Download Anisogamy

The importance of sexual and asexual reproduction
in the recent evolution of Allium vineale
Ceplitis A. EVOLUTION 55 (8): 1581-1591 AUG 2001
Why sex and recombination?
Barton NH, Charlesworth B
SCIENCE 281 (5385): 1986-1990 SEP 25 1998.
Abstract:
Most higher organisms reproduce sexually,
despite the automatic reproductive advantage experienced by asexual variants.
In a population of parthenogenetic females cloning selves:
the pop. can produce babies faster – everyone’s a mom;
& each baby reproduces 100% of a mom’s genes, not 50%;
But babies, families & the pop genetically more homogeneous;
less variable ecology, more vulnerable to coevolving parasites
This implies the operation of selective forces that confer an advantage to
sexuality and genetic recombination, at either the population or individual level.
The effect of sex and recombination {combining isolated beneficial mutations at}
different genetic loci, which increases the efficiency of natural selection,
is likely to be a major factor favoring their evolution and maintenance. …
With sex, isolated mutations at diff loci in diff individuals can be combined in one individual:
beneficial mutations can interact synergistically &
deleterious mutations can be purged in batches
– both processes can speed adaptation and select for sexual recombination.
Abstract:
In the weedy plant species Allium vineale (wild garlic),
individuals may simultaneously produce
sexually and asexually derived offspring,
by seed and bulbils, respectively.
In this study, genetic and genotypic diversity
was determined in samples from 14 European A. vineale populations
using nuclear (RAPD) and cytoplasmic (PCR-RFLP of cpDNA) markers
to investigate the importance of the different reproductive modes. …
Genotypic diversity was significantly lower than expected
under free recombination in almost all populations, indicating that
recruitment into populations is mostly by asexually produced offspring.
Nevertheless, … many … genotypes have originated by sexual recombination …
It is argued that
the weedy habit of A. vineale is likely to have favored bulbil reproduction,
{rapid population growth within habitats by asexual cloning – everyone’s a mom}
whereas sexually generated genotypes may have facilitated local adaptation
during the species' expansion across Europe.
{a little sexual recombination increased the range of habitats invaded}
Recall the famous sweaty T-shirt study!
The Red Queen Principle
(van Valen 1973)
As the Red Queen says
in Lewis Carroll's
Alice through the Looking Glass:
"....it takes all the running you can do, to keep in the same place."
{because environment is moving}
http://instruct.uwo.ca/biology/366b/hist3.html
The idea clearly chimes with recent concepts of evolutionary "arms-races."
… this whole notion goes straight back to
Darwin, in On the Origin of Species:
"........ the most important of all causes of organic change
is one which is almost independent of ......... altered physical conditions,
namely, the mutual relation of organism to organism ....
if some of these many species become modified and improved,
others will have to be improved in a corresponding degree
or they will be exterminated."
If evolution by natural selection
adapts animals to their environments,
WEDEKIND et al. 1995.
MHC-dependent mate preferences in humans.
PROC. ROY. SOC. LOND. B 260: 245-249.
Abstract:
One … benefit of sexual reproduction could be that
it allows animals (including humans) to react rapidly
to a continuously changing environmental selection pressure
such as coevolving parasites.
{ The Red Queen }
This … would be most efficient if the females were able to provide their progeny
with certain allele combinations for loci
which may be crucial in the parasite-host arms race,
for example the MHC (major histocompatibility complex).
Here we show that
the MHC influences both body odours
and body odour preferences in humans …
Asymmetries in Game of Sex begin with gametes.
why are males & females so different?
Anisogamy
Environment includes
social env as well as ecological env.
Males & females are in different social (mating) env’s.
not same-size gametes
The sex with the big gamete is female - by def.
Parasitic male anglerfish
Sex is a game
w/ asymmetries that lead to conflicts
and different optimal tactics
Mating systems begin with sex
but differentiate around parental care.
care
Females begin w/ bigger investment per gamete.
Often (esp. in mammals) females continue
with greater parental investment per offspring.
Biased parental investment creates biased sex ratio,
w/ fewer big investors available to mate at a given time
(w/ females parenting, fertilizable females are scarce)
If one sex has big parental investment per offspring
- selection for parental effort
- choosy about mating
The sex w/ low parental investment per offspring
-selection for mating effort
-less choosy about mating
1
Bateman’s Principle:
Mating is costly for males.
‘Where one sex invests considerably more than the other,
members of the latter will compete among themselves
to mate with members of the former.’
Cordts R, Partridge L. 1996.
Courtship reduces longevity of male Drosophila melanogaster
ANIM. BEHAV. 52: 269-278.
mean # offspring
120
0
0
1
2
# mates
3
In a diploid sexual species, in each generation,
half chromosomes from males & half from females.
Neither sex wins ‘battle of the sexes.’
If one male has more offspring, other males have less,
because this is a is a zero-sum game within the male sex;
males don’t have more offspring on average
if all eggs would have been fertilized anyway, just greater variance.
Mating can be costly to females too
Chapman et al. 1995.
Cost of mating in
Drosophila-melanogaster
females is mediated by
male accessory-gland products
NATURE 373: 241-244.
Abstract:
Female Drosophila melanogaster with environmentally or genetically
elevated rates of mating die younger than controls.
This cost of mating is not attributable to receipt of sperm.
We demonstrate here that seminal fluid products
from the main cells of the male accessory gland
are responsible for the cost of mating in females,
females
and that increasing exposure to these products increases female death rate.
Main-cell products are also involved
in elevating the rate of female eggegg-laying,
in reducing female receptivity to further matings and
in removing or destroying sperm of previous mates .
The cost of mating to females may therefore represent
a side-effect of evolutionary conflict between males {‘toxic semen!’}.
… courtship alone was sufficient to reduce male life span.
Mating itself and production of seminal fluid and sperm,
on the other hand, did not seem to be costly …
Ha
n
p (still alive)
Note that there is
stronger selection on
males - eager ?
than on
females - choosy ?
to mate
more than once.
Fruit flies:
w/
f
em
al
es
in
ch
a
st
ity
be
gi
ng
ou
t
w/
t
he
gu
ys
lts
Mounting attempts may be an indicator of an altered metabolic rate
or hormonal status {stress?} that renders the males more susceptible to death.
Sexual cannibalism and sperm competition in the golden orb-web spider
Nephila plumipes (Araneoidea): female and male perspectives.
Schneider JM, Elgar MA. 2001. Behavioral Ecology 12:547-552.
Abstract:
Mating systems are frequently shaped by conflicts over reproductive interests
between males and females.
Sexual cannibalism can be an especially dramatic manifestation of such conflicts.
Cannibalism may be in the interest of both sexes when females consume males to
improve fecundity and/or males sacrifice their bodies to increase fertilization success.
… in the orb-web spider N. plumipes where 60% of males do not survive copulation.
… males that mate with mated females increase their fertilization success by being
cannibalized. Cannibalized males generally mate for longer, but longer copulations
correspond with increased paternity only in mated females. …
Our data suggest that the conflict between the sexes
differs between virgin and mated females.
Females should always cannibalize a male,
but males only gain from cannibalism
when mating with mated females,
not when mating with virgin females.
Interestingly,
the frequencies of cannibalism are not different
in matings with virgin or mated females.
{ the mating game is not for good of the species!}
Males' evolutionary responses to experimental removal of sexual selection.
Pitnick et al. 2001. Proc Royal Society Of London B 268:1071-1080.
Abstract:
We evaluated the influence of … sexual selection upon
male reproductive traits in a naturally promiscuous species, D. melanogaster.
Sexual selection was
removed in two replicate populations through enforced monogamous mating
with random mate assignment
or retained in polyandrous controls.
Monogamous mating eliminates … mate competition, mate discrimination,
sperm competition, cryptic female choice and, hence, sexual conflict.
Levels of divergence between lines in sperm production and male fitness traits
were quantified after 38-81 generations of selection. …
Monogamy males evolved a smaller body size and the size of their testes and
the number of sperm within the testes were disproportionately further reduced.
… the fitness of monogamous males and their mates was greater
when reproducing in a non-competitive context:
females mated once with monogamous males produced offspring at a faster rate
and produced a greater total number of surviving progeny
than did females mated to control males.
The results indicate that sexual selection
favours the production of increased numbers of sperm in D. melanogaster and …
favours some male traits conferring a direct cost to the fecundity of females.
Intrasexual selection for dominance & access in one sex results in
sexual dimorphism within species
Does size dimorphism in humans
suggest anything about ancestral mating systems?
Across primates,
harem size is
correlated w/
dimorphism
Comparing human dimorphism (1.1) to pattern in primates
suggests humans evolved dominance
and mating system with at least a little polygyny
This is consistent with contemporary prevalence of polygyny
2
More on Primate sexual dimorphism & mating system
(from The Third Chimpanzee, Diamond 1992)
Chimps are not size dimorphic;
males do not control
access to harem
Humans slightly dimorphic;
slightly polygynous;
do guard mate(s)
Inverse correlation between
size dimorphism vs testicle size;
hyp: the more males compete for
& control access to copulation,
the less sperm
Allometry of female breasts
Human female breasts padded w/ fat,
much larger than necessary to nourish babies.
Increasing size dimorphism
correlated with increasing
male-male competition for
Males find ‘shapely’ breasts attractive.
Hyp: Large breasts advertise, youth,
health & high repro. potential.
access to copulation
Gorillas are very dimorphic;
males control access to harem
competition
Human mating system seems to have evolved
w/ some competition for access to copulation and maybe a little sperm competition.
Consider female choices:
Angier (1999)
claims
male obsession
w/ breasts
is maladaptive
Big mystery: Why human male penis so big? J. Diamond hyp: signal to other males?
HILL GE. 1991.
Plumage coloration
is a sexually selected indicator of male quality.
NATURE 350: 337-339
"I'm sorry Roger,
but I've met so meone
else with brighter plumage."
Status symbols signal:
wealth and/or health
‘Goods’ to be delivered:
good genes - indirect benefit
good help - direct benefit
Brightened control lightened
Among unmanipulated males, plumage
coloration was correlated with nest
attentiveness and overwinter survival.
The
survival cost of advertising
is compensated by the
reproductive mating benefits
In addition, there was a positive
correlation between the coloration of
fathers and sons: “sexy sons”
female
male
Social vs genetic mating system:
Results suggest that …
unattractive males accrue fitness gains through high parental investment (PI), ...
attractive males benefit through decreased PI and increased allocation to EPF.
Gill et al. 1999.
Male attractiveness and differential testosterone investment in zebra finch eggs.
SCIENCE 286: 126-128.
… female birds deposit higher amounts of testosterone … in their eggs
when mated to more attractive males {w/ red vs green leg bands}
% resighted
58
% paired
100
Days till pair
12
≈
>
<
50 ≈
60 >
20 <
65
27
28
s
s
Son’s plumage
… Artificially brightened males
paired more quickly and frequently
than sham control or lightened males.
Fitness of signaler
depends on
survival & reproduction
Burley et al. 1996. Sexual selection and extrapair fertilization
in a socially monogamous passerine, the zebra finch ...
Behav. Ecol. 7:218-226.
Abstract:
… the percentage of young reared by a male
that were not his genetic offspring …
as determined … DNA fingerprinting …
… were 16% and 40%
for red- and green-banded males
{females like red-banded males}
… directly proportional to rates of female participation
in unforced extrapair copulations (UEPCs) …
In the house finch (Carpodacus mexicanus),
male plumage coloration is a function of
dietary intake of carotenoids. …
Feeding visits/hr
Often animals
choose mates by
selecting for big signals
or costly advertisements:
“Status
Status symbols”
symbols
s
s
s
s
dull
bright
Barber N. 1995.
The evolutionary psychology of physical attractiveness:
Sexual selection and human morphology.
ETHOL. & SOCIOBIOL. 16: 395-424.
Abstract:
Psychological evidence suggests that sex differences in morphology
have been modified by sexual selection so as to
attract mates (intersexual selection) or intimidate rivals (intrasexual selection):
Women compete with each other for high quality husbands
by advertising reproductive value …
by exaggerating morphological indicators of youthfulness
such as a small nose and small feet and pale, hairless skin.
Low waist-hip ratio is
sexually attractive in women ...
Men's
Men physical appearance tends to
communicate social dominance,
dominance
which has the combined effects of intimidating
reproductive rivals and attracting mates.
The male beard is not obviously
related to phenotypic quality ...
{ Hunt et al. 1997. Red preference lost if filter out UV light }
3
Waynforth, D. & RIM Dunbar. 1995.
Conditional mate choice strategies in humans
- evidence from lonely hearts advertisements.
Behaviour 132: 755-779.
Abstract:
Advertisements from 'Lonely Hearts' columns
in four US newspapers are used to test hypotheses
about mate preferences by male and female humans.
Parental care is costly.
Proximate costs:
costs time, energy
& resources
Ultimate cost:
cost
Ultimate benefit:
benefit increased
current reproduction
We … confirm conventional findings that, in general,
men prefer young women whose reproductive value is high while
women prefer men who are slightly older than themselves ...
men
reduced
future reproduction
women
What ultimately selects for
parental care:
The effort increases survival of
current offspring (benefit)
more than it decreases production
of future offspring (cost)
All adaptive differences in behavior must have some underlying mechanism:
Cantoni D, Brown RE. 1997.
Paternal investment and reproductive success
in the California mouse, Peromyscus californicus.
ANIM. BEHAV. 54: 377-386.
Abstract:
… single females and mated pairs of Peromyscus californicus …
were required to 'forage' for food by running in a wheel. ...
Females alone were able to raise litters of two pups,
but not four pups.
Mated pairs were able to raise a litter of four pups. …
Single females that kept their pups alive
had a mean inter-birth interval of 53 days;
paired females had a mean inter-birth interval of 37 days.
… the presence of the male resulted in four times as many pups
reared over a 74-day period as a female could rear on her own.
So, for male to benefit by abandoning mate to raise offspring,
he would have to expect 4 times as many litters.
Need for ‘babysitter/heater’ while mom forages selects paternal care.
Cellular mechanisms of social attachment.
Young et al. 2001. Hormones and Behavior 40:133-138.
Abstract:
Pharmacological studies in prairie voles have suggested that
the neuropeptides oxytocin and vasopressin {= ADH; see Fig. 45.7}
play important roles in behaviors associated with monogamy,
monogamy
including affiliation, paternal care, and pair bonding. …
Monogamous prairie voles have a higher density of oxytocin receptors the
nucleus accumbens than do nonmonogamous vole species;
blockade of these receptors by site-specific injection of antagonist
in the female prairie vole prevents partner preference formation.
Prairie voles also have a higher density of vasopressin receptors in the
ventral pallidal area,
area which is the major output of the nucleus accumbens,
than montane voles.
Both the nucleus accumbens and ventral pallidum are key relay nuclei
in the brain circuits implicated in reward,
reward such as
the mesolimbic dopamine and opioid systems.
systems
Therefore, we hypothesize that
oxytocin and vasopressin may be facilitating affiliation and social attachment
in monogamous species by modulating these reward pathways.
Fitness depends on lifetime reproductive success:
selection for individuals to
invest in own offspring (or close kin)
rather than unrelated offspring.
‘Discriminative parental solicitude’ in humans?
* Note: 14/1000 = 1.4%
1 natural parent
+ 1 step parent
rel risk = 40.1
(1999)
Increased affiliative response to vasopressin
in mice expressing the V-1a receptor from a monogamous vole.
Young LJ, Nilsen R, Waymire KG, MacGregor GR, Insel TR
NATURE 400 (6746): 766-768 AUG 19 1999
1 natural parent
rel risk = 12.5
2 natural parents
0-4
5-10
11-16
child’s age in years
0
14
Victims / 1000 children in pop
Daly, M & M. Wilson. 1985.
Child abuse and other risks of not living with both parents.
Ethol. & Sociobiol. 6:155-176.
An assessment of
some proposed exceptions
to the phenomenon of
nepotistic discrimination
against stepchildren.
Daly M, Wilson M
ANNALES ZOOLOGICI FENNICI
38 (3-4): 287-296 2001
The presence of a step-parent in
the home is the most powerful
statistical predictor of
severe child abuse and child
homicide yet discovered.
4