Sex: --- understanding its biological significance
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Transcript of Sex: --- understanding its biological significance
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Sex:
--- understanding its biological significance
-- appreciating how genetics was used to understand how it is determined.
… according to Jacob Bronowski in “The Ascent of Man” (1973)Mendel himself was inspired by the clear-cut
difference between males and females and the1:1 sex ratio
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Costs of sex:
(1) Males dilute females’ genetic contribution(the couple is the unit of reproduction)
(2) Seeking a mate and mating takes time and energy -- and is dangerous
(3) Sexual conflicts arise (remember the Haig hypothesis for imprinting)
(4) Sex and its consequence, recombination, break up winning gene teams
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Benefits of sex:
(1) Reduces mutational load
(escape “Muller’s ratchet” -- irreversible loss of genes)
(2) Free good mutations from bad genetic backgrounds
(3) Help to keep ahead of parasites
(there is no “optimal” genotype in the real world)
perhaps males particularly useful (rationale for “maladaptations” from sexual selection)
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“Sex determination genes” determine two qualitativelydifferent things (a distinction not often appreciated, even by thosewho study the genetic programming of sex):
population sex ratio
sexual dimorphism (developmental differences)
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Bonellia viridis
Female: 100 mmMale: 1 mm
larva lands on rock
larva lands on adult female
An extreme example of sexual dimorphism
ESD: environmental sex determination
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relevant variables for ESD:
Host (Bonellia)
Temperature (turtles, alligators)
Neighbor density (parasitic wasps)
“Presence of male” (tropical fish)
vs. GSD: genotypic sex determination
Segregation of alleles (genes) determines sex
best for generating 1:1 sex ratios
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apparant paradox:
Since females are rate-limiting for reproduction,
why see 1:1 sex ratio so often?
In the aggregate, both sexes contribute equallyto the next generation (every female needs a male)
hence, any minority sexon average will make a
disproportionate contribution per individual
Natural selection will favor generation of the minority sex.
At 1:1, no minority sex!
(as usual, Darwin had the answer first)
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Calvin Bridges (1916):
Known for fruit flies: XX females XY males
white daughers (fertile)
red sons (sterile)(primary)
white daughers (fertile)
red sons (fertile!)
expected:w -/w+ (red) daughters
w -/Y (white) sons
XX
XY
XXYX(O)
XXYXY(±Y)
progeny are“secondary” exceptions
x red XY
w -/w - (white eyed) Females X Males (red eyed) w+/Y
“exceptions”:
…but what really determines fly sex?
(xxx & o/Y die)
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for fruit flies:
normal: XX females XY males
abnormal: XXY females XO males
X chromosome number determines sex
Y chromosome does not detemine sex(but is required for male fertility)
Sex-chromosome difference CAUSES(triggers) different sexual development
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XX females XY males
What about X-chromosome number matters?
absolute number: 1=male, 2or more = female
odd vs. even (paired?) XX X=male?
number relative to ploidy (non-sex chromosomes)?
X AA male, but X A female?
…again, genetic exceptions to the rule provide the answer
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Parental types: px + & + sp
Nonparental types:(recombinant)
( 6.5 cM)
+ + & px sp
px bw ++ bw sp
px + spFemales MalesX
expected PROGENY:
(autosomal genes)
ALSO: one unusually large ++ female
px bw ++ bw sppx + sp
(1) Three, not two, parental types recovered:
(2) many intersexual(sterile) progeny
X px bw sp Male
XXXAAA
XXYAAA
(3) normal and jumbo females
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XX AA X:A = 1, female
X AA X:A = 0.5, male
XX(±Y) AAA X:A = 0.67, intersex
XXX AAA X:A= 1, female (large)
X A X:A=1, (dead) female
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XX AA zygote --> XXAA cells / X AA cells
X-chromosome loss generates “gynandromorphs”
GENETIC MOSAICS
(XXAA)
Female(X AA) Male
(XXAA)
Female(X A) Female
XXAA zygote -->XXAA cells/XA cells(“loss” of an entire haploid set)
(XA never reachesadult stage
but mosaics do)
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XX AA X:A = 1, female
X AA X:A = 0.5, male
XX(±Y) AAA X:A = 0.67, intersex
XXX AAA X:A= 1, female (large)
X A X:A=1, (dead) female
GSD by X:A ratio (balance)
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The worm:
XX self-fertilizing hermaphrodite
XO male (heterogametic sex)
Origin of males:
(1) Spontaneous X-chromosome nondisjunction (rare) to make “O” eggs (+ X self sperm)-> XO male
(2) Mating (outcross) of hermaphrodite to male:
X eggs join with X or O male sperm -> 50:50
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The worm:
XX self-fertilizing hermaphrodite
XO male (heterogametic sex)
XX AAA X:A= 0.67 = male
XXX AAAA X:A = 0.75 = hermaphrodite
GSD by X:A ratio
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HUMANS:
XX female XY male
XXY Kleinfeler Syndrome
sterile male (1:1000 men)
XO Turner Syndrome
sterile female (1:2000-5000)
GSD by Active Ydominant masculinizer
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HOUSE FLIES:
m/m female M/m male
GSD by dominant masculinizing allele M
(one of three different GSD systemsin the same species!)
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Birds, moths and butterflies:
ZZ male ZW female
female isthe heterogametic sex
(compare: XY males)
GSD by feminizing W or Z:A ?
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20% of all animals use a very different GSD system:
Eggs fertilized --> Queens (females) or workers (sterile)
Eggs not fertilized --> Drones (males)
Diploid (± royal jelly)
Haploid
GSD by “haplodiploid” system
But is the relevant variable ploidy?
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Let’s encourage inbreeding among the honeybees:
increased homozygosity
suddenly: DIPLOID MALES!
a1/a2 heterozygotes: females (queens and workers)
a1 or a1/a1 hemizygotes and homozygotes: males
a1/a2 Queen X a1 Drone --> a1/a1 & a2/a1
diploiddrones
(fertilization)
GSD by a multiple allele system--- highly “polymorphic” sex gene (many alleles)