Download Genome and sex 10-29

Selfish gene and sex selection
12-7-2015
Random X chromsome inactivation and Calico cat.
X Chromosome Inactivation
XIC – X chromosome Inactivation Center
• XIC controls expression of the XIST gene
• XIST (X-inactive-specific transcript): a non-coding 17
kb RNA molecule
• “Coats” the entire local X-chromosome – cis-acting
Xist RNA-dependent and -independent silencing in mouse
preimplantation embryos
X Chromosome Inactivation
XCI initiation varies in
mammals
Nat Rev Genet. 2014 June ;
15(6): 367–378
TSIX is the Anti-Sense Stand of the XIST gene
Long Noncoding RNAs May Alter Chromosome’s 3D Structure
24 MAY 2013 VOL 340 SCIENCE page 910
從複雜的個体、族群到生態圈
是競爭還是合作？
競爭：生命中無可逃避的宿命！
食物
交配
But…..
陽光和水
生物為什會有利他的行為？
合作的好處：互利互惠！
Wolves hunt in packs and then share their prey.
Reciprocal altruism
Vampire bats: sharing blood
does have costs to the
donor. But they may hope
to get something back: the
next night, they might miss
out and the neighbor they
fed will feed them in return
辨識與記憶能力是必要的條件
(future benefits).
避免白吃的揩油者！
吃虧就是佔便宜！
合作是生物世界中的常態嗎？
當然不是！
任何團体中都會有吃飯不作事的投機份子。
多細胞生物的源起：單個細胞合作的成果！
黏菌世界中的投機分子：只想成為孢子，而不願成為犧牲自已
的支柱細胞！
生物世界如何對付投機分子？
• 投機分子多半由於某些基因的損傷。
• 這些基因多半身負多重功能。
• 黏菌的投機分子單獨存在時，遇到惡劣環
境往往無法正常發育分化成為孢子！
• 此處得了便宜，它處必然得付出代價。
完美的利他主義者
Sterile workers in
social insects: give up
all reproduction for
the benefit of their
mother queen.
如何解釋這種完全
利他的生命行為？
W.D. Hamilton (1936-2000)
Kin selection
Hamilton’s Rule (1964)
Relatedness to partner
r.B > C
Benefit to partner
Personal cost
This rule predicts when a gene for altruism should be selected.
Prediction: cooperation at high relatedness, conflict at low relatedness.
Social insect: haploid male with diploid female
X
C
AB
1
0.5
0,5
Sisters share
genes via father
OR mother, so
average chance is
1 x 0.5 + 0.5 x 0.5 =
0.75
0,5
0,75
AC
BC
AC
BC
Worker
generation
完美的利他主義者
如何解釋這種完全
利他的生命行為？
去幫助蟻后 (姐妹，R = 0.75)；
比照顧自已的後代 (R = 0.5) 更划算！
為什要合作？
都是基因惹的禍:自私的基因！
Richard Dawkins: The Selfish Gene (1976)
無私的個人與自私的基因
Parent birds feed their chick
無私的行為源自於自私的基因！
Altruistic donor individual
Selfish “helping” gene
基因與個体間的関係
DNA
在生殖的過程中
完成複製.
Organism
身體是DNA的載具，它
所有的作為都是在保
存並散播DNA。
The idea of green-beard genes was invented
by Hamilton , and named by Dawkins .
How a gene can spread by recognizing and interacting with
individuals carrying copies of itself?
1, to display an observable trait (the ‘green beard’);
2, to distinguish between individuals that do or do not
display the trait;
3, to be altruistic to those that do.
Different alleles for Gp-9 locus (B and b) in fire ants influence whether
workers will adopt (Bb) or kill queens (BB).
Selfish genes: a green beard in the red fire ant.
Nature 394, 573–575 (1998).
If the metaphor of selfish gene is right,
Why sex reproduction is predominant in the
world of eucaryotes (animal, plant and fungi)?
Sex: the greatest lottery on earth!
•
•
•
•
George Bernard Shaw with actress.
George Wald and Nobel sperm bank.
Sickle cell anaemia
how to keep one good and one bad gene
together?
• Clone or sex reproduction?
Parthenogenesis
Why sex reproduction?
Cost of finding a partner;
Twofold cost of sex;
Give a chance for the cheater (selfish gene);
Syphilis, AIDS etc.
“We do not even in the least know the final cause
of sexuality ; why new beings should be produced
by the union of the two sexual element?”
Darwin, C.R. On the two forms, or dimorphic
condition, in the species Primual, and on their
remarkable sexual relations.
J. Proc. Linn. Soc. (Bot.), 6: 77-96; 1862.
A section of Darwin’s
“notes on marriage”,
1838.
Emma Darwin in
1840
The Wedgwood and Darwin families
Charles Darwin with his son William
Erasmus in 1842
Key steps for sex reproduction:
1, Segregation of chromosomes;
2, Sister chromotid recombination and
3, Cell fusion
A key step in meiosis to make lots of different
sex cells – crossing over!
Crossing-over multiplies the already huge number of different gamete
types produced by independent assortment.
How does sex reproduction arise?
• Sexual reproduction gene: option switch?
• You need genetic diversity to combat crisis (sex on).
• You also need to keep advantageous combinations
of genes to remain intact (sex off).
• The sexy individuals were driving the evolution of
sex reproduction
• Why no new cloner appear over billions of years?
Why asexuality fail?
• The good;
• From ab to Ab or aB to best survival AB
• Why sex is good! Nature 434: 571-2; 2005.
• The bad;
• Why sex? Science 311: 960-1; 2006.
• The ugly!
Low glucose; high temperature and high osmatic pressure (94 vs 80%)
Why asexuality fail?
• 434: 571-2; 2005.
• The bad;
• Why sex? Science 311: 960-1; 2006.
• The ugly!
The water flea, Daphnia pulex, may shift from being sexual to being asexual and
recombination in sexual cycle does lead to fewer deleterious mutations. . They observed
a clear excess of nonsynonymous mutations in the asexual populations. They also
estimated that close to 90% of the nonsynonymous mutations were subject to selection.
Why asexuality fail?
• The good;
• From ab to Ab or aB to best survival AB
• Why sex is good! Nature 434: 571-2; 2005.
• The bad;
• Why sex? Science 311: 960-1; 2006.
• The ugly!
– Conflict between host and parasite.
– Red queen theory
• 程樹德 ：紅皇后與有性生殖 (科學月刊 2011/05/13)
英國作家卡洛（Lewis Carroll）《愛麗絲鏡中奇遇》（Through the
looking glass, and what Alice found there）裡，描寫小女孩愛
麗絲在夢中變成棋子，與紅皇后博弈。紅皇后腳步疾行如風，但人卻
總留在原地，落後於她的愛麗絲喘著大氣說：「在我們家鄉，您走得
這麼快，肯定早不知走到那去了！」紅皇后說：「那是多慢的國度，
在這你光是費勁跑，也只能留在原地。如果想到別的地方去，你至少
得跑兩倍快才行！」
If host (H) and a parasite (p) to coevolve (top series of boxes), then
high rates of sexual reproduction are sustained (graph, right). If
frozen parasite stocks to reinfect each new generation of the host
with a fixed, nonevolving ancestral strain of the parasite (p0 bottom
series of boxes), rates of sexual reproduction can decline.
Science 333: 166-7; 2011.
When coevolving with parasites, all selfing C. elegans populations
became extinct within 20 generations; but not sexual one!
Theory of Sexual Selection
Darwin (1871, p256):
“We are, however, here
concerned only with that kind of
selection, which I have called
sexual selection. This depends
on the advantage which certain
individuals have over other
individuals of the same sex and
species, in exclusive relation to
reproduction.”
Intersexual selection: The peacock’s tail
Extravagant male ornaments
The peacock’s tail greatly impairs his mobility…how could such a trait evolve?
Intrasexual
Selection
Sexual Dimorphism
Why a theory of sexual
selection?
Darwin needed a theory to explain the many
extravagant traits that seem to reduce survival
e.g. the peacock’s tail
Fisher’s runaway model
Survival Selection
Sexual Selection
Total male fitness
(survival + mating)
Fitness
Female choice
adaptive for survival
Fitness due to survival
Tail length
Sexual competition between males
may result in harm to females
( beetle, frog, fly). But what happy if
males are brothers (kin selection)?
Brotherly love benefits females
Nature 505: 626-7; 2014
What happen is the males are AAB?
Sheep with big horn win the chance to mate!
Why not all the males have large horns?
Annual fitness variation and RXFP2 genotype.
Life history trade-offs at a single locus maintain sexually selected genetic
variation Nature 502, 93–95; 2013