The End of Man: Myth or Delightful Truth?

Jessica Krueger's picture

Given that I am entering the culmination of my Bryn Mawr education as a second semester senior, complete with the juggling of thesis and classwork which such an academic role entails, it should come as no surprise that my thoughts are never far from the end of times. Indulging in my fatalism is rapidly becoming my favorite form of procrastination, and whenever I have successfully dodged a volley of questions about my uncertain, post-graduation, future, I find myself visiting my favorite apocalyptic website, Exit Mundi, to give me hope that maybe the world will end tomorrow and I won’t have to worry about assignments, job searches or graduate school. “Giggle!” is by far my favorite end scenario: a world run by women due largely to the fragile nature of that which makes the stronger sex -- the Y-chromosome. Exit Mundi explains that the X-chromosome “hates” the Y and has consequently unleashed an uncompromising attack upon him, reducing him to a mere nubbin of genetic material.(6) But how can coils of genes resent, much less damage, each other? What is going on here? Is the Y-chromosome really under attack, and will this battle result in the eradication of the human male?

It turns out that human sex chromosomes have been dueling for the past few millennia, sort of, and Y has born the brunt of the damage. The Y-chromosome is a mere shadow of his formal self: he only shares 5% of his genetic material with his nearest living relative, the X-chromosome. Up to two-thirds of his overall sequence is considered to be “junk,” deactivated DNA and inert mutations which serve no real purpose but which may be culpable for Y’s tendency to break or get lost during meiosis. (12) One potential source for these compounded errors could be the genetic sweatshop of the human testes where genes find themselves repeatedly unzipped and copied to produce the 150 million sperm men produce daily. The sheer volume of replication offers plenty of opportunity for error (9), but this explanation is unsatisfactory. Most autosomes reduce the damage potential of random mutation through recombination: swapping genetic information with their twin chromosome to produce new combinations and get rid of bad copies during meiosis. (12) Lonely Y’s diminutive size means he has no buddy to trade genes with and no means to correct any errors in his sequence. Why would natural selection allow such a deleterious state of affairs develop around the Y-chromosome? The answer lies in conflict.

Evolutionary conflict occurs when one gene is incredibly successful at propagating itself but at the expense of another individual’s fitness. Sexual reproduction promotes evolutionary conflict because two players are trying to produce the most offspring with different stakes on the table: males tend to be interested in sheer numbers where females must consider quality before investing in reproduction. Evolutionary conflict can establish this sex-behavior dimorphism by developing novel methods of sex determination. Consider a proto-y-chromosome: he looked like any other autosome on the block, save for a new dominant sex-determining region (SRY) which increased his ability to reproduce by making him more masculine. Over generations this region began to collect other modifications to build up his reproductive “hand,” and because his aim is in direct competition with female breeding styles, anti-female mutations also began to accrue along the SRY. (8) As the proto-y-chromosome assembled his formidable genetic hand, females compromised by the anti-female mutations slowly began to die out.

The XXs who remained preserved their reproductive “pot” by developing coping strategies, the easiest being elimination by recombination. Consequently, selection pushed for Y-chromosomes capable of avoiding recombination and hoarding his genetic material to himself. By withdrawing from the game, no female could take his cards away, but he also cannot get new cards from her when something goes awry. (8) The best evidence for this hypothesis of chromosomal evolution come from a study on papayas; researchers claim that male and hermaphroditic papayas have a sex-limited gene which resembles human SRYs from 240-320 million years ago. While male and female papaya chromosomes still bear a strong resemblance to each other, junk DNA appears to be building up as recombination suppression around the genes in question increases, just like the conflict theory predicted. (4) So now we have a Y-chromosome who won’t share his genetic material for fear of losing it and is suffering the consequences, which doesn’t seem like much of an attack so much as a defensive posture. Has the X-chromosome ever given Y cause for such fear?

Yes. One method of overcoming the anti-female mutations on the Y-chromosome is to eliminate it outright, which is exactly what female fruit flies have done. Certain X-chromosomes in fruit flies carry a gene called “Dox” which allows males to produce normal X-laden sperm but damages spermatozoa carrying Ys such that the sex ratio can be driven up to nine females to one male. (1) Chromosomes can also strike each other below the belt by means of a process called “genetic imprinting.” Raising offspring requires serious female investment, and her fitness goals would be best suited by spreading her resources out evenly across all her children. Males, however, rarely trouble themselves with rearing, so their genes are free to demand that the offspring extract as much nutrition and energy from the mother as possible. Thus, genes from both parents, turned on or “imprinted” in accord with their respective genders, are in competition from the moment of conception, and this battle of wills is resolved when the genes turn each other off. (7) The SRY region in humans is what makes men “men.” (9) If X could somehow figure out how to turn various components of the SRY region off, or even shut it down in its entirety, human men would cease to be. It would seem that Y would have good reason to be afraid.

Yet there are some problems with these accounts. Imprinting, for instance, may help turn off genes with conflicting goals, but the more practical, and widely accepted, explanation is that it serves to turn off one of the two X-chromosomes female mammals get, preventing them from producing twice the protein necessary for life. (5) What really makes the case for plucky Y’s robustness came after his genetic sequence was finally decoded in 2003. The Y-chromosome cannot trade genes with anyone else, so he evolved a way to copy them from himself. Most of the genetic information on the Y chromosome is encoded in palindrome form: the individual components read the same forward as they do backward. (11) This means if Y gets a typo somewhere in his man-making blueprints, all he needs to do is bend over and retrieve the backwards information off another arm (3).

There is a small portion of the Y-chromosome which is not replicated upon itself in this clever way, and as such it is subject to the whims and deletions of natural selection. Believers in the stoutness of the Y have looked at the Y-chromosome of one of our nearest cousins, the chimp, to see if we have lost a significant amount of genetic information. If the chimps had genes which we did not, it would portend eventual sadness for Y. Instead, chimps have lost five of their genes (the researchers believe they were unimportant), where human male chromosomes have been holding steady for at least 6 million years. It seems natural selection is fine with genetic “junk,” so long as it doesn’t get in the way. (3) If, however, any mutations were seriously deleterious it is highly unlikely that they would make their way into the gene pool; any negative effects on the SRY would have profound ramifications for the carriers’ fertility. Much as females found means for overcoming anti-female mutations, males whose chromosomes develop clever strategies to improve their fertility would soon outnumber those whose Ys failed to keep up. (2)

So where does that leave our apocalyptic scenario? While I must grant that the Y chromosome has outfoxed humanity until fairly recently, I have my doubts about his future. Namely, most of the theory discussed assume that natural laws of selection still operate on humans. They don’t. As technology races forward in sophistication, our ability to compensate for disease and disorder supports an ever increasingly diverse, and arguably sick, population base. Individuals who would have died fifty years ago now thrive and furthermore wish to have children. Defects like poor vision, mental illness, and mobility disorders no longer preclude participation in the human gene pool. Lest this begin to sound like a social darwinistic diatribe, I don’t necessarily think this is a bad thing. It is something, however, I feel we should be aware of. Advances in our understanding of our own fertility are allowing people who wouldn’t normally be able to reproduce to do so. While definitely a remote possibility, it is a feasibility that in taking conception into our own petri-dishes we may come to support only those Ys which were not powerful enough to fertilize on their own. We could wind up like American Bulldogs, unable to procreate without mechanical intervention. So to say that there is no risk to the Y-chromosome, and consequently to man himself, by appealing to laws which may not apply in an ever increasing amount of cases, particularly when one of our closest relatives is losing genetic ground, is probably a little short sighted. However, even the most catastrophic estimates give us at least 125,000 years to find a solution (9), and given that the hard sciences are still a largely male dominated field I have faith that the issue will be suitably addressed.


Works Cited
1) Anitel, S. (2007, November 7). Sex War: 90% or 50% Females?. Softpedia. Retrieved April 4, 2008, from
http://news.softpedia.com/news/Sex-War-90-Females-or-50-Females-70212.shtml
2) Burbridege, D. (2003, October 7). Adam’s Curse. GeneExpression. Retrieved April 4, 2008, from
http://www.gnxp.com/MT2/archives/001114.html
3) Cameron, D. (2005, August 31). Human Y chromosome stays intact while chimp Y loses genes. Whitehead Institute for Biomedical Research. Retrieved April 4, 2008, from
http://www.wi.mit.edu/news/archives/2005/dp_0831.html
4) Chen, H. (2005, August). The Origin of the Y Chromosome: How the Papaya is Providing Some Interesting Answers to Important Questions. The Science Creative Quarterly. Retrieved April 4, 2008, from
http://www.scq.ubc.ca/the-origin-of-the-y-chromosome-how-the-papaya-is-providing-some-interesting-answers-to-important-questions/
5)DKC., (2000). Another Genetic Battle of the Sexes: Imprinting. An Unfinished Story About the Genesis of Maleness. Retrieved April 4, 2008, from
http://www.hhmi.org/bulletin/bookofy/6.html
6) (n.d.).“Giggle!”. Exit Mundi: a collection of end of world scenarios. Retrieved April 4, 2008, from
http://www.exitmundi.nl/giggle.htm
7) Jirtle, R.J. (2006). What is Genomic Imprinting?. Geneimprint. Retrieved April 4, 2008, from
http://www.geneimprint.com/site/what-is-imprinting
8) Partridge, L. & Hurst, L.D. (1998). Sex and Conflict. Science, 281(5385). Retrieved April 4, 2008, from
http://www.sciencemag.org/cgi/content/full/281/5385/2003
9) Sykes, B. (2003, August 23). Do we need men?. The Guardian. Retrieved April 4, 2008, from
http://www.guardian.co.uk/science/2003/aug/28/genetics.genderissues
10) University of Pennsylvania (2008, March 19). First Sex Chromosome Gene Involved In Meiosis And Male Infertility Identified. ScienceDaily. Retrieved April 4, 2008, from
http://www.sciencedaily.com/releases/2008/03/080314164119.htm (not explicitely cited)
11) Willard, H.F. (2003). Genome Biology: Tales of the Y Chromosome. Nature, 423. Retrieved April 4, 2008, from
http://www.nature.com/nature/journal/v423/n6942/full/423810a.html
12) Z, H.(2007, July 7). Will Men Become Extinct?. AssociatedContent. Retrieved April 4, 2008, from
http://www.associatedcontent.com/article/318550/will_men_become_extinct.html

Comments

Jessica Krueger, B.A.'s picture

I miss you Professor Grobstein

Gene Survival and Death on the Human Y Chromosome
http://mbe.oxfordjournals.org/content/30/4/781.full

Ann Dixon's picture

Hi Jessica, Thanks for adding

Hi Jessica,

Thanks for adding the new citation. I miss him too.

Ann

Paul Grobstein's picture

The Y-chromosome: its struggles and fate

The evolution and future of the human Y-chromosome is an interesting story in its own right. And telling it in this anthropomorphic mode raises some additionally interesting questions. If the Y-chromosome and the X-chromosome, very small and simple entities in comparison to the human organisms of which they are a part, really are engaged in a meaningful "conflict" what does that imply for understanding "conflict" between human beings? Is it the same thing? Something entirely different? Or something that has elements of what goes on between chromosomes and some additional features? Can human behavior be understood in the same terms as chromosome behavior?

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