Pre- and Postmating Reproductive Isolation in
Populations of the Red-sided Garter Snake
By Rachel McAlister
Mentors: Drs. Robert Mason, Suzanne Estes, & Steve Arnold
Department of Zoology
Evolutionary theory predicts that during
a speciation event, premating reproductive isolation will evolve
faster than postmating reproductive isolation in populations
undergoing sexual selection (R.A. Fisher 1930).
Evolutionary theory predicts that during
a speciation event, premating reproductive isolation will evolve
faster than postmating reproductive isolation in populations
undergoing sexual selection.

premating reproductive isolation- behavior preventing
individuals from mating and producing offspring.
Male
Female gold finches choose
mates based on the color
and the brightness of their
plumage.
Evolutionary theory predicts that during
a speciation event, premating reproductive isolation will evolve
faster than postmating reproductive isolation in populations
undergoing sexual selection.

post-mating reproductive isolation- prevents a hybrid
produced by two different species from developing
into a viable, fertile adult.
=
X
Horse
Donkey
Mule
(Sterility)
Postmating reproductive isolation also occurs as a
result of reduced fitness and increased mortality of
hybrid offspring.
Non-hybrid
Hybrid
Evolutionary theory predicts that during
a speciation event, premating reproductive isolation will evolve
faster than postmating reproductive isolation in populations
undergoing sexual selection.

sexual selection- A difference between the mating success of
individuals with a particular phenotype versus individuals
with a different phenotype.
Galapagos Island Marine Iguana
Male-male competition
(Body size and aggressiveness)
Peacock
Female choice
(Tail length and elaboration)

Most research on reproductive isolation is conducted
between species, not between populations—the level
at which speciation generally occurs.
Research Question

Does premating reproductive isolation evolve faster
than postmating reproductive isolation in populations
of the red-sided garter snake?
Red-sided Garter Snake
(Thamnophis sirtalis parietalis)
 Mate after spring emergence
 Females can store sperm
 Highly philopatric-always return
to the same den
 Island-like populations
(geographically distinct
populations)
 Populations differ phenotypically
 Body size
 Coloration
 Female pheromone
composition
 Levels of premating isolation
 Experience sexual selection on
body size
Methods:
Collected males
and females from
2 populations in
Manitoba,
Canada during
May 2003
Snake Island
Inwood
*
*
*
*
Methods: Premating Reproductive Isolation
Simultaneous choice tests
20 Snake Island ♂
1 Snake Island ♀
1 Inwood ♀
20 Inwood ♂
1 Snake Island ♀
1 Inwood ♀
Results: No evidence (yet) for premating isolation
Premating Isolation
1.0
N=6
N=6
Snake Island
males MAY
prefer their
own females
over Inwood
females.
0.5
0.0
-0.5
Inwood
Snake Isl.
Snake
Male Population
Methods: Postmating Reproductive Isolation
Within- and between-population crosses:
within
{
Inwood ♀
X
Inwood ♂
Snake Isl. ♀
X
Snake Isl. ♂
Inwood ♀
X
Snake Isl. ♂
Inwood ♂
X
Snake Isl. ♀
Methods: Postmating Reproductive Isolation
Within- and between-population crosses:
between
{
Inwood ♀
X
Inwood ♂
Snake Isl. ♀
X
Snake Isl. ♂
Inwood ♀
X
Snake Isl. ♂
Inwood ♂
X
Snake Isl. ♀
Methods: Postmating Reproductive Isolation
Body Mass
SVL (Snout-Vent Length) & Tail Length
Snout
Body Condition—regression of SVL on mass
Vent/Genitalia
Results: No evidence (yet) for postmating isolation
(Hybrids)
(Hybrids)
Standardized Means
2.5
2.0
Inwood♀ x
Inwood♂
Inwood♀ x
Snake♂
(Hybrids)
Snake ♀ x
Snake♂
Snake♀ x
Inwood♂
1.5
Mass1
SVL1
1.0
Condition1
Tail length
0.5
0.0
-0.5
Cross
Results: No evidence (yet) for postmating isolation
Postmating isolation  reduction in hybrid fitness correlates
compared to within-population offspring
(Hybrids)
(Hybrids)
Standardized Means
2.5
2.0
Inwood♀ x
Inwood♂
Inwood♀ x
Snake♂
(Hybrids)
Snake ♀ x
Snake♂
Snake♀ x
Inwood♂
1.5
Mass1
SVL1
1.0
Condition1
Tail length
0.5
0.0
-0.5
Cross
Results: No evidence (yet) for postmating isolation
Hybrid Vigor?
(Hybrids)
(Hybrids)
Standardized Means
2.5
2.0
Inwood♀ x
Inwood♂
Inwood♀ x
Snake♂
(Hybrids)
Snake ♀ x
Snake♂
Snake♀ x
Inwood♂
1.5
Mass1
SVL1
1.0
Condition1
Tail length
0.5
0.0
-0.5
Cross
BUT, because females store sperm, paternity of
individual offspring is unknown…
Paternity Assignment

Is postmating isolation data
confounded by multiple
paternity of litters?

How many fathers are
responsible for a given litter?

Sperm precedence: How
many offspring sired by
experimental male vs. stored
sperm?
Methods: Paternity Assignment

Determining paternity using two
microsatellite markers
a) Collect tail tips from
known parents & offspring
b) Tissue digestion
c) DNA extraction
d) DNA amplification
-Polymerase chain
reaction (PCR)
e) Genotyping
f) Paternity assignment
by exclusion
microsatellite
GCATACACACACACGAGGTGAC
Results: Extensive multiple paternity

Multiple paternity was
detected in 68% of the litters
(15/22)
16
14


Most litters sired by at least
2 males
One litter sired by at least 3
males
Frequency
12
10
8
6
4
2
0
1
2
3
Number of sires per litter


Experimental males sire
63% of each litter on
average.
However, for betweenpopulation crosses,
experimental males sire
fewer offspring than
experimental males in
within-population crosses.
Fraction of litters sired by
experimental male
Results: Sperm Precedence
(non-significant)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
N =11
N =11
Between-population
Within-population
0
Cross Type
Conclusions
 Multiple paternity is common in T. s. parietalis
litters
68% of litters were sired by 2 or more males
 Data on postmating reproductive isolation (i.e.,
offspring fitness) is likely confounded by
multiple paternity.
Only about half of each litter was sired by the
experimental male; half was sired by stored sperm
Research Question

Does premating reproductive isolation evolve faster
than postmating reproductive isolation in populations
of the red-sided garter snake?
Future Directions
Reanalyze offspring fitness data taking paternity into
account
(Hybrids)
Standardized Means
2.5
2.0
Inwood♀ x
Inwood♂
Inwood♀ x
Snake♂
(Hybrids)
Snake ♀ x
Snake♂
Snake♀ x
Inwood♂
1.5
Mass1
SVL1
1.0
Condition1
Tail length
0.5
0.0
-0.5
Cross
Future Directions

Within- and between-population crosses were repeated and
offspring await paternity analysis

Mating trials will be repeated in May 2005
Acknowledgements

Dr. Suzanne Estes

Dr. Robert Mason

Dr. Steve Arnold

HHMI

Dr. Kevin Ahern
Premating Isolation
Premating isolation = within-population – between-population
pairings
pairings
total number of pairings

Hybrid (between-population)
litters have more sires
represented than do nonhybrid (within-population)
ones
Average # of sires/litter
Results: Extensive Multiple Paternity
NS
(non-significant)
2.5
2.0
1.5
1.0
0.5
N =11
N =11
Between-population
Within-population
0.0
Cross Type