From the 2001 Holiday Lectures — The Meaning of Sex: Genes and Gender | ||
Evolution of the Y Chromosome How did the human Y chromosome become so small relative to its X counterpart? This animation depicts the 300-million-year odyssey of the sex chromosomes that began when the proto X and Y were an identical pair.5 minutes 38 seconds Play Large: MOV / WMV (11 MB) Play Small: MOV / WMV (6 MB) To download the videos, in Internet Explorer right-click the link and select "Save Target As..." In Firefox right-click and select "Save Link As..." In Safari right-click and select "Download Linked File As..." More About Evolution of The Y Chromosome Background The human X and Y chromosomes are a unique pair. The other chromosome pairs, called the autosomes, appear to be identical twins; they are superficially indistinguishable. In contrast, the X and Y chromosomes appear to be vastly different from one another. Why are the sex chromosomes so different? How did they get that way? The Y chromosome is only one-third the size of the X. Although the Y has a partner in X, only the tips of these chromosomes are able to recombine. Thus, most of the Y chromosome is inherited from father to son in a pattern resembling asexual, not sexual, reproduction. No recombination means no reassortment, so deleterious mutations have no opportunity to be independently selected against. The Y chromosome therefore tends to accumulate changes and deletions faster than the X. Degradation doesn't occur in X chromosomes because during female meiosis, the X has the other X as a full partner in recombination. Clues of how the Y chromosome evolved can be found by comparing the genes and the sequences of X and Y chromosomes as well as homologous genes of different species. One method scientists use to estimate evolutionary time is observing how homologous genes have become different over time in different species. All DNA sequences accumulate random mutations over time, so species that are distant relatives should have more different sequences than close relatives because they have been evolving separately for a longer time. Once recombination stopped between portions of X and Y, genes located on those parts started to evolve separately as homologs. Apparently, this happened in stages, so some X-Y gene pairs are more related than others, meaning they stopped recombining more recently. Also, chunks of genes stopped recombining, and by mapping their positions on the chromosome, one can guess that an event, like an inversion, may have taken place. Travel back in time, when human ancestors were reptile-like forms, and peer into the processes that shaped the X and Y chromosomes. Introduction How did the human Y chromosome become so small relative to its X counterpart? This animation depicts the 300-million-year odyssey of the sex chromosomes that began when the proto X and Y were an identical pair. Over time, structural changes in the Y chromosome resulted in its current form, which is specialized to trigger male development. The evolutionary timescale is represented by positioning the chromosomal remodeling events along an abbreviated vertebrate cladogram, a chart of evolutionary relationships. (Ma = million years ago) Part 1. Sex chromosomes originated as autosomes The sex chromosomes began as an ordinary pair of autosomes. During meiosis. chromosomes replicate their DNA, pair, and exchange genes (recombination; red lines). A mutation in the SOX3 gene produced the SRY gene, a critical determinant of maleness, on the proto Y. While the functions of SRY and SOX3 became very different over time, another gene, RPS4, retained a similar function on both the X and Y chromosomes. Part 2. Inversions restrict recombination between the X and the Y chromosome Inversions, which are internal recombination events, caused a rearrangement of genes on the Y chromosome. These rearrangements meant that large portions of the X and Y chromosome no longer recombined, which made the Y chromosome susceptible to deletions, and it decreased in size. Part 3. Comparison of sex-chromosome recombination in males and females After our lineage diverged from the ancestors of the monotremes, such as the duck-billed platypus, another inversion further scrambled the genes on the proto Y. In males, only the tips of the Y chromosome were left able to recombine with homologous genes on the X chromosome. In contrast, in females, recombination continued to occur across the full length of the two identical X chromosomes. Part 4. Autosomal expansion of X and Y chromosomes About 130 million years ago (Ma), an autosome donated a block of genes that extended the length of both the X and the Y chromosome. The X and Y were able to recombine in these expanded regions of the chromosomes. Subsequently, inversions rearranged the order of genes on the Y chromosome. Additional rearrangements occured almost exclusively on the Y. Without recombination to preserve its integrity, the Y continued to lose genes and, over time, shrank. Part 5. An autosome contributed a copy of the DAZ spermatogenesis gene to the Y chromosome Sometime after squirrel monkeys diverged from the primates that evolved into humans, an autosome contributed a copy of the DAZ spermatogenesis gene to the Y chromosome. The DAZ gene was copied and copied again and now the modern Y chromosome contains four identical DAZ gene sequences. The modern Y chromosome is about one-third the size of its X-chromosome partner. Learn More: Autosomes An autosome is any chromosome that is not a sex chromosome. In ancient reptilian creatures, there was no chromosomal basis for sex determination. Scientists speculate that sex was determined by environmental factors such as temperature. Some modern reptiles, including turtles and crocodiles, still use this mode of sex determination. Learn More: DAZ Many genes essential for the production of sperm are located exclusively on the Y chromosome. One of these genes, DAZ (deleted in azoospermia), was copied from an autosome and was copied twice on the Y chromosome, resulting in four copies of the DAZ gene. The Y chromosome is unique because not only are male spermatogenesis genes sequestered on the Y, but they exist as mutiple copies. In fact, the abundance of multiple copies and mirror images of sequences have led researchers to call the Y chromosome a "hall of mirrors." Although this sequence repetition created great challenges in the sequencing of the Y chromosome, the complex structure also serves an important purpose. Multiple copies of essential spermatogenesis genes ensure that in spite of deletion events, which may result in the loss of a single copy of an essential gene, spermatogenesis can still proceed via proteins produced by remaining copies. Learn More: Deletions Deletions are uncommon, but relative to inversions, they are not rare events. (Recombination, however, is a common event). Deletions occur particularly in regions of the Y chromosome that do not undergo recombination. The chromosome is mutated, causing a section of DNA to be excised, and the two flanking ends of DNA join to form a continuous strand. Learn More: Expansion About 130 Ma, an autosome donated a block of genes that extended the length of both proto X and Y (expansion). The proto X and Y were able to recombine in these expanded regions of the chromosomes. Subsequently, inversions further rearranged the order of genes. Without recombination that preserved the integrity of chromosomes, the proto Y lost genes and, over time, shrank in size. Learn More: Inversions On an evolutionary timescale, large inversions, such as those shown in the animation, are actually very rare events. To occur, the DNA at one end of the chromosome recombines with DNA at the other end of the chromosome, forming a loop. Instead of the loop being eliminated from the chromosome (as happens in other types of recombination events), the loop twists. Therefore, the same DNA sequences are retained in the chromosome, but their orientation is reversed. Scientists speculate that large inversions resulted in the relocation of SRY to the "top" of the Y chromosome relative to its former partner SOX3, which remains near the "bottom" of the X chromosome. Learn More: Meiosis To produce sperm or eggs, germ cells undergo the process called meiosis. Chromosomes replicate and pair up, resulting in a 4n quantity of chromosomes. Two nuclear divisions follow, so that the gametes have a haploid (1n) number of chromosomes. Learn More: Mutation Mutations are alterations in the DNA sequence that occur randomly and can have little or great consequences, depending on the location of the mutation. When a mutation altered SOX3, the testes-determining gene SRY resulted. This mutation had great consequences: The evolution of unique sex chromosomes began. Learn More: Recombination During meiosis, chromosomes with substantial DNA sequence homology will pair and exchange pieces of DNA, a process called genetic recombination. This process provides not only a source of genetic variability but also a way in which deleterious mutations are eliminated by not being passed on to future generations. (Recombination allows for reassortment and thus a way for selecting out deleterious mutations without needing to eliminate the entire set of alleles on which the mutation arose.) In this way, recombination preserves the integrity of the chromosomes. The X and Y chromosomes are notably different with respect to recombination. During female meiosis, the two X chromosomes undergo recombination throughout their entire length (illustrated by red lines). In contrast, during male meiosis, the Y chromosome recombines with the X chromosome only at its tips. Thus, over time, deleterious mutations accumulate in the nonrecombining regions of the Y chromosome. Learn More: RPS4 RPS4 (ribosomal protein small subunit, protein 4) is a gene essential for ribosome formation. Identical copies of RPS4 are found on both the X and the Y chromosome, and the same function has been retained over timeósince before the creation of SRY and through millions of years of divergence between the X and Y. However, the location of RPS4 has changed as a result of a large inversion. Learn More: SRY The SRY gene regulates the formation of the testes from the undifferentiated embryonic gonad. The location of SRY and SOX3 on the X and Y chromosomes of several species has been compared, an analysis that has allowed scientists to roughly date when changes occurred. Gene pairs, such as SRY and SOX3, allow researchers to map changes and rearrangements on chromosomes. Specifically, 300 million years ago (Ma), scientists speculate that SOX3 was in the same location on the proto X and Y chromosomes. A mutation in SOX3 created the gene SRY on the Y chromosome. Researchers found that monotremes are the most ancient mammals that have the SRY gene, whereas all earlier ancestors do not. These data allowed researchers to estimate when the mutation that created SRY occurred. Learn more about SRY at: "Gender Testing of Female Athletes" http://www.hhmi.org/biointeractive/gendertest/gendertest.html (copied without permission from http://www.hhmi.org/biointeractive/gender/Y_evolution.html | ||
Thursday, January 14, 2010
The Meaning of Sex: Genes and Gender
Wednesday, January 13, 2010
Male circumcision is a weapon in the sperm wars
"
Male circumcision is a weapon in the sperm wars
* 18:00 05 June 2008 by Kurt Kleiner
Circumcision and other forms of male genital mutilation have always been a puzzle. The ritual mutilations can leave the man vulnerable to infection and even death. So why do some societies insist on such a risky ritual for their men?
There may be an evolutionary explanation, according to Christopher Wilson, of Cornell University in New York, US. It could function to reduce a young man's potential to father a child with an older man's wife, he says.
Sperm competition theory predicts that males will evolve ways to ensure that their sperm, and not another male's, fertilises a female's eggs. Genital mutilation, in this view, is just another way to win the sperm war.
In some forms of mutilation, the handicap to sperm competition is obvious. There is subincision, for example, where cuts are made to the base of the penis. This causes sperm to be ejaculated from the base rather than the end, and is performed in several Aboriginal Australian societies, says Wilson.
In some African and Micronesian cultures, young men have one of their testicles crushed.
Male genital mutilation makes it less likely that a male will manage to father a child with another man's wife, Wilson says.
Home advantage
Circumcision is one of the less painful forms of mutilation, but it is also less effective at reducing sperm competition. Wilson suggests, however, that the lack of a foreskin could make insertion or ejaculation slower, meaning brief, illicit sex is less likely to come to fruition and lead to a pregnancy.
Younger men, he says, willingly submit to having their reproductive ability reduced because they benefit socially from the older men, by forming alliances, and by gaining access to weapons or tribal lore.
The older men have also gone through the ritual, and seen their own reproductive effectiveness reduced. But if a man with, say, four wives wants to ensure that any children his wives produce are his, there is pressure to make sure other men can't successfully impregnate them.
The husband's own reproductive ability is impaired, but continuous and repeated access to his wives makes up for it, while any genital mutilation is a greater handicap to an interloper trying to sneak brief occasional sex with his wives.
Price of alliance
"An older married man must form alliances, or associate with younger or unmarried men at some point, and it would be better to associate with and invest preferentially in those who are least likely to threaten his paternity, especially in societies where cuckoldry is rife," says Wilson.
"Men who demand genital mutilations as part of the price for alliance and investment would be less vulnerable to exploitation of such relationships and loss of paternity to peers."
Wilson has now tested the idea. If the sperm competition theory is correct, he reasoned, then male genital mutilation should be more common in societies where men tend to have multiple wives, especially those in which the wives live apart from the husband.
The mutilation would also probably be carried out in a public setting, witnessed mostly by other men, and performed by a non-relative. Men who refused would face social sanctions.
Who's the daddy?
Wilson searched anthropological databases and found that his predictions were borne out: 48% of highly polygynous societies practice some form of male genital mutilation, and in societies in which wives live in separate households that increases to 63%.
Only 14% of the monogamous societies in the database practice male genital mutilation.
It might also be the case that selection works at a group level, so that societies that enforce mutilation are more stable because of less conflict over paternity, Wilson says.
David Barash, an evolutionary biologist at the University of Washington in Seattle, US, says that the paper makes a convincing case.
"Wilson has tackled a perplexing question and come up with a persuasive preliminary answer to an evolutionary enigma: why do men submit to procedures that seem to reduce their fitness?" he says.
Journal reference: Evolution and Human Behavior (vol 29 p 149)"
Male circumcision is a weapon in the sperm wars
* 18:00 05 June 2008 by Kurt Kleiner
Circumcision and other forms of male genital mutilation have always been a puzzle. The ritual mutilations can leave the man vulnerable to infection and even death. So why do some societies insist on such a risky ritual for their men?
There may be an evolutionary explanation, according to Christopher Wilson, of Cornell University in New York, US. It could function to reduce a young man's potential to father a child with an older man's wife, he says.
Sperm competition theory predicts that males will evolve ways to ensure that their sperm, and not another male's, fertilises a female's eggs. Genital mutilation, in this view, is just another way to win the sperm war.
In some forms of mutilation, the handicap to sperm competition is obvious. There is subincision, for example, where cuts are made to the base of the penis. This causes sperm to be ejaculated from the base rather than the end, and is performed in several Aboriginal Australian societies, says Wilson.
In some African and Micronesian cultures, young men have one of their testicles crushed.
Male genital mutilation makes it less likely that a male will manage to father a child with another man's wife, Wilson says.
Home advantage
Circumcision is one of the less painful forms of mutilation, but it is also less effective at reducing sperm competition. Wilson suggests, however, that the lack of a foreskin could make insertion or ejaculation slower, meaning brief, illicit sex is less likely to come to fruition and lead to a pregnancy.
Younger men, he says, willingly submit to having their reproductive ability reduced because they benefit socially from the older men, by forming alliances, and by gaining access to weapons or tribal lore.
The older men have also gone through the ritual, and seen their own reproductive effectiveness reduced. But if a man with, say, four wives wants to ensure that any children his wives produce are his, there is pressure to make sure other men can't successfully impregnate them.
The husband's own reproductive ability is impaired, but continuous and repeated access to his wives makes up for it, while any genital mutilation is a greater handicap to an interloper trying to sneak brief occasional sex with his wives.
Price of alliance
"An older married man must form alliances, or associate with younger or unmarried men at some point, and it would be better to associate with and invest preferentially in those who are least likely to threaten his paternity, especially in societies where cuckoldry is rife," says Wilson.
"Men who demand genital mutilations as part of the price for alliance and investment would be less vulnerable to exploitation of such relationships and loss of paternity to peers."
Wilson has now tested the idea. If the sperm competition theory is correct, he reasoned, then male genital mutilation should be more common in societies where men tend to have multiple wives, especially those in which the wives live apart from the husband.
The mutilation would also probably be carried out in a public setting, witnessed mostly by other men, and performed by a non-relative. Men who refused would face social sanctions.
Who's the daddy?
Wilson searched anthropological databases and found that his predictions were borne out: 48% of highly polygynous societies practice some form of male genital mutilation, and in societies in which wives live in separate households that increases to 63%.
Only 14% of the monogamous societies in the database practice male genital mutilation.
It might also be the case that selection works at a group level, so that societies that enforce mutilation are more stable because of less conflict over paternity, Wilson says.
David Barash, an evolutionary biologist at the University of Washington in Seattle, US, says that the paper makes a convincing case.
"Wilson has tackled a perplexing question and come up with a persuasive preliminary answer to an evolutionary enigma: why do men submit to procedures that seem to reduce their fitness?" he says.
Journal reference: Evolution and Human Behavior (vol 29 p 149)"
Tuesday, January 12, 2010
Mysteries of the Human Genome
Gill Bejerano holds a BSc, summa cum laude, in Mathematics, Physics, and Computer Science, and a PhD in Computer Science from the Hebrew University of Jerusalem, Israel. Twice recipient of the RECOMB best paper by a young scientist award, and a former Eshkol pre-doctoral Scholar and HHMI postdoc. As co-discoverer of ultraconserved elements, his research focuses on deciphering the function and evolution of the non-coding regions of the Human Genome. Gill is currently a postdoc with David Haussler at UC Santa Cruz, and in early 2007 he will join Stanford university as an Assistant Professor in the Department of Developmental Biology and the Department of Computer...
Sunday, January 10, 2010
Project For Awesome - Sleeping Children Around The World
"Sophia asked for a bedkit for Christmas in addition to a toy from Santa. I am beaming with pride :) I have decided to upload this video again without the Ready For Bed Week information so that it can stand alone as a video for the charity. (And for the vlogbrothers Project 4 Awesome :) I know you've seen it already but please rate and comment. I will be making an new version today, if Sophia is willing after she comes home from school. Or else I'll upload it later :) Thank you for spreading the word on this wonderful charity!! (Hm. Making a new one for today would be redundant. Maybe I'll hold off on that )
For more information please visit: http://scaw.org or their Youtube channel http://youtube.com/scawweb
A Canadian Charity providing bedkits to kids in underdeveloped countries around the world who sleep on the floor. A bedkit cost $35 and 100% of the money goes to what is inside the bedkit for the child. There are no administration fees and is completely run by VOLUNTEERS!! That is what makes this charity so wonderful!!
Music by http://youtube.com/sequoya
I'm on twitter http://twitter.com/mugglesam
P.S Today from 3pm to 4pm Nerdfighters spammed this video to get it on Youtube's most discussed list. Go to http://ProjectForAwesome.com for more info. So strange to see 31,000 comments and only 500 views. :)
:)"
For more information please visit: http://scaw.org or their Youtube channel http://youtube.com/scawweb
A Canadian Charity providing bedkits to kids in underdeveloped countries around the world who sleep on the floor. A bedkit cost $35 and 100% of the money goes to what is inside the bedkit for the child. There are no administration fees and is completely run by VOLUNTEERS!! That is what makes this charity so wonderful!!
Music by http://youtube.com/sequoya
I'm on twitter http://twitter.com/mugglesam
P.S Today from 3pm to 4pm Nerdfighters spammed this video to get it on Youtube's most discussed list. Go to http://ProjectForAwesome.com for more info. So strange to see 31,000 comments and only 500 views. :)
:)"
Wednesday, January 6, 2010
BBC: Visions of the Future: The Intelligence Revolution. Hosted by Michio Kaku
Visions of the Future: The Intelligence Revolution. 1st part of 3 part miniseries on the BBC hosted by Michio Kaku. In this new three-part series, leading theoretical physicist and futurist Dr Michio Kaku explores the cutting edge science of today, tomorrow, and beyond. He argues that humankind is at a turning point in history. In this century, we are going to make the historic transition from the 'Age of Discovery' to the 'Age of Mastery', a period in which we will move from being passive observers of nature to its active choreographers. This will give us not only unparalleled possibilities but also great responsibilities.
Sunday, January 3, 2010
Modular robot reassembles when kicked apart
A robot developed by roboticists at the University of Pennsylvania is made of modules that can recognise each other.
Robots with a mind of their own!
Scientists are now building a new kind of robot capable of self-assembly and doing tasks too difficult or too dangerous for human beings.
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