Two Generations of Hybrids between Even- and Odd-Year Pink Salmon (Oncorhynchus gorbuscha): A Test for Outbreeding Depression?

Abstract

Hybrids of genetically isolated odd- and even-year pink salmon (Oncorhynchus gorbuscha) from the same stream were made by fertilizing eggs with cryopreserved milt. Anadromous first-generation (F1) hybrids and controls returned to the hatchery at equal rates (153 of 5483 and 160 of 5492, respectively), on the same average date, and with the same size. However, variances of F1size (female length and weight and male length) exceeded variances of control sizes, suggesting increased genetic variation in F1's. Only 11 of 5165 F2's returned. F2's were similar meristically and in size to fish of their parents' generation, but were bilaterally more asymmetric in number of gill rakers and in combined numbers of gill rakers and of branchiostegals. Increased F1variation followed by low F2returns and increased bilateral asymmetry is a pattern to be expected when coadapted allele complexes are disrupted by outbreeding depression.