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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
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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).
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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
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post-mating reproductive isolation- prevents a hybrid produced by two different species from developing into a viable, fertile adult. Mule = Donkey X Horse (Sterility) Evolutionary theory predicts that during a speciation event, premating reproductive isolation will evolve faster than postmating reproductive isolation in populations undergoing sexual selection.
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Postmating reproductive isolation also occurs as a result of reduced fitness and increased mortality of hybrid offspring. Hybrid Non-hybrid
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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.
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Most research on reproductive isolation is conducted between species, not between populations—the level at which speciation generally occurs.
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Research Question Does premating reproductive isolation evolve faster than postmating reproductive isolation in populations of the red-sided garter snake?
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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 * * * Methods: Collected males and females from 2 populations in Manitoba, Canada during May 2003
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20 Inwood ♂ 1 Snake Island ♀ 1 Inwood ♀ 20 Snake Island ♂ 1 Snake Island ♀ 1 Inwood ♀ Methods: Premating Reproductive Isolation Simultaneous choice tests
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Results: No evidence (yet) for premating isolation Male Population Premating Isolation N=6 Snake Island males MAY prefer their own females over Inwood females. Snake Isl.
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Inwood ♀ X Inwood ♂ Snake Isl. ♀ XSnake Isl. ♂ Inwood ♀ XSnake Isl. ♂ Inwood ♂ XSnake Isl. ♀ Methods: Postmating Reproductive Isolation Within- and between-population crosses: { within
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Inwood ♀ X Inwood ♂ Snake Isl. ♀ XSnake Isl. ♂ Inwood ♀ XSnake Isl. ♂ Inwood ♂ XSnake Isl. ♀ Methods: Postmating Reproductive Isolation Within- and between-population crosses: { between
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Methods: Postmating Reproductive Isolation Body Mass SVL (Snout-Vent Length) & Tail Length Body Condition—regression of SVL on mass Snout Vent/Genitalia
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Results: No evidence (yet) for postmating isolation (Hybrids) Cross Standardized Means Inwood♀ x Inwood♂ Inwood♀ x Snake♂ Snake ♀ x Snake♂ Snake♀ x Inwood♂ (Hybrids)
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Results: No evidence (yet) for postmating isolation (Hybrids) Cross Standardized Means Inwood♀ x Inwood♂ Inwood♀ x Snake♂ Snake ♀ x Snake♂ Snake♀ x Inwood♂ (Hybrids) Postmating isolation reduction in hybrid fitness correlates compared to within-population offspring
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Results: No evidence (yet) for postmating isolation (Hybrids) Cross Standardized Means Inwood♀ x Inwood♂ Inwood♀ x Snake♂ Snake ♀ x Snake♂ Snake♀ x Inwood♂ (Hybrids) Hybrid Vigor?
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BUT, because females store sperm, paternity of individual offspring is unknown…
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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?
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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
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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 Number of sires per litter Frequency
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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. (non-significant) Cross Type Fraction of litters sired by experimental male N =11
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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
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Research Question Does premating reproductive isolation evolve faster than postmating reproductive isolation in populations of the red-sided garter snake?
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(Hybrids) Reanalyze offspring fitness data taking paternity into account (Hybrids) Future Directions Cross Standardized Means Inwood♀ x Inwood♂ Inwood♀ x Snake♂ Snake ♀ x Snake♂ Snake♀ x Inwood♂
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Future Directions Within- and between-population crosses were repeated and offspring await paternity analysis Mating trials will be repeated in May 2005
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Acknowledgements Dr. Suzanne Estes Dr. Robert Mason Dr. Steve Arnold HHMI Dr. Kevin Ahern
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Premating Isolation Premating isolation = within-population – between-population pairings pairings total number of pairings
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Results: Extensive Multiple Paternity Hybrid (between-population) litters have more sires represented than do non- hybrid (within-population) ones NS (non-significant) Cross Type Average # of sires/litter N =11
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