A brief history of sperm and egg

A brief history of sperm and egg
• Spermatozoa discovered by Anton Leeuwenhoek (1632-1723)
• homemade lens magnified 300 fold
• “What I investigate is only what, without sinfully defiling myself,
remains as a residue after conjugal coitus.”
coitus.”
• followed by observation of fertilization
by Lazzaro Spallazani (1637-1680)
who worked with frogs
• made frog “trousers”
trousers” to prevent semen
from mixing with eggs
• Eggs were not fertilized “for want of
being bedewed with semen.”
semen.”
• he moved onto dog studies with
cocker spaniels and performed the
first recorded successful
artificial insemination
1
A brief history of sperm and egg (part 2)
• Spallanzani showed semen was essential for fertilization
• Newport showed frog sperm penetrating an egg in 1853
• ‘ovist’
ovist’ believed the egg contained a fully formed embryo
• ‘spermists’
spermists’ believed sperm contained the embryo
• Hartsoeker was a spermist - his view was a bit cramped:
• but what of all the wasted sperm w/ homunculus inside?
• how could each tiny homunculus have it’
it’s own tiny
sperm containing even smaller honumculi ad infinitum?
• the spermists argument fell apart
• ovists argument suffered from the same criticism
• can’
can’t be like a Russian doll
going on forever…
forever…
matrioshka
A brief history of sperm and egg (part 3)
• epigenesis:
epigenesis: embryo results from fusion of male+female sex cells
• championed by William Harvey
• Oscar Hertwig (1875) showed the sperm head fused with the
female genetic material in sea urchins, giving a conceptual
basis for genetic inheritance
2
Sex cells: the sperm compared to the ova
• sperm: small, produced by male
• ova: large, produced by female
• anisogamy:
anisogamy: difference in size of male and female gametes
• sperm essentially parsitize resources contained in ova
• sperm: replenished throughout life
• ova: females start life with fixed quota, many never ovulate
• ova decrease throughout lifespan: 6 million (embryo), half million
(birth), 50,000 (puberty), 400 produced in lifespan
• atresia:
atresia: degeneration of ova throughout lifespan of female
• ova: immobile, loaded with nutrients that take up volume
• sperm: specialized to deliver haploid chromosome payload
• highly mobile, limited internal resources
• the male produces and delivers sperm
Spermatozoa: variations on a specialized theme
• three parts: a head, midpiece,
midpiece, and tail
• acrosome at head tip to digest through outer layers of egg
• midpiece contains mitochondria - energy for flagellum propulsion
• tail consists of flagellum to provide mechanical thrust
• enormous variation in design between species
• biggest variation is in tail length (from 0.3 mm to 6 cm)
Fruit fly
(only long version fertilizes egg)
Opossum
(operate as pairs for efficiency)
Plains rat
(marsupial with strange hooked
head and finger-like projections)
projections)
Nematode
(knobby false legs are used to crawl)
3
Drosophila: the giant sperm machine
a)
b)
c)
d)
A male with one testis unwound
around him
Male reproductive system with
multiple accessory glands
(paragonial gland “p” stores
sperm)
sperm store showing individual
coiled up giant sperm that are
released to the female one at a
time
a single partially unraveled sperm
at 58 mm long - many times the
length of the owner. It takes over
2 weeks to produce a single
sperm, but the giant tail is
destined to be digested and
eventually excreted by the
fertilized female
Spermatozoa: why all the variation?
three main ideas to account for variation:
1. Reproductive isolating mechanism - prevents cross
species fertilization (probably wrong, since this occurs well
before the sperm comes close to the egg)
2. Females prefer larger sperm (nutrition for zygote? Fruit
fly?) Females get some benefit from larger sperm
(genetic benefit for offspring, longer sperm more ‘attractive’
attractive’
and selected for in her reproductive tract), If sperm
‘attractiveness’
attractiveness’ was heritable, then there would be selection
for larger and more well developed sperm to fertilize future
generations of eggs.
eggs.
3. ‘Attractiveness’
Attractiveness’ of sperm accurately reflects the viability
of the sperm’
sperm’s producer.
producer. Females will choose sperm
most likely to produce high quality offspring.
and one more possibility…
possibility…
4
Spermatozoa variation: male-male competition?
Maybe larger sperm are more competitive in the female
reproductive tract and better able to fertilize the egg.
1. Faster swimmers? (always fertilize better than slow ones)
2. greater energy reserves to survive longer?
3. more efficient at evading female counter-adaptations?
Studies to examine these three possibilities have shown that the
first two are not correct: no evidence that bigger sperm are
faster or survive better than shorter sperm
However, the third possibility may be supported by studies
showing a linear relationship between
sperm length and the size of sperm
storage tubules in female birds
Co-evolutionary response of males
attempting to get to egg and females
selecting for a specific need
Spermatozoa variation: not every sperm is sacred?
• marked variation within a single ejaculate
• in some butterflies many sperm have no nucleus and
are incapable of fertilizing the egg (up to 90%)
• what are shorter / infertile sperm used for?
Three possibilities:
1. Help transport nucleated sperm to egg
2. Provide nourishment for nucleated sperm
3. Serve a role in sperm competition
Again, the third possibility is supported by research in
army worm moths. The male semen contains about
95% non-nucleate sperm, yet these non-nucleated
sperm retained in the female determine the time until
she mated again. Non-nucleate sperm are fillers.
5
Sperm profligacy: why use many if one is enough?
•
•
•
•
•
•
•
profligacy is counter-intuitive unless there’
there’s an advantage
we know now that sperm competition is one reason
if >1 male copulates with a female, inseminating more sperm
is like buying more lottery tickets
species with high sperm competition invest with large testes
yet the ratio of sperm:egg presentation varies widely
some insects provide <100 sperm per egg
in humans millions of sperm are provided per egg
Crossing over: a reason to make more sperm?
•
•
•
•
•
•
•
•
•
•
•
maybe all those extra sperm are due to production errors?
with each crossing over event there is a 20% chance something will go
wrong ending in faulty sperm
humans have ~50 crossing over events / sperm, so chance of an error is
high
thus, many of the sperm produced may be incapable of fertilization (Jack
Cohen, ~1960)
if true, females must select for best sperm, weed out duds
as we saw in the last lecture, this is exactly the way it works
if crossing over necessitates production of vast numbers to overcome
errors, the same should be true for eggs
there’
there’s no correlation between egg redundancy and crossover frequency
Maybe crossing over is good - reduces production errors by mixing up the
genetic material and lowering the risk from bad combinations of genes
if mutations arise that decrease sperm competitiveness, then the individual
sperm within the same ejaculate will compete - not all are equal
Then there’
there’s competition with other ejaculates if present, with increased
selection pressure for crossing over to create variation and enhance sperm
performance to get most viable sperm with maximum fertilization ability
6
Ejaculate control: males exercise choice
•
•
•
•
•
•
•
•
•
•
in species where males are promiscuous,
promiscuous, reproductive success is
enhanced by transferring maximum numbers of sperm
without sperm competition, selection favors males with modest
ejaculates
even so, sperm numbers in ejaculates vary widely, even in the same
individual. Why would this variation and potential for control exist?
sperm competition varies from situation to situation
inseminating a virgin, males aren’
aren’t concerned with sperm competition
with a non-virgin, more sperm would be recommended to compete
an evolutionary advantage to providing males with mechanism to
adjust ejaculate size and sperm content depending upon the situation
would require males to be capable of discriminating virgins from nonvirgins
alternatively, would require that females are monogamous and
sperm competition is accordingly reduced
under these circumstances, males could reduce sperm delivery
without risking loss of reproductive fitness
Ejaculate control in the real world
•
•
•
•
blue crab males are very sensitive to insemination status of females
Males will inseminate with more sperm for a female that has
copulated with other males versus a virgin female
may be able to sense prior insemination using pleiopods
(specialized appendages used for transfer of spermatophores)
spermatophores) that
are placed inside of female and detect sperm store status
copulation goes on from 5-12 hours depending (in part) upon
whether prior sperm are present
7
Ejaculate control in the real world (part 2)
•
•
•
•
•
•
•
•
•
•
•
•
blueheaded wrasse is an external fertilizer
males secure territory for entire breeding life
(3 months), then dies
dozens of females turn up daily for afternoon
spawning
territorial males under huge pressure to produce
lots of sperm
no other males around, no sperm competition
males gauge sperm delivery based upon female
size, which is closely related to the number of eggs she will release
he ejaculates fewer sperm than required to fertilize all the eggs
by conserving sperm he is able to fertilize eggs from more females
rare case of eggs outnumbering sperm - greater cost to female
when raiders sneak in and try to ejaculate at the same time, the resident male
increases the sperm he delivers - up to a point
if too many raiding males arrive, the resident male decreases his sperm output when competition is intense and chances of success are low, no point in wasting
your seed
Ejaculate control based upon integrating information about female size and male
competition - Not a conscious decision - natural selection eliminated those who
couldn’
couldn’t compete effectively
Seminal magic
•
•
•
•
•
mammalian sperm does 3 tricks: swim, swim fast, and eat a hole in the egg
protective layer
human sperm travel 15-20 cm from insemination point to fertilization in the oviduct
transport is a combination of swimming and active transport by female tract
sperm transport is complex and sperm change as they ascend the escalator
in mammals, undergo capacitation when reaching the end of the journey - begin
hyperactive swimming wrenching free of the oviduct lining and increasing chances of
bumping into an egg
A human egg with single
sperm attached to
exterior moments before
fusion of male and
female DNA. The sperm
has just penetrated the
egg and the purple
shapes to the right are
female chromosomes
Egg punching
Bull sperm
(clasped by female oviduct)
(sperm holes)
Giant Drosophila sperm
inside the egg
8
A commotion in the semen
•
•
•
•
•
•
•
more to seminal fluid than just sperm
ejaculate extremes: turkey (0.22 ul;
ul; 1,600 sperm) vs.
vs. pig (500
ml; 100,000 mil sperm)
seminal fluid protects and nurtures sperm
also manipulates the female response to copulation: in some
species facilitates transport in the oviduct, stimulates ovulation,
or other benefits
example:
example: house fly semen mimics female hormones to
stimulate ovulation and also an anti-aphrodesiac that puts the
female off wanting sex - permanently
works to the male’
male’s favor under conditions of otherwise
promiscuous female flies
similar substances occur in some insects and snakes, but they
only delay return to receptivity, not kill off sex drive completely
Killer semen: why fruit fly females play hard to get
•
•
•
•
•
•
•
•
•
•
•
ultimate ejaculate: one that kills or diasbles rival sperm
in many species ‘last sperm precendence’
precendence’ is the rule (as in flies)
also, female fruit flies that copulate frequently die sooner
males suffer no penalty for frequent sex
answer: being inseminated causes reduction in female lifespan
seminal fluid (not the sperm) was damaging to females
mating spermless males to females causes same reduced lifespan
‘last sperm precendence’
precendence’ and reduced female lifespan were linked
fruit fly semen contains toxins responsible for disabling rival sperm
these toxins, like spider venom, kill the female over time
To the male, as long as she lives to lay eggs fertilized with his sperm,
her shortened lifespan is of little concern
9
Battle for control of reproduction
•
•
•
•
•
•
•
why would females allow for this kind of treatment by males?
perhaps difficult to evolve countermeasures
co-evolution of Drosophila reproductive tracts is a race
example:
example: when female flies are not allowed to co-evolve with males,
males acquire sperm displacement abilities that are even more
damaging to females
under normal conditions, males evolve strategies to enhance
reproductive success and females evolve countermeasure to regain
control
when fruit flies were forced to be monogamous, selection favored
those males with less toxic ejaculates who could produce more
offspring before the females died
Much more is known about ejaculates than ova because they are
easier to study, not because they are more important
10