Long-term experimental hybrid swarms between moderately incompatible Tigriopus californicus populations: hybrid inferiority in early generations yields to hybrid superiority in later generations

The deleterious effects of hybridization are a serious concern for the conservation and management of species, particularly when populations mix as a result of human activity. Outbreeding depression is the typical result observed in early-generation interpopulation hybrids of Tigriopus californicus. We examined both controlled crosses and long-term, freely-mating, experimental hybrid populations composed of southern California populations Royal Palms (RP) and San Diego (SD). Controlled crosses included parentals plus all reciprocal F1, F2, F3 and backcross cohorts, and only F2 cohorts showed significant declines in fitness compared to midparent values, indicating recovery in the F3. For long-term studies, four treatment groups were initiated: 100% RP, 100% SD, 50% RP: 50% SD, and 80% RP: 20% SD. Replicates were surveyed at 3-month intervals for morphometric, census and fitness measures. Fitness of hybrid treatments showed declines relative to midparent values followed by rapid recovery, with two hybrid replicates ultimately showing higher fitness than parentals at the final 15-month time-point (up to 20 generations). In contrast, both males and females in hybrid treatments were larger than the midparent for several morphometric characters at the first time-point, and smaller than the midparent at the final time-point, indicating a possible tradeoff between fitness and body size. Microsatellites for a subset of samples revealed extensive introgression in hybrid treatments. This adds to previous evidence that hybrid breakdown in early generations may be a temporary phenomenon followed by the persistence of highly fit recombinant genotypes.