Pre- and Postmating Reproductive Isolation in Populations of the Red-sided Garter Snake

By Rachel McAlisterMentors: Drs. Robert Mason, Suzanne Estes, & Steve ArnoldDepartment 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).

premating reproductive isolation- behavior preventing individuals from mating and producing offspring.

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.

Male

post-mating reproductive isolation- prevents a hybrid produced by two different species from developing into a viable, fertile adult.

Mule

=Donkey

XHorse (Sterility)

Evolutionary theory predicts that during a speciation event, premating reproductive isolation will evolve faster than postmating reproductive isolation in populations undergoing sexual selection.

Postmating reproductive isolation also occurs as a result of reduced fitness and increased mortality of hybrid offspring.

HybridNon-hybrid

sexual selection- A difference between the mating success of individuals with a particular phenotype versus individuals with a different phenotype.

Galapagos Island Marine Iguana Peacock

(Tail length and elaboration)(Body size and aggressiveness)Male-male competition Female choice

Evolutionary theory predicts that during a speciation event, premating reproductive isolation will evolve faster than postmating reproductive isolation in populations undergoing sexual selection.

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

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Snake Island *Inwood *

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*

Methods:

Collected males and females from 2 populations in Manitoba, Canada during May 2003

20 Inwood ♂1 Snake Island ♀1 Inwood ♀

20 Snake Island ♂1 Snake Island ♀1 Inwood ♀

Methods: Premating Reproductive Isolation

Simultaneous choice tests

Results: No evidence (yet) for premating isolation

Male Population

Inwood Snake

-0.5

0.0

0.5

1.0

Pre

ma

tin

g I

sola

tio

n

N=6 N=6

Snake Islandmales MAY prefer theirown femalesover Inwoodfemales.

Snake Isl.

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:

{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

Methods: Postmating Reproductive Isolation

Body Mass SVL (Snout-Vent Length) & Tail Length

Body Condition—regression of SVL on mass

Snout Vent/Genitalia

Results: No evidence (yet) for postmating isolation

(Hybrids)

Cross

Sta

nd

ard

ize

d M

ean

s

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Mass1

SVL1

Condition1

Tail length

Inwood♀ x Inwood♂

Inwood♀ x Snake♂

Snake ♀ x Snake♂

Snake♀ x Inwood♂

(Hybrids) (Hybrids)

Results: No evidence (yet) for postmating isolation

(Hybrids)

Cross

Sta

nd

ard

ize

d M

ean

s

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Mass1

SVL1

Condition1

Tail length

Inwood♀ x Inwood♂

Inwood♀ x Snake♂

Snake ♀ x Snake♂

Snake♀ x Inwood♂

(Hybrids) (Hybrids)

Postmating isolation reduction in hybrid fitness correlatescompared to within-population offspring

Results: No evidence (yet) for postmating isolation

(Hybrids)

Cross

Sta

nd

ard

ize

d M

ean

s

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Mass1

SVL1

Condition1

Tail length

Inwood♀ x Inwood♂

Inwood♀ x Snake♂

Snake ♀ x Snake♂

Snake♀ x Inwood♂

(Hybrids) (Hybrids)

Hybrid Vigor?

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

GCATACACACACACGAGGTGAC

microsatellite

Results: Extensive multiple paternity

Multiple paternity was detected in 68% of the litters (15/22)

Most litters sired by at least 2 males

One litter sired by at least 3 males

0

2

4

6

8

10

12

14

16

1 2 3

Number of sires per litter

Fre

qu

en

cy

Results: Sperm Precedence

Experimental males sire 63% of each litter on average.  

However, for between-population crosses, experimental males sire fewer offspring than experimental males in within-population crosses.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Between-population Within-population

(non-significant)

Cross Type

Fra

ctio

n o

f lit

ters

sir

ed b

y e

xpe

rim

en

tal m

ale

N =11 N =11

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?

(Hybrids)

Reanalyze offspring fitness data taking paternity into account

(Hybrids)

Future Directions

Cross

Sta

nd

ard

ize

d M

ean

s

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Mass1

SVL1

Condition1

Tail length

Inwood♀ x Inwood♂

Inwood♀ x Snake♂

Snake ♀ x Snake♂

Snake♀ x Inwood♂

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

Results: Extensive Multiple Paternity

Hybrid (between-population) litters have more sires represented than do non-hybrid (within-population) ones

0.0

0.5

1.0

1.5

2.0

2.5

Between-population Within-population

NS(non-significant)

Cross Type

Av

era

ge

# o

f si

res

/litt

er

N =11 N =11