Effects of outbreeding depression on meristics and bilateral asymmetry in hybrids of spatially separated populations of pink salmon (Oncorhynchus gorbuscha)

Abstract

Different populations of a species distributed over diverse conditions adapt to their local environments to improve their ability to survive or reproduce. Intraspecific hybridization can alter the locally adapted population, resulting in reduced fitness, causing outbreeding depression. Manifestations of outbreeding depression in Pacific salmon include decreases in survival, fitness, and/or fitness-related traits. Many animals have paired morphological structures, resulting from canalization during development, which promote the animal's fitness; more symmetrical individuals often have faster growth, higher fecundity, or better survival. Meristic traits, such as the number of gill rakers in fish, can be easily determined. This study examined the potential effects of outbreeding depression on morphological meristic characteristics. Variation in fish size and meristic counts of returning F₁ and F₂ hybrids of spatially separated populations of pink salmon was compared to those of controls. There was no evidence for increased fluctuating asymmetry in hybrids. Directional asymmetry was significant for branchiostegals and pectoral fin ray counts. No single character consistently had sire or interaction effects except gill rakers; the few significant effects probably result from maternal environment effects. Canalization of bilateral asymmetry seems to be relatively unaffected by outbreeding depression.