Females discriminate against heterospecific sperm in a natural hybrid zone.
When hybridization is maladaptive, species-specific mate preferences are selectively favored, but low mate availability may constrain species-assortative pairing. Females paired to heterospecifics may then benefit by copulating with multiple males and subsequently favoring sperm of conspecifics. Whether such mechanisms for biasing paternity toward conspecifics act as important reproductive barriers in socially monogamous vertebrate species remains to be determined. We use a combination of long-term breeding records from a natural hybrid zone between collared and pied flycatchers (Ficedula albicollis and F. hypoleuca), and an in vitro experiment comparing conspecific and heterospecific sperm performance in female reproductive tract fluid, to evaluate the potential significance of female cryptic choice. We show that the females most at risk of hybridizing (pied flycatchers) frequently copulate with multiple males and are able to inhibit heterospecific sperm performance. The negative effect on heterospecific sperm performance was strongest in pied flycatcher females that were most likely to have been previously exposed to collared flycatcher sperm. We thus demonstrate that a reproductive barrier acts after copulation but before fertilization in a socially monogamous vertebrate. While the evolutionary history of this barrier is unknown, our results imply that there is opportunity for it to be accentuated via a reinforcement-like process.
Natural History Museum, University of Oslo, 0318, Oslo, Norway. erc25@cornell.edu.|Current Address: Smithsonian Migratory Bird Center, PO Box 37012 MRC5503, Washington, District of Columbia 20008, and Cornell Lab of Ornithology, Ithaca, New York, 14850. erc25@cornell.edu.
Emily R A
ER
Cramer
Department of Ecology and Genetics, Animal Ecology, Uppsala University, Uppsala, 75236, Sweden.
Murielle
M
Ålund
Department of Ecology and Genetics, Animal Ecology, Uppsala University, Uppsala, 75236, Sweden.
S Eryn
SE
McFarlane
Natural History Museum, University of Oslo, 0318, Oslo, Norway.
Arild
A
Johnsen
Department of Ecology and Genetics, Animal Ecology, Uppsala University, Uppsala, 75236, Sweden.
Anna
A
Qvarnström
D000818:Animals; D005260:Female; D005306:Fertilization; D006824:Hybridization, Genetic; D008297:Male; D020308:Songbirds / Q000235:genetics / Q000502:physiology*; D013094:Spermatozoa / Q000502:physiology*
Cryptic female choice; hybrid zones; postcopulatory prezygotic barriers; reinforcement; sexual selection; speciation