Transcript Lecture_31_Mar 26_ sex

A characteristic feature of parasites is their high reproductive output.
Other organisms that have hazardous phases in their life cycles, such
as planktonic larvae, produce vast numbers of offspring. Organisms that
live in transient environments, such as rainwater pools, have, like
parasites, developed resistant and dispersal stages and are often
capable of asexual reproduction.
But even in parasites there is a relationship between fecundity and
parental care. The monogeneans for example produce few wellprovisioned eggs, tsetse vectors produce very few, but well provisioned
larvae.
How many eggs should be produced… how should they be produced?
Trade offs……………
Asex vs SEX…
Why do organisms have sex?
Sex is an embarrassing subject for biologists….
Most organisms have sex. Why is it so common?
Sex is very expensive
Sex is not necessary for reproduction (is an "interruption" of
mitosis)
Costs time and energy finding a mate
Big genetic cost
Evolution of Sex
Bell (1982) commented that “Sex is the queen
of problems in evolutionary biology.
Perhaps no other natural phenomenon has
aroused so much interest; certainly none
has sowed as much confusion”.
The two-fold cost of sex (Maynard Smith, 1978)
Sexually generated offspring are only 50% related to
parent ("cost of meiosis") + only half of offspring
reproductive ("cost of males"; slower rate of increase).
Mutations for asexuality (e.g. "cut out meiosis") should
rapidly spread: are passed on with 100% efficiency +
100% of offspring are reproductive
Ratio of asexual to sexual females should double per
generation
It has been argued that unless sexual offspring are
significantly fitter, sex is not an Evolutionary Stable
Strategy
So, all other things being equal, sexual forms should
be easily outcompeted by parthenogens
There is a consistent trend for sex to be most prevalent
where the habitat is not subject to dramatic
fluctuations
Parthenogens are more common early in succession
and at extreme latitudes and altitudes in both animals
and plants
The vast majority of eukaryotes reproduce sexually. This
poses a central problem in evolutionary biology: a
parthenogenetic female passes twice as many of her
genes to progeny as a sexually reproducing female. Why
are not all organism parthenogenetic?
How can we explain sex persisting in populations?
Are these explanations powerful enough relative to individual
selection?
Need sufficiently large short-term advantage to offset two-fold
cost per generation
Five Costs of Sex
1. Recombination scrambles genotypes, disrupting favorably adapted gene
combinations, whereas parthenogenesis preserves advantageous
genotypes
2. Meiosis and syngamy takes longer than mitosis, slowing the pace of
reproduction - decreased population growth rates
3. Courtship and mating may be risky; risk from predation or sexually
transmitted disease - there may be waste of gametes and costs
associated with maintenance of sexual dimorphism and sexual
competition
4. At low population densities sexual reproduction may be difficult to
coordinate. Parthenogenesis ensures that reproduction will be possible
at any time or place
5. Most important of all, sexual females suffer from genome dilution.
Consider parthenogenetic and sexual females producing eggs at the
same size and rate. Because the sexual female contributes only half
the genetic material to each egg, she suffers a greater (two-fold) cost
relative to the parthenogen
Muller’s Ratchet Hypothesis: Recombination/sex is
directly beneficial by purging deleterious mutations
Fisher-Muller Hypothesis: Sex may facilitate response to
environmental change by generating new gene
combinations allowing populations to track a dynamic
environment: This is because adaptive favorable mutations can be
combined horizontally through a population
Segregation Hypothesis: Similar to the Fisher-Muller
Parthenogenetic clones cannot acquire favorable mutations
in the population: once a favorable mutant A1A1 - A1A2
arises the adaptive transition A1A2 - A2A2 is not possible
without segregation
Tangled Bank Hypothesis: heterogeneity in which a genotype
favored at one location may perform poorly elsewhere: it
may be advantageous to break up genotypes of offspring
dispersing to random sites nearby or distant
Red Queen Hypothesis: A related hypothesis attributes
environmental heterogeneity to species interactions
For example, sex would be favored in host-parasite interactions
because it generates diverse progeny, some of which may
have novel resistance genotypes and be able to withstand
parasite attack or disease
The high incidence of sex in physically stable environments
where species diversity is high is consistent with both the
Tangled Bank and Red Queen hypotheses where both
intraspecific competition and interspecific interactions should
be most intense
Fisher-Muller Hypothesis
Sex may facilitate response to environmental change by generating new gene combinations
allowing populations to track a dynamic environment
– This is because adaptive favorable mutations can be combined horizontally through a
population
Asexual
A
AB
time
Sexual
A
AB ABC
B BC
C
time
ABC
Tangled Bank Hypothesis
Sexual groups may be able to exploit a greater number of microsites
than a limited number of parthenogenetic clones
In addition to the greater number of microsites accessed by sexuals,
siblings will compete less due to specialization in different microsites
Red Queen Hypothesis
Attributes environmental heterogeneity to species interactions
For example, sex would be favored in host-parasite interactions because it
generates diverse progeny, some of which may have novel resistance
genotypes and be able to withstand parasite attack or disease
If sib competition is important, (Tangled Bank Hypothesis) then
recombination should be most prevalent when litter size is high, when
within-brood variation is possible (higher recombination being
necessary for greater diversification)
However, if parasites are important, (Red Queen Hypothesis) then
recombination should be more important in long-lived species
where the number of parasite generations will be greater than
the number of host generations
The high incidence of sex in physically stable environments where
species diversity is high is consistent with both the Tangled Bank
and Red Queen hypotheses where both intraspecific competition
and interspecific interactions should be most intense
Burt and Bell (1987)
Overall, there is support for the Red Queen Hypothesis since there
is a positive correlation between recombination and generation
time
40
3
020
10
. .
. . . .
. . .
10 50
100
.
.
500 1000 5000
R2 = 0.76
24 species of
nondomesticated
mammals
10000
Age to Maturity (Days) - Semilog
Sex
Asex
All genotypes are new
New genotypes formed at
mutation rate
Combination of alleles from
many genomes
Each allele shares descent with
entire genome
Associations between alleles
halved per generation
Associations between alleles
maintained (linkage
disequilibrium increased per
mutation)
Lots of variation between
offspring
Only mutational variation
between offspring
What does sex actually do?
Sex can’t cause adaptation directly: i.e. can’t change allele
frequencies. Instead, it affects the distribution of alleles in a
population; breaks up linkage disequilibrium (non-random
associations between alleles in the population). It stops
offspring being the same as their parent.
Sex breaks up these associations between alleles and
either maintains heritable variation, so allowing continual
responses to variable selection, or (long-term) speeds up
spread of of "good alleles“ or prevents the accumulation of
deleterious alleles or less fit genotypes.
Sex shuffles alleles between genomes
A GENERALLY ACCEPTED RULE:
Sex increases the rate of adaptation
Sex increases rate of spread of good alleles, allows you to generate
composite genomes; mutations that arise independently can spread
together
Asexual genomes = "selfish inventors"
Sexual genomes = share technology/innovation, Reduces "waiting time"
for good combination of alleles
BUT: This is a long-term advantage…
Does Natural selection act to favour long-term evolvability of populations
(group selection vs individual selection)
At individual level, asexuality should still spread within populations
Parasites and the Red Queen
Parasites are a big source of mortality in nature (Spanish influenza; 25 Million
people in four months, Plague in Europe 1300s); also evidence that
immune systems genes evolve v. quickly relative to other genes
Coevolution/arms race could provide appropriate timescale for constant
advantage of sex
"Best defence is the first that the enemy will counter" = constant need to switch
from rare to common alleles (frequency dependent selection)
Sustained oscillations in gene frequency between parasites and host: direction
of selection changes every generation
Empirical support for Red Queen
Potomopyrgus: freshwater snail: (Lively, 1987). Sexual or asexual forms: sexual
more common when parasite present (Red Queen)
Correlation between number and age to maturity (Burt and Bell, 1987); longer
generation time - need more recombination per generation to counter
parasites
Long lived trees are sexual (e.g. Douglas fir), only short-lived annuals tend to be
asexual; longer time between generations for parasites to evolve
More anecdotal: e.g. clonal crops are v. susceptible to sexual parasites e.g.
potato blight fungus, Ireland, 1848. All potatoes grown were clones of single
line.
There is a 50% cost of sex per generation. Asexual mutants should spread rapidly.
In contrast, sex is very widespread.
Sex breaks up associations between alleles, so maintains variation for selection to
act on. Clearly advantageous in long term…asexuals are "selfish inventors"
To be continually advantageous, need constantly changing selection: Coevolution
with parasites (Red Queen) may be a candidate, also explains evolutionary stasis.
Coevolution between hosts and parasites could maintain sex and affect cyclic
increases and decreases in frequency of resistance and defence alleles
Requires close coupling of parasite and host (specialists), and severe fitness costs
of infection (otherwise not much cost to not being resistant)
Old defences need to periodically become useful again (so that sex prevents
extinction of temporarily useless alleles); defence against one makes you
susceptible to the other
Sex is widespread and very important in evolution, also has
a big effect on how evolution works (e.g. speciation, sexual
selection, intragenomic conflict). However, it is difficult to
explain as it is clearly costly.
Our understanding will benefit from future approaches - into
the nature and rate of mutations and coevolution between
parasites and hosts, and the genetic architecture of immune
systems
Have parasites been involved in maintaining sexual
reproduction?
What factors can influence evolution?
The joy of sex - courtesy of parasites by Kate Melville
Why do we have sex? From an evolutionary perspective, the
answer is not as obvious as we might think. And now, a fascinating
new study in American Naturalist suggests that sex may have
evolved primarily as a defense against parasites.
Sex is something of an evolutionary mystery to biologists, as reproducing without
sex - as microbes, some plants and even a few reptiles do - would seem like a
much more efficient way to go. Every individual in an asexual species has the
ability to reproduce on its own. But in sexual species, two individuals have to
combine in order to reproduce one offspring. That gives each generation of
asexuals twice the reproductive capacity of sexuals. Why then is sex the dominant
strategy when the do-it-yourself approach is so much more efficient?
One theory is that parasites keep asexual organisms from getting too plentiful.
When an asexual creature reproduces, it creates clones - exact genetic copies of
itself. Since each clone has the same genes, each has the same genetic
vulnerabilities to parasites. If a parasite emerges that can exploit those
vulnerabilities, it can wipe out the whole population. On the other hand, sexual
offspring are genetically unique. So a parasite that can destroy some can't
necessarily destroy all. That, in theory, should help sexual populations maintain
stability, while asexual populations face extinction at the hands of parasites.
Now, thanks to Potamopyrgus antipodarum, a snail common in fresh water lakes in
New Zealand, scientists have a chance to test this parasite theory. The snails exist
in both sexual and asexual versions, thus providing biologists with an opportunity
to compare the two versions side-by-side in nature.
Researchers began observing several populations of these snails for ten years
beginning in 1994. They monitored the number of sexuals, the number asexuals,
and the rates of parasitic infection for both.
They found that while clones were plentiful at the beginning of the study, they
became more susceptible to parasites over time. And as parasite infections
increased, the once plentiful clones dwindled dramatically in number. Meanwhile,
sexual snail populations remained much more stable over time.
This, the authors say, is exactly the pattern predicted by the parasite hypothesis.
"The rise and fall of these female-only lineages was surprisingly fast and
consistent with the prediction of the parasite hypothesis for sex," Jokela said.
"These results suggest that sexual reproduction provides an evolutionary
advantage in parasite rich environments."
Mites Re-Evolve Sexual Reproduction by Kate Melville
Researchers from the University of Darmstadt in Germany and the SUNY College
of Environmental Science and Forestry reported this week on a family of mites that
have forsaken asexual reproduction and re-evolved to reproduce sexually.
Reported in the Proceedings of the National Academy of Sciences, the revival of a
complex trait such as sexual reproduction after it had been dormant for millions of
years raises interesting questions about our understanding of evolutionary biology.
"They found a way to re-evolve sex," Norton said. "We're talking about something
that involves a lot of elements in both males and females."
Mainly found in soils in the southern hemisphere, oribatid mites perform a vital role
as decomposition agents, grazing on fungi, decaying leaves and other organic
matter. The critters are also found on trees and rocks and Norton speculates that
moving from their ancestral home in the ground, where organic material is plentiful,
to trees and rocks, where food can be scarce, could account for the change in
reproduction. "The increased exposure and competition for food may create a
need for more responsive genetic defense mechanisms," he said.
The discovery raises some intriguing questions. Why do some organisms continue
to reproduce asexually, given the distinct evolutionary advantages - especially
defenses against parasites, predators and competitors - from reproducing sexually
and mixing genomes? And how can an organism jump-start a group of genes such as those specific to sexual reproduction - after many millions of years of not
being used?
Sex – Evolution’s Janitor
by Kate Melville
Asexual reproduction leads to a faster accumulation of bad mutations, says a
report in this week's issue of Science. Indiana University evolutionary biologists
used the water flea (Daphnia pulex) to establish their findings, which support the
hypothesis that sex is an evolutionary housekeeper that efficiently reorders genes
and removes deleterious gene mutations. Interestingly, the study also suggests
that sexual reproduction maintains its own existence by "punishing" individuals of
a species that meander into asexuality. The researchers say that the ability to
reproduce asexually may be useful to organisms that can't get mates, but its longterm benefits are questionable.
"It is known that sex is common in plants and animals, and that asexual species
are typically short-lived, but why this should hold throughout evolutionary time is a
great mystery," said study leader Susanne Paland. "Our results show that asexual
deviants are burdened by an ever-increasing number of genetic changes that
negatively affect the function of their proteins. It appears sex is important because
it rids genomes of harmful mutations."
Sexual reproduction is a complicated, biologically costly business. In mammals,
sex is usually preceded by intricate mating behaviors. It requires the compatibility
of sexual structures, an insertion event, fertile eggs and sperm, and the successful
unification of egg and sperm into a viable zygote. All of this adds up to a big energy
investment - energy an organism might have used for other purposes. It's no
surprise then, that scientists have long pondered what it is about sex that justifies
such a big energy investment.
One of the most widely accepted explanations has been that sexual reproduction
confers the benefit of unlinking genes, so that bad versions of genes won't always
get to hitch a ride with the good versions. This theory contends that natural
selection operates optimally when parts of the genome are free to shuffle about.
Sexual populations have recently and repeatedly spun off asexual strains. By
comparing rates of protein evolution, the researchers found that the asexual lines
accumulated bad mutations four times faster than sexual lines.
If a switch to asexuality causes a big increase in the number of protein defects, a
mechanism for removing those defects must somehow be missing when sex, too,
is missing. The present report supports the notion that it is sex - or the genetic
recombination that is a component of sexual reproduction - which is the purifying
force that helps get rid of genetic mishaps that harm the overall evolutionary health
of a population.
Sex Ratio Variation
Why is it that we often observe a sex ratio of 1/1 or
one close to a 1/1?
• Fisher (1930) found that he could explain the 1/1 sex
ratio in terms of selection at the level of the individual
• Fisher reasoned that if there were, say, 10 females
per male, selection would favor the production of
males since the fitness of the parent would be
enhanced by production of males as opposed to
females
Sex Ratio Variation
• The rare sex is also favored when one reverses the
situation to 10 males per female, with the female having
10 times the average reproductive success of a male
• Since a female produces an egg(s) that can be fertilized
by a single male, chances are that the remaining 9 males
will not breed. Therefore, it would be to the parents
benefit to produce female offspring, ensuring that their
genes are passed on
Sex Ratio Variation
• In either case, the sex ratio would not be evolutionarily
stable because a gene causing parents to bias the sex
ratio towards a greater number of the rarer sex, would
rapidly spread
• Only when the sex ratio is exactly 1/1 will the expected
success of a male and a female be equal and the
population stable
Investment
• If a male costs twice as much to produce in terms of energy
invested and a female half as much (and yet both males and
females produced the same number of young) it would be
advantageous for the parent to produce females as opposed
to males
• In this case the stable strategy would balance out when the
population became skewed to a 2/1 (female/male) sex ratio
• Therefore, in terms of numbers one would expect a 1/1 ratio
when the sexes are of equal cost but a skewed ratio in terms
of numbers when there is a differential cost between males
and females
• However, there will be a 1/1 ratio in terms of investment even
though the sex ratio is skewed in terms of numbers
Trivers and Willard (1973)
• Assume that a female in good condition is better able to bear and nurse
her calf than is a female in poor condition, so that at the end of the period
of parental investment, the healthiest, strongest and heaviest calves will
tend to be the offspring of the adult females who were in the best
condition during the period of parental investment
• Assume that there is some tendency for differences in the condition of
calves at the end of the period of parental investment to be maintained
into adulthood
• Assume that such adult differences in condition affect male reproductive
success more strongly than female reproductive success
• In this case, male caribou in good condition tend to exclude other males
from breeding, thereby inseminating many more females themselves,
while females in good condition, through their greater ability to invest in
their young, show only a moderate increase in reproductive success
• Under these assumptions, an adult female in good condition who
produces a son will leave more surviving grandchildren than a similar
female who produces a daughter, while an adult female in poor condition
who produces a daughter will leave more surviving grandchildren than a
similar female who produces a son
• So, as maternal condition declines, the adult female tends to produce a
lower ratio of males to females
Can Parasites alter behaviour?
Can Parasites affect sex?
Sex…
Can parasites affect mate selection?
Can one gender determine the parasite status of a potential mate before
deciding?
In the animal world why are males usually larger and more colorful?
What are the costs? What are the benefits?