the sperm cell non-viable. So it's more about creating a floor than getting the creme de la cremepie.
Not at all: we've been doing IVF for a while now, and completely immobile sperm produce healthy normal babies.
It's the peacock's tail effect: what relevance does a brightly colored tail have to a male peacock's actual fitness?
The tail’s appearance is a meaningful proxy for the state of health of its holder.
It's not, the guy is just making stuff up.
A lot of what's behind that "selection" there is still unknown; in principle all sperm are, more or less, the same.
There's also so many external effects in play that no single sperm cell may actually have a significant advantage over others; e.g. the behavior of the seminal fluid (ph, viscosity), the physical location of the egg, etc.
The cartoonish image of sperm swimming towards the egg is pretty much that ... a cartoon. In reality, they're pretty much drifting and their movement is much more like brownian motion than anything else [1].
Reminds me of this sperm race thing that took the spotlight a month ago, after watching the videos [2] ... come on, man.
Only someone who is extremely ignorant and/or is lacking severely on their mental abilities (bordering on idiocy), would believe that thing was true.
1: When the sperm is really close to the egg, however, there seems to be a hormone gradient that guides the sperm, preferentially, towards it.
Not really, the reason so many sperm are needed is because a woman's reproductive tract requires an aggressive immunological response to foreign bodies (which sperm are). The vagina provides a direct route for pathogens through the cervix and uterus to the fallopian tubes (which can be scarred by inflammation resulting in infertility) and they themselves open up directly into the peritoneal cavity (potentially exposing a woman to septic shock or death if an infection reaches it). To protect against that, the vaginal environment is highly acidic, has layers of mucus that shields the cervix, and a high concentration of immune cells proliferate throughout. Men need to produce so many sperm because they need to be able to temporarily overwhelm these defenses.
What you say is right, but it does not contradict the parent poster.
Both reasons for the high count of sperm cells are true.
There must be many sperm cells to survive the adverse conditions, but there is also intense competition with the sibling sperm cells.
The DNA of the sperm cells is generated by a random generator, which is the meiosis mechanism, which randomly shuffles then randomly discards half of the father DNA.
The sperm competition then discards the random choices that happened to be bad, implementing an optimum search algorithm.
The sperm competition is only a first filter for rejecting bad random choices. Many embryos will die very soon, without ever developing, rejecting other bad random choices.
You are all wrong :)
There is a competition between sperm from different males.
as to the large number of sperm required, it is note worthy that excess male germ cell's is the norm for many?, most?, all?, species, and that in a lot of cases there is a number of complications involved in transfering them to a female, and altogether improbable solutions, also the basic mechanisms for germ cells are conserved by plants and animals, "pollen" bieng tailess sperm.....except in the case of ginko trees, that have motile sperm so the real questions are around why did evolution produce, and stick to this mechanism, and not so much about our species rather mundane take on it.
I'm broadly speculating here, but I tend to view most mechanisms like this as evolution's "desire" for a well tuned, but imperfect, CRC check.
Ignoring whether or not it would even be possible, a perfect CRC is antithetical to evolution itself, wiggle room for mutation must always be possible, but too much mutation gives you cancer and systemic malfunction. So you end up with these bizarre processes that allow just the right amount of imperfection.
With sperm specifically it ends up closer to a signature check than just a CRC, if the sperm doesn't exhibit behavior that falls under a certain umbrella of expected behaviors, it's rejected by the surrounding environment. The difficulty to comprehend it could even be a feature of the process in many respects, especially when you consider everything in this realm risks getting "hacked" if precautions are not put into place.
So when I see huge numbers like this, I see it as an indirect measure of the precision of the overall process. To put it another way, it's like brute forcing a password you don't know, but happen to have a lot of hints to (since obviously, we are all still the same species at the end of the day).
I’ve actually had this question myself, “is there evolutionary pressure for imperfect genetic copying and repair mechanisms, since perfect ones would halt evolution and leave a species unable to adapt?”
I’ve asked this question to multiple evolutionary biologists, and all of them answered “no” very strongly, strongly enough that I’m inclined to believe it. Apparently the frequency of deleterious mutations is many orders of magnitude greater than the frequency of beneficial ones, meaning there’s little chance perfect copying could be maladaptive. And in any event, evolution always selects for the fitness of the individual, not the species— group selection is a controversial topic in evobio, but the general consensus is that it does not happen, and that the rare things which kinda look like group selection (e.g. eusociality in bees) actually aren’t and can be explained without it.
I suspect there could be a misalignment in semantics here, and not necessarily a disagreement. When a biologist says "evolutionary pressure" perhaps they have a different way of modeling what that means to them?
To me evolutionary pressure isn't an on/off thing, it's like a signal in the noise. It's a vector with a direction and magnitude, facing varying levels of environmental resistance.
To be more specific, if there was enough "magnitude", evolution could potentially arrive at a perfect CRC. But the "resistance" requires a "magnitude" higher than evolution is willing to pay to reach that perfection. Likely in part due to the implicit complexity slope. Considering the systemic malfunction mutation can cause, one might assume this magnitude would be higher than it currently is. However, this is entirely speculation, and not falsifiable.
So when I think of evolutionary pressure I'm considering it as a component of the final vector, where a biologist might more pragmatically consider the total sum of vectors instead. This way of thinking is likely more productive for what they are doing.
As for evolution always selecting for the individual over the group, I'm surprised this is controversial when it's so obviously happening? If that was true how could multicellular organisms even exist? I'm very much not an expert on any of this, but this sounds like perhaps an over focus on DNA itself and not evolution as a whole, but maybe I'm misunderstanding something?
> This way of thinking is likely more productive for what they are doing.
Which is… attempting a rigorous understanding of evolutionary biology, rather than idly ruminating.
I hate to use a dork-ism like “update your priors”, but this is actually maybe a situation where it applies? If you’re serious about a subject it’s more interesting to really incorporate the likelihood that you’re wrong than it is to wave it away as semantics or point of view.
I find evolution interesting because I like algorithms, so I view it through that lens. There is absolutely nothing wrong with this perspective. Clarifying my mental model is by no means simple hand waving.
Tell me this, which is more productive in an open discussion? My idle rumination? Or the lazy dismissal that lacks any substantive contribution to the ongoing discussion? I can't help but agree with your distaste for "dorkisms".
Imperfect copying is only one reason for having to reject many sperm cells, and a less important one.
Meiosis, which produces both the sperm cells and the egg cells, is not a copying mechanism, unlike mitosis, which generates the other cells of a body.
Meiosis is a random generator, it randomly shuffles the DNA of the grandparents, which is stored in the father's cells, then it randomly selects half of it, producing a unique combination of genes in each sperm cell or egg cell.
The random genome generator together with the following filtering steps that will reject the bad variants, implement an optimum search algorithm for the fittest descendants.
Meiosis has greatly accelerated the evolution of the nucleated living beings (eukaryotes). Because favorable mutations are extremely rare, the probability of a living being accumulating multiple favorable mutations would have been negligible. With meiosis, if in a population 5 individuals have 1 favorable mutation each, there are good chances that soon some individuals will appear who have all 5 favorable mutations, then their descendants may become dominant and replace all others.
This is one of those issues where I think that the experts in the field saying that are just plain wrong. I know how that is a bad assumption to start out with; but I also know that obviously there is an amount of mutations that is beneficial, and organisms with a rate lower than that will be outcompeted. It could be no other way.
So is it just misunderstanding? Maybe they don't understand the question, or we don't understand the answer? Or they're hidebound, or incentivized somehow to be blind to the truth? Epistemological questions abound.
> And in any event, evolution always selects for the fitness of the individual, not the species— group selection is a very controversial topic in evobio, but the general consensus is that it does not happen
To me that seems more to me like a group of academics who just can’t see “how” it’d work. We’ve seen that in field after field where practitioners rarely look outside their own field.
There’s no “magic” in evolution to make it work only on individuals. Evolution is going to follow game theory. In some scenarios the evolution of the group will over time be far more adaptive than that of an individual. It’s a math question, not a biology question.
And it's not like you need 50 million to fertilize the egg. It just gives a better chance of out-competing the other guy's sperm!
Then: how many of these "male" individuals should we produce? 10%? Could probably get away with 5%... Nah, let's do 50%.
A better way to think about it is just how much surface area of uterus the ovum could actually be in.
It's not that you need millions of sperm or that millions of sperm are competing, its that those sorts of numbers are necessary just to make it probably at least 1 sperm even finds the egg while it's still got energy.
Sperm are actually quite good at finding the egg if they make it to the fallopian tube, they have their own chemoreceptors that can detect very small changes in chemical gradients, but only a few hundred make it through the preceding gauntlet.
Come on lads! Don't give up, we're only just past the tonsils!
There's this fascinating phenomenon called "micro chimerism"where somehow they've found that some of the cells in women's bodies are actually descendents of cells from past sexual partners, and they can be found in places far removed from the reproductive tract. the relation to your comment is that the tonsils and throat are known to be susceptible to this.
Fun rabbit hole, in since cases they think it's the result of cells from offspring winding up in the wrong side of the umbilical but there are also cases where there was never a pregnancy in which case it has to be wayward sperm but that's absolutely bizarre and far too orthogonal to the sperms primary objective.
And AFAIK they don't have any idea of why this is beneficial to the woman or even to the man who created the invasive cells.
>That is the big, hidden world. I think the female reproductive tract is the greatest unexplored frontier for sexual selection, theory and speciation
I too have encountered severe difficulties in exploring the female reproductive tract.
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