Sunday, February 28, 2010

moss and gender

So if you're like me, you've often thought "Why am I not more like moss?" The possibilities of being hydrostatic aside, why are our gametes so pathetic? Why don't they live independent lives?

Note to self. Write science fiction with characters who can be completely dehydrated and revived. Also, with gametes that pull their own weight.

The cheery answer is that sometimes human gametes do. Recall high school sex education, where they explained that sperm contain a chemical that causes girls to fall in love? That is a lie and I did not know that until just now! I swear I have other examples!

Right. When the sperm and the egg fuse, some proteins from the gametes are still present. These proteins have startling effects on placental and embryo development. Sperm proteins determine the degree of implantation of the placenta- its ability to obtain nutrients from the bloodstream. (This is one of the major problems with egg fusion. No egg is going to encourage aggressive nutrient use on the part of the embryo. ) Egg proteins determine head size.

Oh, but that's not really gamete function at all- we want to anthropomorphize our gametes whilst they are still alive. Fine, fine, here. It's an article on how men will someday become extinct!

Except they won't. At least, not until everyone else does.

You'll have to excuse my terrible lack of citations here- all my science books are in storage. While Google is great, it cannot search for "that thing about 3/4 of the way through the book, next to the mostly green illustration". As a quick refresher- all eggs have an X chromosome. The chromosome present in the sperm (X or Y) will determine the gender of the embryo (female or male). Thus, most males have XY, most females have XX, and intersex persons have a dizzying array of possible chromosomal and hormonal variations. When fertilization occurs, there's a swap meet among the chromosomes- known as crossing over. This has less impact on the person who will get these genes than previously thought- you use all your DNA- but it makes a big difference to that person's offspring, because they get only the one copy. Oh genes, always thinking ahead.

The article above states that as Y chromosomes cannot recombine, single detrimental alleles are not exposed to selective pressure and eliminated. As such, most of the Y is nasty junk DNA, with only a cheery SRY (order: make testicles!) gene cluster maintaining some level of professionalism. (But there's evidence that they can recombine with themselves. Cool.) Since the DNA on Y chromosomes is crap, they are steadily shrinking. Soon they will vanish!

Well, okay. They'll vanish as soon as the SRY gene cluster moves to another chromosome. And all of the Y chromosome data becomes useless. Then males as we know them will vanish, as the genes that make them male will be on a different chromosome. (Italics for sarcasm.)

But we've strayed from our discussion about gamete adventures.

All living DNA is mutating at a steady rate. Since there are three times as many X chromosomes as there are Y chromosomes, X chromosomes have more time to experiment. (This is one of my favorite evolution facts. Larger populations mean a higher chance of an interesting mutation. Higher survival rates mean a smaller likelihood that the awesome will be lost due to random chance. This is why I am a liberal: because social supports might foster cool mutations.) Every so often, an X chromosome will decide that it can't trust a chromosome it can't recombine with- so all eggs containing this X chromosome will reject all sperm with Y chromosomes. Think of those amazon eggs, rejecting the possibility of becoming male! The excitement!

That's my gamete adventures Hail Mary pass. Sometimes eggs reject sperm with Y chromosomes. One of these shows up about once every... million years.

But let's think about the implications! The mutation can arise in a male, but after the first male passes it along to his daughters, there will be no males with that gene. (Well, until the magical circumstances that I'll talk about later.) Without males passing along this X chromosome, there can't be females who are homozygous for the trait. Thus, even if the gene gives you superhuman speed and the ability to eat rocks, it can't be present in more than 50% of all eggs. You'd end up with a male to female ratio of about 1:3. And what an advantage to the X chromosome! A 50% increase in available carriers! After a few generations as a zero sum gene, becoming 50% of all the mammals in a population!

But maybe a plucky sperm (with a Y) gets hit by a magic bolt of cosmic radiation. It's enough to knock off the DNA that marks it as a possessor of a Y chromosome, so it manages to fertilize an amazon egg. The resulting dude is a biological jackpot. Since his sons can carry the amazon gene, their daughters can be homozygous for that trait. Their sons will have the magic Y chromosome by default. Sperm from any other source is quietly selected against, because an ever growing portion of the population will be unable to bear sons with the old Y. Soon, everyone is a descendant of this dude. He wasn't even that awesome. (Everyone is also a descendant of the first person with the amazon X)

Man, that's another great science fiction short story. Oh, and platypus gender is freaky.

So this process has happened several times in our past. Every time, we've shaved a little bit more off the Y chromosome. Right now it is tiny. It's basically the SRY gene complex (testicles!) the SRY gene complex upside down, in case recombining is gonna happen, and about 50 million base pairs of mad ramblings. (The X has 150 million base pairs that code for things like blood clotting and color vision.) In the future, there will probably be further shaving of those mad ramblings. If the next amazon X recognizes the SRY complex as the marker of Y-ness, then we'll end up with that old 3:1 gender ratio. There can be no magic supersperm to remedy that, because the resulting person will have no sperm.

Unless the SRY complex is swapped to a different gene. See above sarcasm.

My, this is a long post.

What advantage do the other chromosomes gain by letting the X chromosome be picky about what sperm to accept? Well, at one time there were lots of human like things wandering around so it paid to be choosy about which sperm to accept. Mammalian hybrids don't survive very well- another way that moss is cooler than us- and since female mammals bear almost the entire cost of producing babies, X chromosomes in eggs were under significant pressure to reject dicey sperm.

One last thing. Over the last five million years, we've shaved 100 base pairs off the Y chromosome. The thousands of genes that once separated males and females have been reduced to less than ten. Human males are basically ladies with testicles.

C and I debate about this all the time. That's why I make him look at orangutans when we are at the zoo.


2 comments:

Drewscriver said...

I can't say I've ever thought "Why am I not more like moss?", but you've definitely raised the issue to my attention. I also love the "someday I'll regret posting this" tag.

Janeric said...

Oh, I was thinking about the look one gets from parents now and again, "Where the heck did that come from?" Thought I'd play up the intergenerational barrier. I do love the gametes that wander off and then you have a baby... I think I've read teen novels which approach it pretty much the same way.

Really? I wonder why I'm not as awesome as moss every day. Should my next science post be on polyploidy (plants are awesome) or poorly supported rantings about reproductive strategies?