Download - Y Chromosome Evolution BB
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EVOLUTION OF THE Y CHROMOSOME
PROVIDES SOME
INTERESTING INSIGHTS
INTO MAMMALIAN AND
EVEN HUMAN
EVOLUTIONARY
PATTERNS. WE AREGOING TO DISCUSS
WHAT IS KNOWN AND
WHAT CONCLUSIONS
ARE SUGGESTED BY
THE EVIDENCE.
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What is the Y Chromosome?
23rd pair of chromosomes One of two sex chromosomes in mammals Absence/presence determines whether an
individual is male or female
Determined by the SRY gene Fetus develops into a male Formation of testes
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More About the Chromosome Contains about 50-60 genes providing
instructions for making proteins
Composed of about 59 million base pairs 2% of total DNA of a human man Genes passed down from father to son are
called holandric traits, only found on Y
chromosome
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Y Linked Traits
Genes located only on the Y chromosome Affected men will never have an affected
daughter
100% affected sons
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Lead Research Scientists First researchers to discover the Y chromosome and its function were
Nettie Stevens and Edmund Beecher Wilson
In 1905, Nettie Stevens, unlike other researchers, discovered that the Y chromosome determined gender.
The same year, Edmund Beecher Wilson concluded the same ideas as Nettie Stevens.
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Lead Research Scientists Nettie Stevens
Her work on sex determination was first published in 1905
While investigating mealworms, she discovered that females contained 20 chromosomes and males contained 19 chromosomes and an extra small
chromosome
Later found that males sex cells contained X and Y chromosomes, and females only made X chromosomes
Also concluded that sex is a chromosomal factor and males determine the gender
http://www.nature.com/scitable/topicpage/nettie-stevens-a-discoverer-of-sex-chromosomes-6580266
http://www.geneticstv.org/scientists/stevens.htm
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Lead Research Scientists Edmund Beecher Wilson
Advanced the theory that gender is inherited by the Y chromosomes produced only by males
In 1905 his Studies of Chromosomes paper was published
This paper was also based on insect chromosomes
http://www.geneticstv.org/scientists/wilson.htm
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Recent Y chromosome discoveries One Y chromosome lineage found in South Carolina expanded the Y
chromosome tree
Consumer genetic testing company identified that the lineage didnt match any existing Y chromosome trees on file
Research concludes that pockets of genetically isolated communities in the world such as this one did not match those previously discovered
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Recent Y chromosome discoveries
(cont.)
The DNA sample matched the Y chromosome DNA of 11 men all from a small region of western Cameroon in sub Saharan Africa.
However, these individuals with variable sequences did not descend from the same grandfather.
The Y chromosome is much older than we thought.
This recent discovery and analysis of this rare African American Y chromosome indicates that the lineage actually diverged about 338,000 years ago.
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So how old is the Y chromosome?
Through this discovery, researchers and geneticists at the University of Arizona have concluded that this is the oldest known genetic branch of
the Y chromosome.
What does it mean? This discovery pushes back the time that the last known common ancestor
with Y chromosome lived by almost 70% (338,000 years ago).
The Neanderthals are believed to have split from ancestral human lineage around 300,000 years ago and modern humans did not appear on the fossil
record until 100,000 years later.
This time actually predates the oldest known anatomically modern human fossils! (Stolte, 2013)
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Evolution of the Y chromosome
Scientists speculate that around 300 million years ago the SOX3 gene was on the same location on the proto X and Y chromosomes which both
began as autosomes.
Since a mutation in the SOX3 gene created the gene SRY on the Y chromosome, researchers also conclude that monotremes are the most
ancient mammals with the SRY gene.
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Evolution of the Y chromosome (cont.) Since the Y chromosome is 1/3 the size of the X chromosome and is only
able to recombine via the tips of the chromosome, the Y chromosome
tends to accumulate changes as well as deletions much faster.
In addition, the repetition of sequences of male spermatogenesis genes on the Y chromosome ensure that spermatogenesis will proceed via proteins
produced with remaining genes despite any deletions that may occur.
The Y chromosome is here to stay.
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Y Chromosome Facts mutation in the SOX3 gene produced SRY gene
SOX3 gene is associated with regulation of embryonic development and determination of cell fate
SRY gene: regulates formation of testes from the undifferentiated embryonic gonad
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Y Chromosome Facts cont.
autosomes donate a block of genes that extended the length of both the X and Y chromosome
RSP4 retained similar function on both X and Y chromosomes
RPS4 (ribosomal protein small subunit, protein 4): essential for ribosome formation
although function remained the same, location changed due to inversion
inversions which are internal recombination events account for most of the rearrangement on Y chromosomes
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Y Chromosome Facts Cont. Y chromosome became susceptible to deletions, resulting in size decrease
only the tips of the Y chromosome are able to recombine with homologous genes on X chromosome
while recombination can occur across full length of two identical X chromosomes in females (XX)
without recombination to preserve integrity, Y lost more genes and shrank more
modern Y chromosome is 1/3 the size of X chromosome
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Y Chromosome Facts cont. autosomes contribute a copy of DAZ spermatogenesis gene to Y chromosome
in modern Y chromosomes there are four DAZ sequences
DAZ: essential for production of sperm
spermatogenesis: process by which spermatozoa are produced from male primordial germ cells by way of mitosis and meiosis
Y Chromosome sometimes referred to as hall of mirrors because of the four identical DAZ genes
due to multiple copies of DAZ genes, deletion can occur but spermatogenesis will still be possible
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History of the Y Chromosome 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 assortment, 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 ashomologous 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.
History of the Y Chromosome ! Over the last century, scientists have used various methods to study
the Y-chromosome and why it maintains a different structure than its
par tner X-chromosome, unlike other autosomes
! Specifically they have focused on
! DNA sequences of a wide range of organisms
! Ancestral genes and gene pools
! Rates of recombination amongst generations
! The role that the Y-chromosome has had in sex determination
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Mammalian Evolutionary Patterns
The Y chromosome of placental mammals
Sex was determined by a variety of means
Recombination of proto-sex-chromosomes
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Mammalian Evolutionary Patterns
(CONT.)
Recombination between the X and Y for male genes became disadvantageous
Fitness advantage and its result in natural selection
Y chromosomes that hardly recombined with the X chromosomes
Female and male sex determination
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Mutation
Mutation and the integrity of the Y chromosome
Mutations building up over time
Genetic load
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Human Evolution Of Y Chromosome
- Over time, structural changes in the Y chromosome resulted in its current form,
which is specialized to trigger male development.
- 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.
- Most of the Y chromosome is inherited from father to son in a pattern
resembling asexual, not sexual, reproduction.
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Cont.. No recombination means no re-assortment, so 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.
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Sources University of Arizona. "Human Y chromosome much older than previously
thought." ScienceDaily. ScienceDaily, 5 March 2013. www.sciencedaily.com/releases/2013/03/130305145821.htm
http://www.hhmi.org/biointeractive/evolution-y-chromosome
"Mammalian Y Chromosomes Retain Widely Expressed Dosage-sensitive Regulators." Nature. N.p., 23 Apr. 2014. Web. 3 Mar. 2015.
"Why the Y Is Here to Stay." Why the Y Is Here to Stay. N.p., May 2014. Web. 01 Apr. 2015.
"Result Filters." National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 01 Apr. 2015.
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More Sources http://www.hhmi.org/biointeractive/evolution-y-chromosome
http://www.learner.org/courses/biology/textbook/gender/gender_4.html
http://petrov.stanford.edu/pdfs/47.pdf