Topping off: A mechanism of firstmale sperm precedence in a vertebrate
Adam G. Jones, Erika M. Adams, and
Stevan J. Arnold
1
Introduction
Materials and Methods
Result
Discussion
2
Introduction
Materials and Methods
Result
Discussion
3
Sexual selection is an important facet of the
evolutionary process.
-male male competition & female choice
Many of key aspects of sexual selection occur
before mating, the competition among sperm
within a female’s reproductive tract.
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Sperm competition research in recent years has
been to quantify the proportion of offspring
sired by the second of two males mated
sequentially to a female, a value known as P2.
Multiple distinct mechanisms can lead to the
same value of P2, so such an approach to the
problem often says very little about the actual
mechanism of sperm competition.
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Taricha granulosa
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Due to the three following reasons
Females receive sperm during a short receptive
period, then it lay eggs after several weeks to
months, fertilizing them with stored sperm.
Females lay large numbers of eggs, a
characteristic that permits a description of the
change in P2 over time.
The transfer of sperm in newts is indirect.
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Sperm competition appears to be rare or
nonexistent in Taricha.
(Halliday,1998)
The extrapolation from laboratory observation
may be premature!
-no data from natural population
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Last-male precedence?
If postinsemination period of amplexus
represents mate guarding, then we might
expect such a behavior to evolve in a species
with last-male sperm precedence.
Some aspects of the mating behavior of
related newts of the genus Triturus imply that
these species experience last-male precedence.
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Introduction
Materials and Methods
Result
Discussion
10
Paternity in Seminatural Breeding
Assemblages
collected unmated females newt on rainy night
collected males by submerging plastic minnow
trap
8♂+8♀
Water-filled tanks
╳6
8♂+3♀
Water-filled tanks
╳6
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Each female given an injection of 10µl of a
0.5mg/ml LH-RH analog to induce egg laying.
57 of them laid eggs. They collected either
hatching or containing well-developed embryo
for microsatellite analysis.
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Mating Patterns in a Natural
Population
Field-mated female newts were collected from
a natural population during the egg-laying
phase by the trap.
Each female given an injection of 10µl LH-RH.
30 of them laid eggs. Hatching for
microsatellite analysis.
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Sperm Precedence Experiment
Female was placed with male in a water-filled
38-liter aquarium, and were scored every 12h
for insemination.
They removed the males and waited until the
sperm cap was no longer visible in the
female’s cloaca, at which time a second male
was placed in the tank with the female.
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Females successfully inseminated twice in
isolation for at least 20 days and then injected
them with 10 l of LH-RH.
They collect eggs in temporal groups by
checking a female’s tank daily for the presence
of eggs.
Eggs raised to hatching for microsatellite
analysis.
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Microsatellite Analyses
In first two experiments, we assayed a random
sample of an average of 24 offspring from
each female.
Paternity was assigned for 99.1% of the
offspring genotyped, resulting in a very precise
estimate of multiple mating by females
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Natural Population:
three loci :Tgr14, Tgr10, and Tgr06
computer program GERUDSIM1.0
Sperm Precedence Experiment:
first assayed all males and females by using six
microsatellite loci
chose the loci that would differentiate the two
experimental males and assayed all hatchlings
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Introduction
Materials and Methods
Result
Discussion
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This result prove that T. granulosa mate with
multiple males during the course of the
breeding season.
The majority of multiply inseminated females
appeared to have mated with just two males.
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tested for a trend in P2 over time
linear model for categorical data
(procedure CATMOD in SAS ) (P=0.09)
failed to reject the null hypothesis that P2 remained
constant (P=0.09)
no significant relationships between P2 and other
measured variables (insemination interval,first male
size, second male size, male size difference)
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Introduction
Materials and Methods
Result
Discussion
24
clearly that the potential for sperm competition is very
high in natura populations of T. granulosa
The idea that each female typically receives sperm
from a single male was based on an extrapolation
from laboratory observations. Such extrapolations
plainly should be treated with caution.
Molecular markers should play a central role in future
efforts to connect the laboratory observations.
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Female accepts sperm from her first mate, and,
if additional space remains in her
spermathecae, she until she has no need for
further sperm.
‘‘ Topping off ’’
This interpretation is also consistent with
anatomical observations of sperm storage in
Notophthalmus and Triturus, close relatives of
Taricha.
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Female should accept sperm only from highquality males.
Sperm from different males in her
spermathecae should have equivalent value to
her. There should be no selective pressure for a
female to engage in cryptic female choice.
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The pattern of sperm competition that we have
documented in T. granulosa differs from the
patterns described for other vertebrate taxa.
The closest vertebrate analog ‘‘passive sperm
loss’’ (PSL) model. (birds)
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Same feather:
passive and lack sperm stratification.
Different feather:
PSL model typically produces last-male
advantage, whereas topping-off model
produces first-male advantage.
PSL model, the timing of inseminations has a
profound effect on the outcome of sperm
competition.
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Technique of assaying offspring in temporal
groups provides far more information
regarding the mechanisms of sperm usage than
simply measuring P2.
This study is consistent with the idea that
sexual dialectics may explain an important
aspect of the evolution of rough-skinned newt
reproductive ecology.
Study contributes to debates regarding the
importance of cryptic female choice.
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