| Summary: | Hybridization is a complex process beginning with the mating of two species. However, hybrid offspring frequency does not predict hetero-specific mating frequency, as post-mating, both pre-zygotic and post-zygotic barriers influence their occurrence. Post-zygotic outbreeding depression usually results in poor embryo-juvenile survival or the production of sterile hybrid offspring. Females have more to lose with each hybrid fertilization than males, and thus should avoid it. Even if females choose con-specific males as preferred mates, they often cannot control which males release sperm during spawning. Polyandry is ubiquitous and may result in hetero-specific sperm competition. In such cases, cryptic female choice (the ability to bias paternity towards certain males under sperm competition) is the last line of defence to prevent hybridization of her eggs, and is highly adaptive if it enables con-specific sperm preference. Such seems to be the case with hybridization of Atlantic salmon (Salmo salar) and brown trout (S. trutta) in their native Europe. Under hetero-specific sperm competition, hybrid fertilizations in these fish are reported to be reduced via ovarian fluid mediated cryptic female choice. It is not known however whether the strength of this mechanism is dependent on reinforcement, and thus historical sympatry/allopatry of hybridizing populations. Brown trout are one of the world’s worst invasive species. Ecological impacts arise through competition with other species (e.g., Galaxids in the southern hemisphere, Oncorhynchus in western North America). Eastern North America is unique in containing native salmonids that evolved in the absence of brown trout, but have gametes that are compatible. The 140 year-old brown trout invasion of Newfoundland is ground zero to study these potential interactions. Their relatively low spread rate across the island may be the result of inherent poor productivity, but data suggest it could also be a function of hybridization with native Atlantic salmon and brook char ...
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