Download document

Sex Differences
Biological Sex
• Recombination of genes
• Sexual reproduction involves multiple
individuals
– Usually 2
• Male vs Female
What determines male and female?
In ye old days…
• Aristotle
– Sex of infant determined by how much heat
was in a man’s sperm
Having a 2 of the same
Chromosome?
Edmund Beecher Wilson & Nettie Stevens 1905
chromosomal XY sex-determination system
Lyonisation
• Mary Lyon 1962
• In Females each somatic cell has 2 X
chromosomes
– One X chromosome is inactivated
– Random inactivation
• Shortened and condensed
• Females are genetic mosaics
Lyonisation
• Occurs only in females
• X chromosome has fur color
control gene
• 2 alleles
– Black & Orange
• White results from genetic
condition of unpigmented hair
What About…
• Genotypic sex
determination
– Birds & mammals
• Homogametic
– Humans female
• Heterogametic
– Human male
– Bird female
Temperature-dependent Sex
Determination
• Temperature eggs exposed to during
development determines biological sex of
offspring
• Thermosensitive period during
development
Turtles
• Males produced typically at
lower incubation
temperatures
– 22.5-27C
• Females higher
temperatures
– 30C females
• Lizards and crocodiles
opposite pattern
American Alligator
• Females result from eggs incubated at
both high and low temps
• Males develop at intermediate
temperatures
Genotypic Sex Determination
• Humans exhibit heterogametic
genotypic sex determination
• All embryos have genes for
both testes and ovary
production
– Female is the “Default” sex
• Presence of a Y chromosome
will stimulate the embryo to
develop into a male offspring
Genotypic Sex Determination
• Sex determining region of Y chromosome
– SRY gene complex
• Gene expression of SRY gene will result in the
production of Testes determining factor (TDF)
•
Testes Determining Factor
• TDF triggers activation of
genes found on somatic
chromosomes
– ~7 weeks after fertilization
• Activated genes responsible
for the development of testes
• Developing testes will produce
Mullerian inhibiting substance
– Causes regression of female
gonads
Definitions of Male & Female
• Gender is a social categorization
• Biological categories have to apply to all
forms of life
– Algae, humans, sea stars, plants
Biological Definition
• Based on gamete size
• Male
– Small gametes
– Sperm (spermatozoa)
• Female
– Large gametes
– Egg (oocyte)
Isogamy
• Gametes same size
Mating types
•
•
•
•
Characteristic of isogametic species
Differences in cell surfaces
Algae, fungi, protozoans
No male/ female
Clamydomonas sp
Anisogamy
•
•
•
•
“not same gametes”
Gametes different size
Usually 2 sizes- small & large
Rarely some species have multiple
sized gametes
– Chlamydomonas euchlora
– Divides 4-64 times any inbetween
• 4 = big gametes
• 64 = small gametes
Why 2 sexes?
• Multiple mating types can exist
– Ex Slime molds..15 types
• Fungi +/• The magic number two
– More than one ensures self does not breed with self
• Produces variable offspring
• Inbreeding avoidance
– Smallest number possible to introduce diversity
• Minimizes time invested finding mates
Why Two Sexes
• Cytoplasm control
• Energy investment strategies
Cytoplasm Control
• Sperm evolved to prevent mixing of incompatible
cytoplasm
• Sperm very little other than DNA. Mitochondria &
other organelles from sperm are typically
excluded
• Selection against incompatibilities in cytoplasm
fusion drive evolution of a small & large gamete
Evolution of Anisogamy
• Isogamy ancestral condition
• Anisogamy independently evolved many
times
• Driving force is energy investment
– Resulted in disruptive selection
Reproduction is an energy
investment
• Different strategies exist for reproductive
investment
• Trade-off exists between
– size & number of gametes
• Lot of small
– Size & quality of gametes
• Few high quality
– Results in a selection force for 2 fertility
strategies
The Scenario
• In a population diversity
in gamete size exists
(bell shaped curve)
• Some individuals make
small nutrient poor
gamete
• Some make an inbetween gamete sizes
• Some make large
nutrient rich gametes
Small Nutrient Poor Gamete
• Proto-male
• Pro
– can produce lots of cheap gametes
– Increases number of reproductive events
• Greater reproductive potential
– Greater fitness & more individuals in the population
• Con
– 2 nutrient poor gametes together reduce survival
Large Nutrient Rich Gametes
• Proto-female
• Pro
– Offspring produced have greater probability of
survival
• Con
– Produce fewer gametes
– Fewer reproductive events
– less prevalent in the population
Some make an in-between gamete
sizes
• No selective advantage
• Gamete not big enough to gain selective
advantage of having lots of nutrients
• Gametes not produced in large enough
numbers to increase reproductive success
Proto-Male Strategy
• Once gametes begin to diverge anisogamy itself
creates pressure that advanced differentiation
• Protomales produced in large numbers
• Success is limited by ability to find rarer
protofemales
• Selection forces act on Proto-male to increase
adaptations for finding proto-female
Proto-Female Strategy
• Reproductive success driven by producing
fewer high quality gametes
Consequences
• Males
– Lots of gametes
– Little energy per gamete produced
– Evolution favors mechanisms for
encountering female
– Adaptations
• Motile
• Locate- egg tracking systems
• Females
– Fewer gametes
– Lots energy per gamete produced
– Evolution favors mechanisms for being
encountered
– Adaptations
• Chemical attractants
• Selection
Coevolution of Males & Females
Male vs Female
What happens when there is no
hope….
• Fruit fly’s that mated
repeatedly for several
days no preference for
alcohol
• Males without access to
females strongly
preferred food mixed
with alcohol
• Alcohol satisfied desire
for physical reward
Bateman’s Principle
Mated fruit fly’s in
monogamous &
promiscuous groups
Male reproductive success
increases with number of
mates
Female reproductive
success does not
increase with multiple
mates
Bateman’s Principle
• Bateman 1948
• The sex which invests the
most in producing offspring
becomes a limiting resource
over which the other sex will
compete
– Male strategy
• Promiscuous
– Female strategy
• Choosy
– Males should be eager,
females…
“The female, with the rarest exceptions, is less eager than the male…she is
coy, and may often be seen endeavoring for a long time to escape.”
Robert Trivers
• 1972
• Expanded Bateman’s principle to extend
beyond gamete scope to include all
aspects of reproductive effort
– Gestation, feeding, protection
• The sex that invests more should be
passive & discriminating
Terminology
• Reproductive effort
– Proportion of the total energy budget of an organism
that is devoted to reproduction
• Parental Investment (effort)
– Any investment made by parent to individual offspring
to increase the survivability of the offspring
• Mating effort
– Any investment made in by individual to increase
reproductive event
Consequence of Anisogamy
• Male
– Increased proportion of reproductive effort
directed into mating effort
• Female
– Increased proportion of reproductive effort
directed into Parental effort
Sex Conflict
• Conflict of interest as two sexes
reproductive strategies change over the
course of evolution
• Ex sponges sex strategies similar among
sexes
• Vs Peacock & peahen
Effects
• Results in morphological differences
• Results in behavioral differences
Isogamous Species
Anisogamous Species
Sources
• http://www.sciencedaily.com/releases/201
2/03/120315145415.htm