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.

:

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

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

Y Linked Traits

Genes located only on the Y chromosome Affected men will never have an affected

daughter

100% affected sons

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.

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

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

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

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.

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)

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.

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.

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

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

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

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

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

Mammalian Evolutionary Patterns

The Y chromosome of placental mammals

Sex was determined by a variety of means

Recombination of proto-sex-chromosomes

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

Mutation

Mutation and the integrity of the Y chromosome

Mutations building up over time

Genetic load

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.

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.

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.

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